Is there any fast way to find the largest power of 10 smaller than a given number?
I'm using this algorithm, at the moment, but something inside myself dies anytime I see it:
10**( int( math.log10(x) ) ) # python
pow( 10, (int) log10(x) ) // C
I could implement simple log10 and pow functions for my problems with one loop each, but still I'm wondering if there is some bit magic for decimal numbers.
An alternative algorithm is:
i = 1;
while((i * 10) < x)
i *= 10;
Log and power are expensive operations. If you want fast, you probably want to look up the IEEE binary exponent in table to get the approximate power of ten, and then check if the mantissa forces a change by +1 or not. This should be 3 or 4 integer machine instructions (alternatively O(1) with a pretty small constant).
Given tables:
int IEEE_exponent_to_power_of_ten[2048]; // needs to be 2*max(IEEE_exponent)
double next_power_of_ten[600]; // needs to be 2*log10(pow(2,1024)]
// you can compute these tables offline if needed
for (p=-1023;p>1023;p++) // bounds are rough, see actual IEEE exponent ranges
{ IEEE_exponent_to_power_of_ten[p+1024]=log10(pow(2,p)); // you might have to worry about roundoff errors here
next_power_of_ten[log10(pow(2,p))+1024]=pow(10,IEEE_exponent_to_power_of_ten[p+1024]);
}
then your computation should be:
power_of_ten=IEEE_exponent_to_power_of_10[IEEE_Exponent(x)+1023];
if (x>=next_power_of_ten[power_of_ten]) power_of_ten++;
answer=next_power_of_ten[power_of_ten];
[You might really need to write this as assembler to squeeze out every last clock.]
[This code not tested.]
However, if you insist on doing this in python, the interpreter overhead may swamp the log/exp time and it might not matter.
So, do you want fast, or do you want short-to-write?
EDIT 12/23: OP now tells us that his "x" is integral. Under the assumption that it is a 64 (or 32) bit integer, my proposal still works but obviously there isn't an "IEEE_Exponent". Most processors have a "find first one" instruction that will tell you the number of 0 bits on the left hand (most significant) part of the value, e.g., leading zeros; you likely This is in essence 64 (or 32) minus the power of two for the value. Given exponent = 64 - leadingzeros, you have the power of two exponent and most of the rest of the algorithm is essentially unchanged (Modifications left for the reader).
If the processor doesn't have a find-first-one instruction, then probably the best bet is a balanced discrimination tree to determine the power of ten. For 64 bits, such a tree would take at most 18 compares to determine the exponent (10^18 ~~ 2^64).
Create an array of powers of 10. Search through it for the largest value smaller than x.
If x is fairly small, you may find that a linear search provides better performance than a binary search, due in part to fewer branch mis-predictions.
The asymptotically fastest way, as far as I know, involves repeated squaring.
func LogFloor(int value, int base) as int
//iterates values of the form (value: base^(2^i), power: 2^i)
val superPowers = iterator
var p = 1
var c = base
while c <= value
yield (c, p)
c *= c
p += p
endwhile
enditerator
//binary search for the correct power
var p = 0
var c = 1
for val ci, pi in superPowers.Reverse()
if c*ci <= value
c *= ci
p += pi
endif
endfor
return p
The algorithm takes logarithmic time and space in N, which is linear to N's representation size. [The time bound is probably a bit worse because I simplified optimistically]
Note that I assumed arbitrarily large integers (watch out for overflow!), since the naive times-10-until-over algorithm is probably fast enough when dealing with just 32-bit integers.
I think the fastest way is O(log(log(n))^2), the while loop takes O(log(log(n)) and it can be recursive call finite time (we can say O(c) where see is constant), first recursive call is takes log(log(sqrt(n))) time second takes .. and the number of sqrt in sqrt(sqrt(sqrt....(n)) < 10 is log(log(n)) and constant, because of machine limitations.
static long findPow10(long n)
{
if (n == 0)
return 0;
long i = 10;
long prevI = 10;
int count = 1;
while (i < n)
{
prevI = i;
i *= i;
count*=2;
}
if (i == n)
return count;
return count / 2 + findPow10(n / prevI);
}
In Python:
10**(len(str(int(x)))-1)
Given that this is language independent, if you can get the power of two that this number is significant to, eg y in x*2^y (which is the way the number is stored, though I'm not sure I have seen an easy way to access y in any language I have used) then if
z = int(y/(ln(10)/ln(2)))
(one floating point division)
10^z or 10^(z+1) will be your answer, though 10^z is still is not so simple (beg to be corrected).
I timed the methods with the following variations in C++ for the value a being a size_t type (inlining improves performance but does not change relative ordering).
Try 1: Multiply until find number.
size_t try1( size_t a )
{
size_t scalar = 1ul;
while( scalar * 10 < a ) scalar *= 10;
return scalar;
}
Try 2: Multiway if (could also be programmed using a lookup table).
size_t try2( size_t a )
{
return ( a < 10ul ? 1ul :
( a < 100ul ? 10ul :
( a < 1000ul ? 100ul :
( a < 10000ul ? 1000ul :
( a < 100000ul ? 10000ul :
( a < 1000000ul ? 100000ul :
( a < 10000000ul ? 1000000ul :
( a < 100000000ul ? 10000000ul :
( a < 1000000000ul ? 100000000ul :
( a < 10000000000ul ? 1000000000ul :
( a < 100000000000ul ? 10000000000ul :
( a < 1000000000000ul ? 100000000000ul :
( a < 10000000000000ul ? 1000000000000ul :
( a < 100000000000000ul ? 10000000000000ul :
( a < 1000000000000000ul ? 100000000000000ul :
( a < 10000000000000000ul ? 1000000000000000ul :
( a < 100000000000000000ul ? 10000000000000000ul :
( a < 1000000000000000000ul ? 100000000000000000ul :
( a < 10000000000000000000ul ? 1000000000000000000ul :
10000000000000000000ul )))))))))))))))))));
}
Try 3: Modified from findPow10 of #Saaed Amiri, which uses squaring to more rapidly find very large powers than Try 1.
size_t try3( size_t a )
{
if (a == 0)
return 0;
size_t i, j = 1;
size_t prev = 1;
while( j != 100 )
{
i = prev;
j = 10;
while (i <= a)
{
prev = i;
i *= j;
j *= j;
}
}
return prev;
}
Try 4: Lookup table indexed using count leading zeros instruction as per #Ira Baxter.
static const std::array<size_t,64> ltable2{
1ul, 1ul, 1ul, 1ul, 1ul, 10ul, 10ul, 10ul,
100ul, 100ul, 100ul, 1000ul, 1000ul, 1000ul,
1000ul, 10000ul, 10000ul, 10000ul, 100000ul,
100000ul, 100000ul, 1000000ul, 1000000ul,
1000000ul, 1000000ul, 10000000ul, 10000000ul,
10000000ul, 100000000ul, 100000000ul,
100000000ul, 1000000000ul, 1000000000ul,
1000000000ul, 1000000000ul, 10000000000ul,
10000000000ul, 10000000000ul, 100000000000ul,
100000000000ul, 100000000000ul, 1000000000000ul,
1000000000000ul, 1000000000000ul, 1000000000000ul,
10000000000000ul, 10000000000000ul, 10000000000000ul,
100000000000000ul, 100000000000000ul, 100000000000000ul,
1000000000000000ul, 1000000000000000ul, 1000000000000000ul,
1000000000000000ul, 10000000000000000ul, 10000000000000000ul,
10000000000000000ul, 100000000000000000ul, 100000000000000000ul,
100000000000000000ul, 100000000000000000ul, 1000000000000000000ul,
1000000000000000000ul };
size_t try4( size_t a )
{
if( a == 0 ) return 0;
size_t scalar = ltable2[ 64 - __builtin_clzl(a) ];
return (scalar * 10 > a ? scalar : scalar * 10 );
}
Timing is as follows (gcc 4.8)
for( size_t i = 0; i != 1000000000; ++i) try1(i) 6.6
for( size_t i = 0; i != 1000000000; ++i) try2(i) 0.3
for( size_t i = 0; i != 1000000000; ++i) try3(i) 6.5
for( size_t i = 0; i != 1000000000; ++i) try4(i) 0.3
for( size_t i = 0; i != 1000000000; ++i) pow(10,size_t(log10((double)i)))
98.1
The lookup/multiway-if beats everything in C++, but requires we know integers are a finite size. try3 is slower than try1 in this test for smaller values of the loop end value, for large numbers try3 beats try1. In python things are made difficult because integers are not limited so I would combine try2 with try3 to quickly process numbers up to a fixed limit then handle the possibly very large numbers.
In python I think lookup using a list comprehension is probably faster than a multiway-if.
# where we previously define lookuptable = ( 1, 10, 100, ..... )
scalar = [i for i in lookuptable if i < a][-1]
I need help solving problem N from this earlier competition:
Problem N: Digit Sums
Given 3 positive integers A, B and C,
find how many positive integers less
than or equal to A, when expressed in
base B, have digits which sum to C.
Input will consist of a series of
lines, each containing three integers,
A, B and C, 2 ≤ B ≤ 100, 1 ≤ A, C ≤
1,000,000,000. The numbers A, B and C
are given in base 10 and are separated
by one or more blanks. The input is
terminated by a line containing three
zeros.
Output will be the number of numbers,
for each input line (it must be given
in base 10).
Sample input
100 10 9
100 10 1
750000 2 2
1000000000 10 40
100000000 100 200
0 0 0
Sample output
10
3
189
45433800
666303
The relevant rules:
Read all input from the keyboard, i.e. use stdin, System.in, cin or equivalent. Input will be redirected from a file to form the input to your submission.
Write all output to the screen, i.e. use stdout, System.out, cout or equivalent. Do not write to stderr. Do NOT use, or even include, any module that allows direct manipulation of the screen, such as conio, Crt or anything similar. Output from your program is redirected to a file for later checking. Use of direct I/O means that such output is not redirected and hence cannot be checked. This could mean that a correct program is rejected!
Unless otherwise stated, all integers in the input will fit into a standard 32-bit computer word. Adjacent integers on a line will be separated by one or more spaces.
Of course, it's fair to say that I should learn more before trying to solve this, but i'd really appreciate it if someone here told me how it's done.
Thanks in advance, John.
Other people pointed out trivial solution: iterate over all numbers from 1 to A. But this problem, actually, can be solved in nearly constant time: O(length of A), which is O(log(A)).
Code provided is for base 10. Adapting it for arbitrary base is trivial.
To reach above estimate for time, you need to add memorization to recursion. Let me know if you have questions about that part.
Now, recursive function itself. Written in Java, but everything should work in C#/C++ without any changes. It's big, but mostly because of comments where I try to clarify algorithm.
// returns amount of numbers strictly less than 'num' with sum of digits 'sum'
// pay attention to word 'strictly'
int count(int num, int sum) {
// no numbers with negative sum of digits
if (sum < 0) {
return 0;
}
int result = 0;
// imagine, 'num' == 1234
// let's check numbers 1233, 1232, 1231, 1230 manually
while (num % 10 > 0) {
--num;
// check if current number is good
if (sumOfDigits(num) == sum) {
// one more result
++result;
}
}
if (num == 0) {
// zero reached, no more numbers to check
return result;
}
num /= 10;
// Using example above (1234), now we're left with numbers
// strictly less than 1230 to check (1..1229)
// It means, any number less than 123 with arbitrary digit appended to the right
// E.g., if this digit in the right (last digit) is 3,
// then sum of the other digits must be "sum - 3"
// and we need to add to result 'count(123, sum - 3)'
// let's iterate over all possible values of last digit
for (int digit = 0; digit < 10; ++digit) {
result += count(num, sum - digit);
}
return result;
}
Helper function
// returns sum of digits, plain and simple
int sumOfDigits(int x) {
int result = 0;
while (x > 0) {
result += x % 10;
x /= 10;
}
return result;
}
Now, let's write a little tester
int A = 12345;
int C = 13;
// recursive solution
System.out.println(count(A + 1, C));
// brute-force solution
int total = 0;
for (int i = 1; i <= A; ++i) {
if (sumOfDigits(i) == C) {
++total;
}
}
System.out.println(total);
You can write more comprehensive tester checking all values of A, but overall solution seems to be correct. (I tried several random A's and C's.)
Don't forget, you can't test solution for A == 1000000000 without memorization: it'll run too long. But with memorization, you can test it even for A == 10^1000.
edit
Just to prove a concept, poor man's memorization. (in Java, in other languages hashtables are declared differently) But if you want to learn something, it might be better to try to do it yourself.
// hold values here
private Map<String, Integer> mem;
int count(int num, int sum) {
// no numbers with negative sum of digits
if (sum < 0) {
return 0;
}
String key = num + " " + sum;
if (mem.containsKey(key)) {
return mem.get(key);
}
// ...
// continue as above...
// ...
mem.put(key, result);
return result;
}
Here's the same memoized recursive solution that Rybak posted, but with a simpler implementation, in my humble opinion:
HashMap<String, Integer> cache = new HashMap<String, Integer>();
int count(int bound, int base, int sum) {
// No negative digit sums.
if (sum < 0)
return 0;
// Handle one digit case.
if (bound < base)
return (sum <= bound) ? 1 : 0;
String key = bound + " " + sum;
if (cache.containsKey(key))
return cache.get(key);
int count = 0;
for (int digit = 0; digit < base; digit++)
count += count((bound - digit) / base, base, sum - digit);
cache.put(key, count);
return count;
}
This is not the complete solution (no input parsing). To get the number in base B, repeatedly take the modulo B, and then divide by B until the result is 0. This effectively computes the base-B digit from the right, and then shifts the number right.
int A,B,C; // from input
for (int x=1; x<A; x++)
{
int sumDigits = 0;
int v = x;
while (v!=0) {
sumDigits += (v % B);
v /= B;
}
if (sumDigits==C)
cout << x;
}
This is a brute force approach. It may be possible to compute this quicker by determining which sets of base B digits add up to C, arranging these in all permutations that are less than A, and then working backwards from that to create the original number.
Yum.
Try this:
int number, digitSum, resultCounter = 0;
for(int i=1; i<=A, i++)
{
number = i; //to avoid screwing up our counter
digitSum = 0;
while(number > 1)
{
//this is the next "digit" of the number as it would be in base B;
//works with any base including 10.
digitSum += (number % B);
//remove this digit from the number, square the base, rinse, repeat
number /= B;
}
digitSum += number;
//Does the sum match?
if(digitSum == C)
resultCounter++;
}
That's your basic algorithm for one line. Now you wrap this in another For loop for each input line you received, preceded by the input collection phase itself. This process can be simplified, but I don't feel like coding your entire answer to see if my algorithm works, and this looks right whereas the simpler tricks are harder to pass by inspection.
The way this works is by modulo dividing by powers of the base. Simple example, 1234 in base 10:
1234 % 10 = 4
1234 / 10 = 123 //integer division truncates any fraction
123 % 10 = 3 //sum is 7
123 / 10 = 12
12 % 10 = 2 //sum is 9
12 / 10 = 1 //end condition, add this and the sum is 10
A harder example to figure out by inspection would be the same number in base 12:
1234 % 12 = 10 //you can call it "A" like in hex, but we need a sum anyway
1234 / 12 = 102
102 % 12 = 6 // sum 16
102/12 = 8
8 % 12 = 8 //sum 24
8 / 12 = 0 //end condition, sum still 24.
So 1234 in base 12 would be written 86A. Check the math:
8*12^2 + 6*12 + 10 = 1152 + 72 + 10 = 1234
Have fun wrapping the rest of the code around this.
Is there an algorithm for accurately multiplying two arbitrarily long integers together? The language I am working with is limited to 64-bit unsigned integer length (maximum integer size of 18446744073709551615). Realistically, I would like to be able to do this by breaking up each number, processing them somehow using the unsigned 64-bit integers, and then being able to put them back together in to a string (which would solve the issue of multiplied result storage).
Any ideas?
Most languages have functions or libraries that do this, usually called a Bignum library (GMP is a good one.)
If you want to do it yourself, I would do it the same way that people do long multiplication on paper. To do this you could either work with strings containing the number, or do it in binary using bitwise operations.
Example:
45
x67
---
315
+270
----
585
Or in binary:
101
x101
----
101
000
+101
------
11001
Edit: After doing it in binary I realized that it would be much simpler (and faster of course) to code using bitwise operations instead of strings containing the base-10 numbers. I've edited my binary multiplying example to show a pattern: for each 1-bit in the bottom number, add the top number, bit-shifted left the position of the 1-bit times to a variable. At the end, that variable will contain the product.
To store the product, you'll have to have two 64-bit numbers and imagine one of them being the first 64 bits and the other one the second 64 bits of the product. You'll have to write code that carries the addition from bit 63 of the second number to bit 0 of the first number.
If you can't use an existing bignum library like GMP, check out Wikipedia's article on binary multiplication with computers. There are a number of good, efficient algorithms for this.
The simplest way would be to use the schoolbook mechanism, splitting your arbitrarily sized numbers into chunks of 32-bit each.
Given A B C D * E F G H (each chunk 32-bit, for a total 128 bit)
You need an output array 9 dwords wide.
Set Out[0..8] to 0
You'd start by doing: H * D + out[8] => 64 bit result.
Store the low 32-bits in out[8] and take the high 32-bits as carry
Next: (H * C) + out[7] + carry
Again, store low 32-bit in out[7], use the high 32-bits as carry
after doing H*A + out[4] + carry, you need to continue looping until you have no carry.
Then repeat with G, F, E.
For G, you'd start at out[7] instead of out[8], and so forth.
Finally, walk through and convert the large integer into digits (which will require a "divide large number by a single word" routine)
Yes, you do it using a datatype that is effectively a string of digits (just like a normal 'string' is a string of characters). How you do this is highly language-dependent. For instance, Java uses BigDecimal. What language are you using?
This is often given as a homework assignment. The algorithm you learned in grade school will work. Use a library (several are mentioned in other posts) if you need this for a real application.
Here is my code piece in C. Good old multiply method
char *multiply(char s1[], char s2[]) {
int l1 = strlen(s1);
int l2 = strlen(s2);
int i, j, k = 0, c = 0;
char *r = (char *) malloc (l1+l2+1); // add one byte for the zero terminating string
int temp;
strrev(s1);
strrev(s2);
for (i = 0;i <l1+l2; i++) {
r[i] = 0 + '0';
}
for (i = 0; i <l1; i ++) {
c = 0; k = i;
for (j = 0; j < l2; j++) {
temp = get_int(s1[i]) * get_int(s2[j]);
temp = temp + c + get_int(r[k]);
c = temp /10;
r[k] = temp%10 + '0';
k++;
}
if (c!=0) {
r[k] = c + '0';
k++;
}
}
r[k] = '\0';
strrev(r);
return r;
}
//Here is a JavaScript version of an Karatsuba Algorithm running with less time than the usual multiplication method
function range(start, stop, step) {
if (typeof stop == 'undefined') {
// one param defined
stop = start;
start = 0;
}
if (typeof step == 'undefined') {
step = 1;
}
if ((step > 0 && start >= stop) || (step < 0 && start <= stop)) {
return [];
}
var result = [];
for (var i = start; step > 0 ? i < stop : i > stop; i += step) {
result.push(i);
}
return result;
};
function zeroPad(numberString, zeros, left = true) {
//Return the string with zeros added to the left or right.
for (var i in range(zeros)) {
if (left)
numberString = '0' + numberString
else
numberString = numberString + '0'
}
return numberString
}
function largeMultiplication(x, y) {
x = x.toString();
y = y.toString();
if (x.length == 1 && y.length == 1)
return parseInt(x) * parseInt(y)
if (x.length < y.length)
x = zeroPad(x, y.length - x.length);
else
y = zeroPad(y, x.length - y.length);
n = x.length
j = Math.floor(n/2);
//for odd digit integers
if ( n % 2 != 0)
j += 1
var BZeroPadding = n - j
var AZeroPadding = BZeroPadding * 2
a = parseInt(x.substring(0,j));
b = parseInt(x.substring(j));
c = parseInt(y.substring(0,j));
d = parseInt(y.substring(j));
//recursively calculate
ac = largeMultiplication(a, c)
bd = largeMultiplication(b, d)
k = largeMultiplication(a + b, c + d)
A = parseInt(zeroPad(ac.toString(), AZeroPadding, false))
B = parseInt(zeroPad((k - ac - bd).toString(), BZeroPadding, false))
return A + B + bd
}
//testing the function here
example = largeMultiplication(12, 34)
console.log(example)
I want to print the first 10000 prime numbers.
Can anyone give me the most efficient code for this?
Clarifications:
It does not matter if your code is inefficient for n >10000.
The size of the code does not matter.
You cannot just hard code the values in any manner.
The Sieve of Atkin is probably what you're looking for, its upper bound running time is O(N/log log N).
If you only run the numbers 1 more and 1 less than the multiples of 6, it could be even faster, as all prime numbers above 3 are 1 away from some multiple of six.
Resource for my statement
I recommend a sieve, either the Sieve of Eratosthenes or the Sieve of Atkin.
The sieve or Eratosthenes is probably the most intuitive method of finding a list of primes. Basically you:
Write down a list of numbers from 2 to whatever limit you want, let's say 1000.
Take the first number that isn't crossed off (for the first iteration this is 2) and cross off all multiples of that number from the list.
Repeat step 2 until you reach the end of the list. All the numbers that aren't crossed off are prime.
Obviously there are quite a few optimizations that can be done to make this algorithm work faster, but this is the basic idea.
The sieve of Atkin uses a similar approach, but unfortunately I don't know enough about it to explain it to you. But I do know that the algorithm I linked takes 8 seconds to figure out all the primes up to 1000000000 on an ancient Pentium II-350
Sieve of Eratosthenes Source Code: http://web.archive.org/web/20140705111241/http://primes.utm.edu/links/programs/sieves/Eratosthenes/C_source_code/
Sieve of Atkin Source Code: http://cr.yp.to/primegen.html
This isn't strictly against the hardcoding restriction, but comes terribly close. Why not programatically download this list and print it out, instead?
http://primes.utm.edu/lists/small/10000.txt
In Haskell, we can write down almost word for word the mathematical definition of the sieve of Eratosthenes, "primes are natural numbers above 1 without any composite numbers, where composites are found by enumeration of each prime's multiples":
import Data.List.Ordered (minus, union)
primes = 2 : minus [3..] (foldr (\p r -> p*p : union [p*p+p, p*p+2*p..] r)
[] primes)
primes !! 10000 is near-instantaneous.
References:
Sieve of Eratosthenes
Richard Bird's sieve (see pp. 10,11)
minus, union
The above code is easily tweaked into working on odds only, primes = 2 : 3 : minus [5,7..] (foldr (\p r -> p*p : union [p*p+2*p, p*p+4*p..] r) [] (tail primes)). Time complexity is much improved (to just about a log factor above optimal) by folding in a tree-like structure, and space complexity is drastically improved by multistage primes production, in
primes = 2 : _Y ( (3:) . sieve 5 . _U . map (\p -> [p*p, p*p+2*p..]) )
where
_Y g = g (_Y g) -- non-sharing fixpoint combinator
_U ((x:xs):t) = x : (union xs . _U . pairs) t -- ~= nub.sort.concat
pairs (xs:ys:t) = union xs ys : pairs t
sieve k s#(x:xs) | k < x = k : sieve (k+2) s -- ~= [k,k+2..]\\s,
| otherwise = sieve (k+2) xs -- when s⊂[k,k+2..]
(In Haskell the parentheses are used for grouping, a function call is signified just by juxtaposition, (:) is a cons operator for lists, and (.) is a functional composition operator: (f . g) x = (\y -> f (g y)) x = f (g x)).
GateKiller, how about adding a break to that if in the foreach loop? That would speed up things a lot because if like 6 is divisible by 2 you don't need to check with 3 and 5. (I'd vote your solution up anyway if I had enough reputation :-) ...)
ArrayList primeNumbers = new ArrayList();
for(int i = 2; primeNumbers.Count < 10000; i++) {
bool divisible = false;
foreach(int number in primeNumbers) {
if(i % number == 0) {
divisible = true;
break;
}
}
if(divisible == false) {
primeNumbers.Add(i);
Console.Write(i + " ");
}
}
#Matt: log(log(10000)) is ~2
From the wikipedia article (which you cited) Sieve of Atkin:
This sieve computes primes up to N
using O(N/log log N) operations with
only N1/2+o(1) bits of memory. That is
a little better than the sieve of
Eratosthenes which uses O(N)
operations and O(N1/2(log log N)/log
N) bits of memory (A.O.L. Atkin, D.J. Bernstein, 2004). These asymptotic
computational complexities include
simple optimizations, such as wheel
factorization, and splitting the
computation to smaller blocks.
Given asymptotic computational complexities along O(N) (for Eratosthenes) and O(N/log(log(N))) (for Atkin) we can't say (for small N=10_000) which algorithm if implemented will be faster.
Achim Flammenkamp wrote in The Sieve of Eratosthenes:
cited by:
#num1
For intervals larger about 10^9,
surely for those > 10^10, the Sieve of
Eratosthenes is outperformed by the
Sieve of Atkins and Bernstein which
uses irreducible binary quadratic
forms. See their paper for background
informations as well as paragraph 5 of
W. Galway's Ph.D. thesis.
Therefore for 10_000 Sieve of Eratosthenes can be faster then Sieve of Atkin.
To answer OP the code is prime_sieve.c (cited by num1)
Using GMP, one could write the following:
#include <stdio.h>
#include <gmp.h>
int main() {
mpz_t prime;
mpz_init(prime);
mpz_set_ui(prime, 1);
int i;
char* num = malloc(4000);
for(i=0; i<10000; i++) {
mpz_nextprime(prime, prime);
printf("%s, ", mpz_get_str(NULL,10,prime));
}
}
On my 2.33GHz Macbook Pro, it executes as follows:
time ./a.out > /dev/null
real 0m0.033s
user 0m0.029s
sys 0m0.003s
Calculating 1,000,000 primes on the same laptop:
time ./a.out > /dev/null
real 0m14.824s
user 0m14.606s
sys 0m0.086s
GMP is highly optimized for this sort of thing. Unless you really want to understand the algorithms by writing your own, you'd be advised to use libGMP under C.
Not efficient at all, but you can use a regular expression to test for prime numbers.
/^1?$|^(11+?)\1+$/
This tests if, for a string consisting of k “1”s, k is not prime (i.e. whether the string consists of one “1” or any number of “1”s that can be expressed as an n-ary product).
I have adapted code found on the CodeProject to create the following:
ArrayList primeNumbers = new ArrayList();
for(int i = 2; primeNumbers.Count < 10000; i++) {
bool divisible = false;
foreach(int number in primeNumbers) {
if(i % number == 0) {
divisible = true;
}
}
if(divisible == false) {
primeNumbers.Add(i);
Console.Write(i + " ");
}
}
Testing this on my ASP.NET Server took the rountine about 1 minute to run.
Sieve of Eratosthenes is the way to go, because of it's simplicity and speed. My implementation in C
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <math.h>
int main(void)
{
unsigned int lim, i, j;
printf("Find primes upto: ");
scanf("%d", &lim);
lim += 1;
bool *primes = calloc(lim, sizeof(bool));
unsigned int sqrtlim = sqrt(lim);
for (i = 2; i <= sqrtlim; i++)
if (!primes[i])
for (j = i * i; j < lim; j += i)
primes[j] = true;
printf("\nListing prime numbers between 2 and %d:\n\n", lim - 1);
for (i = 2; i < lim; i++)
if (!primes[i])
printf("%d\n", i);
return 0;
}
CPU Time to find primes (on Pentium Dual Core E2140 1.6 GHz, using single core)
~ 4s for lim = 100,000,000
Here is a Sieve of Eratosthenes that I wrote in PowerShell a few days ago. It has a parameter for identifying the number of prime numbers that should be returned.
#
# generate a list of primes up to a specific target using a sieve of eratosthenes
#
function getPrimes { #sieve of eratosthenes, http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
param ($target,$count = 0)
$sieveBound = [math]::ceiling(( $target - 1 ) / 2) #not storing evens so count is lower than $target
$sieve = #($false) * $sieveBound
$crossLimit = [math]::ceiling(( [math]::sqrt($target) - 1 ) / 2)
for ($i = 1; $i -le $crossLimit; $i ++) {
if ($sieve[$i] -eq $false) {
$prime = 2 * $i + 1
write-debug "Found: $prime"
for ($x = 2 * $i * ( $i + 1 ); $x -lt $sieveBound; $x += 2 * $i + 1) {
$sieve[$x] = $true
}
}
}
$primes = #(2)
for ($i = 1; $i -le $sieveBound; $i ++) {
if($count -gt 0 -and $primes.length -ge $count) {
break;
}
if($sieve[$i] -eq $false) {
$prime = 2 * $i + 1
write-debug "Output: $prime"
$primes += $prime
}
}
return $primes
}
Adapting and following on from GateKiller, here's the final version that I've used.
public IEnumerable<long> PrimeNumbers(long number)
{
List<long> primes = new List<long>();
for (int i = 2; primes.Count < number; i++)
{
bool divisible = false;
foreach (int num in primes)
{
if (i % num == 0)
divisible = true;
if (num > Math.Sqrt(i))
break;
}
if (divisible == false)
primes.Add(i);
}
return primes;
}
It's basically the same, but I've added the "break on Sqrt" suggestion and changed some of the variables around to make it fit better for me. (I was working on Euler and needed the 10001th prime)
The Sieve seems to be the wrong answer. The sieve gives you the primes up to a number N, not the first N primes. Run #Imran or #Andrew Szeto, and you get the primes up to N.
The sieve might still be usable if you keep trying sieves for increasingly larger numbers until you hit a certain size of your result set, and use some caching of numbers already obtained, but I believe it would still be no faster than a solution like #Pat's.
The deque sieve algorithm mentioned by BenGoldberg deserves a closer look, not only because it is very elegant but also because it can occasionally be useful in practice (unlike the Sieve of Atkin, which is a purely academical exercise).
The basic idea behind the deque sieve algorithm is to use a small, sliding sieve that is only large enough to contain at least one separate multiple for each of the currently 'active' prime factors - i.e. those primes whose square does not exceed the lowest number currently represented by the moving sieve. Another difference to the SoE is that the deque sieve stores the actual factors into the slots of composites, not booleans.
The algorithm extends the size of the sieve window as needed, resulting in fairly even performance over a wide range until the sieve starts exceeding the capacity of the CPU's L1 cache appreciably. The last prime that fits fully is 25,237,523 (the 1,579,791st prime), which gives a rough ballpark figure for the reasonable operating range of the algorithm.
The algorithm is fairly simple and robust, and it has even performance over a much wider range than an unsegmented Sieve of Eratosthenes. The latter is a lot faster as long its sieve fits fully into the cache, i.e. up to 2^16 for an odds-only sieve with byte-sized bools. Then its performance drops more and more, although it always remains significantly faster than the deque despite the handicap (at least in compiled languages like C/C++, Pascal or Java/C#).
Here is a rendering of the deque sieve algorithm in C#, because I find that language - despite its many flaws - much more practical for prototyping algorithms and experimentation than the supremely cumbersome and pedantic C++. (Sidenote: I'm using the free LINQPad which makes it possible to dive right in, without all the messiness with setting up projects, makefiles, directories or whatnot, and it gives me the same degree of interactivity as a python prompt).
C# doesn't have an explicit deque type but the plain List<int> works well enough for demonstrating the algorithm.
Note: this version does not use a deque for the primes, because it simply doesn't make sense to pop off sqrt(n) out of n primes. What good would it be to remove 100 primes and to leave 9900? At least this way all the primes are collected in a neat vector, ready for further processing.
static List<int> deque_sieve (int n = 10000)
{
Trace.Assert(n >= 3);
var primes = new List<int>() { 2, 3 };
var sieve = new List<int>() { 0, 0, 0 };
for (int sieve_base = 5, current_prime_index = 1, current_prime_squared = 9; ; )
{
int base_factor = sieve[0];
if (base_factor != 0)
{
// the sieve base has a non-trivial factor - put that factor back into circulation
mark_next_unmarked_multiple(sieve, base_factor);
}
else if (sieve_base < current_prime_squared) // no non-trivial factor -> found a non-composite
{
primes.Add(sieve_base);
if (primes.Count == n)
return primes;
}
else // sieve_base == current_prime_squared
{
// bring the current prime into circulation by injecting it into the sieve ...
mark_next_unmarked_multiple(sieve, primes[current_prime_index]);
// ... and elect a new current prime
current_prime_squared = square(primes[++current_prime_index]);
}
// slide the sieve one step forward
sieve.RemoveAt(0); sieve_base += 2;
}
}
Here are the two helper functions:
static void mark_next_unmarked_multiple (List<int> sieve, int prime)
{
int i = prime, e = sieve.Count;
while (i < e && sieve[i] != 0)
i += prime;
for ( ; e <= i; ++e) // no List<>.Resize()...
sieve.Add(0);
sieve[i] = prime;
}
static int square (int n)
{
return n * n;
}
Probably the easiest way of understanding the algorithm is to imagine it as a special segmented Sieve of Eratosthenes with a segment size of 1, accompanied by an overflow area where the primes come to rest when they shoot over the end of the segment. Except that the single cell of the segment (a.k.a. sieve[0]) has already been sieved when we get to it, because it got run over while it was part of the overflow area.
The number that is represented by sieve[0] is held in sieve_base, although sieve_front or window_base would also be a good names that allow to draw parallels to Ben's code or implementations of segmented/windowed sieves.
If sieve[0] contains a non-zero value then that value is a factor of sieve_base, which can thus be recognised as composite. Since cell 0 is a multiple of that factor it is easy to compute its next hop, which is simply 0 plus that factor. Should that cell be occupied already by another factor then we simply add the factor again, and so on until we find a multiple of the factor where no other factor is currently parked (extending the sieve if needed). This also means that there is no need for storing the current working offsets of the various primes from one segment to the next, as in a normal segmented sieve. Whenever we find a factor in sieve[0], its current working offset is 0.
The current prime comes into play in the following way. A prime can only become current after its own occurrence in the stream (i.e. when it has been detected as a prime, because not marked with a factor), and it will remain current until the exact moment that sieve[0] reaches its square. All lower multiples of this prime must have been struck off due to the activities of smaller primes, just like in a normal SoE. But none of the smaller primes can strike off the square, since the only factor of the square is the prime itself and it is not yet in circulation at this point. That explains the actions taken by the algorithm in the case sieve_base == current_prime_squared (which implies sieve[0] == 0, by the way).
Now the case sieve[0] == 0 && sieve_base < current_prime_squared is easily explained: it means that sieve_base cannot be a multiple of any of the primes smaller than the current prime, or else it would have been marked as composite. I cannot be a higher multiple of the current prime either, since its value is less than the current prime's square. Hence it must be a new prime.
The algorithm is obviously inspired by the Sieve of Eratosthenes, but equally obviously it is very different. The Sieve of Eratosthenes derives its superior speed from the simplicity of its elementary operations: one single index addition and one store for each step of the operation is all that it does for long stretches of time.
Here is a simple, unsegmented Sieve of Eratosthenes that I normally use for sieving factor primes in the ushort range, i.e. up to 2^16. For this post I've modified it to work beyond 2^16 by substituting int for ushort
static List<int> small_odd_primes_up_to (int n)
{
var result = new List<int>();
if (n < 3)
return result;
int sqrt_n_halved = (int)(Math.Sqrt(n) - 1) >> 1, max_bit = (n - 1) >> 1;
var odd_composite = new bool[max_bit + 1];
for (int i = 3 >> 1; i <= sqrt_n_halved; ++i)
if (!odd_composite[i])
for (int p = (i << 1) + 1, j = p * p >> 1; j <= max_bit; j += p)
odd_composite[j] = true;
result.Add(3); // needs to be handled separately because of the mod 3 wheel
// read out the sieved primes
for (int i = 5 >> 1, d = 1; i <= max_bit; i += d, d ^= 3)
if (!odd_composite[i])
result.Add((i << 1) + 1);
return result;
}
When sieving the first 10000 primes a typical L1 cache of 32 KiByte will be exceeded but the function is still very fast (fraction of a millisecond even in C#).
If you compare this code to the deque sieve then it is easy to see that the operations of the deque sieve are a lot more complicated, and it cannot effectively amortise its overhead because it always does the shortest possible stretch of crossings-off in a row (exactly one single crossing-off, after skipping all multiples that have been crossed off already).
Note: the C# code uses int instead of uint because newer compilers have a habit of generating substandard code for uint, probably in order to push people towards signed integers... In the C++ version of the code above I used unsigned throughout, naturally; the benchmark had to be in C++ because I wanted it be based on a supposedly adequate deque type (std::deque<unsigned>; there was no performance gain from using unsigned short). Here are the numbers for my Haswell laptop (VC++ 2015/x64):
deque vs simple: 1.802 ms vs 0.182 ms
deque vs simple: 1.836 ms vs 0.170 ms
deque vs simple: 1.729 ms vs 0.173 ms
Note: the C# times are pretty much exactly double the C++ timings, which is pretty good for C# and ìt shows that List<int> is no slouch even if abused as a deque.
The simple sieve code still blows the deque out of the water, even though it is already operating beyond its normal working range (L1 cache size exceeded by 50%, with attendant cache thrashing). The dominating part here is the reading out of the sieved primes, and this is not affected much by the cache problem. In any case the function was designed for sieving the factors of factors, i.e. level 0 in a 3-level sieve hierarchy, and typically it has to return only a few hundred factors or a low number of thousands. Hence its simplicity.
Performance could be improved by more than an order of magnitude by using a segmented sieve and optimising the code for extracting the sieved primes (stepped mod 3 and unrolled twice, or mod 15 and unrolled once) , and yet more performance could be squeezed out of the code by using a mod 16 or mod 30 wheel with all the trimmings (i.e. full unrolling for all residues). Something like that is explained in my answer to Find prime positioned prime number over on Code Review, where a similar problem was discussed. But it's hard to see the point in improving sub-millisecond times for a one-off task...
To put things a bit into perspective, here are the C++ timings for sieving up to 100,000,000:
deque vs simple: 1895.521 ms vs 432.763 ms
deque vs simple: 1847.594 ms vs 429.766 ms
deque vs simple: 1859.462 ms vs 430.625 ms
By contrast, a segmented sieve in C# with a few bells and whistles does the same job in 95 ms (no C++ timings available, since I do code challenges only in C# at the moment).
Things may look decidedly different in an interpreted language like Python where every operation has a heavy cost and the interpreter overhead dwarfs all differences due to predicted vs. mispredicted branches or sub-cycle ops (shift, addition) vs. multi-cycle ops (multiplication, and perhaps even division). That is bound to erode the simplicity advantage of the Sieve of Eratosthenes, and this could make the deque solution a bit more attractive.
Also, many of the timings reported by other respondents in this topic are probably dominated by output time. That's an entirely different war, where my main weapon is a simple class like this:
class CCWriter
{
const int SPACE_RESERVE = 11; // UInt31 + '\n'
public static System.IO.Stream BaseStream;
static byte[] m_buffer = new byte[1 << 16]; // need 55k..60k for a maximum-size range
static int m_write_pos = 0;
public static long BytesWritten = 0; // for statistics
internal static ushort[] m_double_digit_lookup = create_double_digit_lookup();
internal static ushort[] create_double_digit_lookup ()
{
var lookup = new ushort[100];
for (int lo = 0; lo < 10; ++lo)
for (int hi = 0; hi < 10; ++hi)
lookup[hi * 10 + lo] = (ushort)(0x3030 + (hi << 8) + lo);
return lookup;
}
public static void Flush ()
{
if (BaseStream != null && m_write_pos > 0)
BaseStream.Write(m_buffer, 0, m_write_pos);
BytesWritten += m_write_pos;
m_write_pos = 0;
}
public static void WriteLine ()
{
if (m_buffer.Length - m_write_pos < 1)
Flush();
m_buffer[m_write_pos++] = (byte)'\n';
}
public static void WriteLinesSorted (int[] values, int count)
{
int digits = 1, max_value = 9;
for (int i = 0; i < count; ++i)
{
int x = values[i];
if (m_buffer.Length - m_write_pos < SPACE_RESERVE)
Flush();
while (x > max_value)
if (++digits < 10)
max_value = max_value * 10 + 9;
else
max_value = int.MaxValue;
int n = x, p = m_write_pos + digits, e = p + 1;
m_buffer[p] = (byte)'\n';
while (n >= 10)
{
int q = n / 100, w = m_double_digit_lookup[n - q * 100];
n = q;
m_buffer[--p] = (byte)w;
m_buffer[--p] = (byte)(w >> 8);
}
if (n != 0 || x == 0)
m_buffer[--p] = (byte)((byte)'0' + n);
m_write_pos = e;
}
}
}
That takes less than 1 ms for writing 10000 (sorted) numbers. It's a static class because it is intended for textual inclusion in coding challenge submissions, with a minimum of fuss and zero overhead.
In general I found it to be much faster if focussed work is done on entire batches, meaning sieve a certain range, then extract all primes into a vector/array, then blast out the whole array, then sieve the next range and so on, instead of mingling everything together. Having separate functions focussed on specific tasks also makes it easier to mix and match, it enables reuse, and it eases development/testing.
In Python
import gmpy
p=1
for i in range(10000):
p=gmpy.next_prime(p)
print p
Here is my VB 2008 code, which finds all primes <10,000,000 in 1 min 27 secs on my work laptop. It skips even numbers and only looks for primes that are < the sqrt of the test number. It is only designed to find primes from 0 to a sentinal value.
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles
Button1.Click
Dim TestNum As Integer
Dim X As Integer
Dim Z As Integer
Dim TM As Single
Dim TS As Single
Dim TMS As Single
Dim UnPrime As Boolean
Dim Sentinal As Integer
Button1.Text = "Thinking"
Button1.Refresh()
Sentinal = Val(SentinalTxt.Text)
UnPrime = True
Primes(0) = 2
Primes(1) = 3
Z = 1
TM = TimeOfDay.Minute
TS = TimeOfDay.Second
TMS = TimeOfDay.Millisecond
For TestNum = 5 To Sentinal Step 2
Do While Primes(X) <> 0 And UnPrime And Primes(X) ^ 2 <= TestNum
If Int(TestNum / Primes(X)) - (TestNum / Primes(X)) = 0 Then
UnPrime = False
End If
X = X + 1
Loop
If UnPrime = True Then
X = X + 1
Z = Z + 1
Primes(Z) = TestNum
End If
UnPrime = True
X = 0
Next
Button1.Text = "Finished with " & Z
TM = TimeOfDay.Minute - TM
TS = TimeOfDay.Second - TS
TMS = TimeOfDay.Millisecond - TMS
ShowTime.Text = TM & ":" & TS & ":" & TMS
End Sub
The following Mathcad code calculated the first million primes in under 3 minutes.
Bear in mind that this would be using floating point doubles for all of the numbers and is basically interpreted. I hope the syntax is clear.
Here is a C++ solution, using a form of SoE:
#include <iostream>
#include <deque>
typedef std::deque<int> mydeque;
void my_insert( mydeque & factors, int factor ) {
int where = factor, count = factors.size();
while( where < count && factors[where] ) where += factor;
if( where >= count ) factors.resize( where + 1 );
factors[ where ] = factor;
}
int main() {
mydeque primes;
mydeque factors;
int a_prime = 3, a_square_prime = 9, maybe_prime = 3;
int cnt = 2;
factors.resize(3);
std::cout << "2 3 ";
while( cnt < 10000 ) {
int factor = factors.front();
maybe_prime += 2;
if( factor ) {
my_insert( factors, factor );
} else if( maybe_prime < a_square_prime ) {
std::cout << maybe_prime << " ";
primes.push_back( maybe_prime );
++cnt;
} else {
my_insert( factors, a_prime );
a_prime = primes.front();
primes.pop_front();
a_square_prime = a_prime * a_prime;
}
factors.pop_front();
}
std::cout << std::endl;
return 0;
}
Note that this version of the Sieve can compute primes indefinitely.
Also note, the STL deque takes O(1) time to perform push_back, pop_front, and random access though subscripting.
The resize operation takes O(n) time, where n is the number of elements being added. Due to how we are using this function, we can treat this is a small constant.
The body of the while loop in my_insert is executed O(log log n) times, where n equals the variable maybe_prime. This is because the condition expression of the while will evaluate to true once for each prime factor of maybe_prime. See "Divisor function" on Wikipedia.
Multiplying by the number of times my_insert is called, shows that it should take O(n log log n) time to list n primes... which is, unsurprisingly, the time complexity which the Sieve of Eratosthenes is supposed to have.
However, while this code is efficient, it's not the most efficient... I would strongly suggest using a specialized library for primes generation, such as primesieve. Any truly efficient, well optimized solution, will take more code than anyone wants to type into Stackoverflow.
Using Sieve of Eratosthenes, computation is quite faster compare to "known-wide" prime numbers algorithm.
By using pseudocode from it's wiki (https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes), I be able to have the solution on C#.
/// Get non-negative prime numbers until n using Sieve of Eratosthenes.
public int[] GetPrimes(int n) {
if (n <= 1) {
return new int[] { };
}
var mark = new bool[n];
for(var i = 2; i < n; i++) {
mark[i] = true;
}
for (var i = 2; i < Math.Sqrt(n); i++) {
if (mark[i]) {
for (var j = (i * i); j < n; j += i) {
mark[j] = false;
}
}
}
var primes = new List<int>();
for(var i = 3; i < n; i++) {
if (mark[i]) {
primes.Add(i);
}
}
return primes.ToArray();
}
GetPrimes(100000000) takes 2s and 330ms.
NOTE: Value might vary depend on Hardware Specifications.
Here is my code which finds
first 10,000 primes in 0.049655 sec on my laptop, first 1,000,000 primes in under 6 seconds and first 2,000,000 in 15 seconds
A little explanation. This method uses 2 techniques to find prime number
first of all any non-prime number is a composite of multiples of prime numbers so this code test by dividing the test number by smaller prime numbers instead of any number, this decreases calculation by atleast 10 times for a 4 digit number and even more for a bigger number
secondly besides dividing by prime, it only divides by prime numbers that are smaller or equal to the root of the number being tested further reducing the calculations greatly, this works because any number that is greater than root of the number will have a counterpart number that has to be smaller than root of the number but since we have tested all numbers smaller than the root already, Therefore we don't need to bother with number greater than the root of the number being tested.
Sample output for first 10,000 prime number
https://drive.google.com/open?id=0B2QYXBiLI-lZMUpCNFhZeUphck0 https://drive.google.com/open?id=0B2QYXBiLI-lZbmRtTkZETnp6Ykk
Here is the code in C language,
Enter 1 and then 10,000 to print out the first 10,000 primes.
Edit: I forgot this contains math library ,if you are on windows or visual studio than that should be fine but on linux you must compile the code using -lm argument or the code may not work
Example: gcc -Wall -o "%e" "%f" -lm
#include <stdio.h>
#include <math.h>
#include <time.h>
#include <limits.h>
/* Finding prime numbers */
int main()
{
//pre-phase
char d,w;
int l,o;
printf(" 1. Find first n number of prime numbers or Find all prime numbers smaller than n ?\n"); // this question helps in setting the limits on m or n value i.e l or o
printf(" Enter 1 or 2 to get anwser of first or second question\n");
// decision making
do
{
printf(" -->");
scanf("%c",&d);
while ((w=getchar()) != '\n' && w != EOF);
if ( d == '1')
{
printf("\n 2. Enter the target no. of primes you will like to find from 3 to 2,000,000 range\n -->");
scanf("%10d",&l);
o=INT_MAX;
printf(" Here we go!\n\n");
break;
}
else if ( d == '2' )
{
printf("\n 2.Enter the limit under which to find prime numbers from 5 to 2,000,000 range\n -->");
scanf("%10d",&o);
l=o/log(o)*1.25;
printf(" Here we go!\n\n");
break;
}
else printf("\n Try again\n");
}while ( d != '1' || d != '2' );
clock_t start, end;
double cpu_time_used;
start = clock(); /* starting the clock for time keeping */
// main program starts here
int i,j,c,m,n; /* i ,j , c and m are all prime array 'p' variables and n is the number that is being tested */
int s,x;
int p[ l ]; /* p is the array for storing prime numbers and l sets the array size, l was initialized in pre-phase */
p[1]=2;
p[2]=3;
p[3]=5;
printf("%10dst:%10d\n%10dnd:%10d\n%10drd:%10d\n",1,p[1],2,p[2],3,p[3]); // first three prime are set
for ( i=4;i<=l;++i ) /* this loop sets all the prime numbers greater than 5 in the p array to 0 */
p[i]=0;
n=6; /* prime number testing begins with number 6 but this can lowered if you wish but you must remember to update other variables too */
s=sqrt(n); /* 's' does two things it stores the root value so that program does not have to calaculate it again and again and also it stores it in integer form instead of float*/
x=2; /* 'x' is the biggest prime number that is smaller or equal to root of the number 'n' being tested */
/* j ,x and c are related in this way, p[j] <= prime number x <= p[c] */
// the main loop begins here
for ( m=4,j=1,c=2; m<=l && n <= o;)
/* this condition checks if all the first 'l' numbers of primes are found or n does not exceed the set limit o */
{
// this will divide n by prime number in p[j] and tries to rule out non-primes
if ( n%p[j]==0 )
{
/* these steps execute if the number n is found to be non-prime */
++n; /* this increases n by 1 and therefore sets the next number 'n' to be tested */
s=sqrt(n); /* this calaulates and stores in 's' the new root of number 'n' */
if ( p[c] <= s && p[c] != x ) /* 'The Magic Setting' tests the next prime number candidate p[c] and if passed it updates the prime number x */
{
x=p[c];
++c;
}
j=1;
/* these steps sets the next number n to be tested and finds the next prime number x if possible for the new number 'n' and also resets j to 1 for the new cycle */
continue; /* and this restarts the loop for the new cycle */
}
// confirmation test for the prime number candidate n
else if ( n%p[j]!=0 && p[j]==x )
{
/* these steps execute if the number is found to be prime */
p[m]=n;
printf("%10dth:%10d\n",m,p[m]);
++n;
s = sqrt(n);
++m;
j=1;
/* these steps stores and prints the new prime number and moves the 'm' counter up and also sets the next number n to be tested and also resets j to 1 for the new cycle */
continue; /* and this restarts the loop */
/* the next number which will be a even and non-prime will trigger the magic setting in the next cycle and therfore we do not have to add another magic setting here*/
}
++j; /* increases p[j] to next prime number in the array for the next cycle testing of the number 'n' */
// if the cycle reaches this point that means the number 'n' was neither divisible by p[j] nor was it a prime number
// and therfore it will test the same number 'n' again in the next cycle with a bigger prime number
}
// the loops ends
printf(" All done !!\n");
end = clock();
cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
printf(" Time taken : %lf sec\n",cpu_time_used);
}
I spend some time writing a program calculating a lot of primes and this is the code I'm used to calculate a text file containing the first 1.000.000.000 primes. It's in German, but the interesting part is the method calcPrimes(). The primes are stored in an array called Primzahlen. I recommend a 64bit CPU because the calculations are with 64bit integers.
import java.io.*;
class Primzahlengenerator {
long[] Primzahlen;
int LastUnknown = 2;
public static void main(String[] args) {
Primzahlengenerator Generator = new Primzahlengenerator();
switch(args.length) {
case 0: //Wenn keine Argumente übergeben worden:
Generator.printHelp(); //Hilfe ausgeben
return; //Durchfallen verhindern
case 1:
try {
Generator.Primzahlen = new long[Integer.decode(args[0]).intValue()];
}
catch (NumberFormatException e) {
System.out.println("Das erste Argument muss eine Zahl sein, und nicht als Wort z.B. \"Tausend\", sondern in Ziffern z.B. \"1000\" ausgedrückt werden.");//Hinweis, wie man die Argumente angeben muss ausgeben
Generator.printHelp(); //Generelle Hilfe ausgeben
return;
}
break;//dutchfallen verhindern
case 2:
switch (args[1]) {
case "-l":
System.out.println("Sie müsen auch eine Datei angeben!"); //Hilfemitteilung ausgeben
Generator.printHelp(); //Generelle Hilfe ausgeben
return;
}
break;//durchfallen verhindern
case 3:
try {
Generator.Primzahlen = new long[Integer.decode(args[0]).intValue()];
}
catch (NumberFormatException e) {
System.out.println("Das erste Argument muss eine Zahl sein, und nicht als Wort z.B. \"Tausend\", sondern in Ziffern z.B. \"1000\" ausgedrückt werden.");//Hinweis, wie man die Argumente angeben muss ausgeben
Generator.printHelp(); //Generelle Hilfe ausgeben
return;
}
switch(args[1]) {
case "-l":
Generator.loadFromFile(args[2]);//Datei Namens des Inhalts von Argument 3 lesen, falls Argument 2 = "-l" ist
break;
default:
Generator.printHelp();
break;
}
break;
default:
Generator.printHelp();
return;
}
Generator.calcPrims();
}
void printHelp() {
System.out.println("Sie müssen als erstes Argument angeben, die wieviel ersten Primzahlen sie berechnen wollen."); //Anleitung wie man das Programm mit Argumenten füttern muss
System.out.println("Als zweites Argument können sie \"-l\" wählen, worauf die Datei, aus der die Primzahlen geladen werden sollen,");
System.out.println("folgen muss. Sie muss genauso aufgebaut sein, wie eine Datei Primzahlen.txt, die durch den Aufruf \"java Primzahlengenerator 1000 > Primzahlen.txt\" entsteht.");
}
void loadFromFile(String File) {
// System.out.println("Lese Datei namens: \"" + File + "\"");
try{
int x = 0;
BufferedReader in = new BufferedReader(new FileReader(File));
String line;
while((line = in.readLine()) != null) {
Primzahlen[x] = new Long(line).longValue();
x++;
}
LastUnknown = x;
} catch(FileNotFoundException ex) {
System.out.println("Die angegebene Datei existiert nicht. Bitte geben sie eine existierende Datei an.");
} catch(IOException ex) {
System.err.println(ex);
} catch(ArrayIndexOutOfBoundsException ex) {
System.out.println("Die Datei enthält mehr Primzahlen als der reservierte Speicherbereich aufnehmen kann. Bitte geben sie als erstes Argument eine größere Zahl an,");
System.out.println("damit alle in der Datei enthaltenen Primzahlen aufgenommen werden können.");
}
/* for(long prim : Primzahlen) {
System.out.println("" + prim);
} */
//Hier soll code stehen, der von der Datei mit angegebenem Namen ( Wie diese aussieht einfach durch angeben von folgendem in cmd rausfinden:
//java Primzahlengenerator 1000 > 1000Primzahlen.txt
//da kommt ne textdatei, die die primzahlen enthält. mit Long.decode(String ziffern).longValue();
//erhält man das was an der entsprechenden stelle in das array soll. die erste zeile soll in [0] , die zweite zeile in [1] und so weiter.
//falls im arry der platz aus geht(die exception kenn ich grad nich, aber mach mal:
//int[] foo = { 1, 2, 3};
//int bar = foo[4];
//dann kriegst ne exception, das ist die gleiche die man kriegt, wenn im arry der platzt aus geht.
}
void calcPrims() {
int PrimzahlNummer = LastUnknown;
// System.out.println("LAstUnknown ist: " + LastUnknown);
Primzahlen[0] = 2;
Primzahlen[1] = 3;
long AktuelleZahl = Primzahlen[PrimzahlNummer - 1];
boolean IstPrimzahl;
// System.out.println("2");
// System.out.println("3");
int Limit = Primzahlen.length;
while(PrimzahlNummer < Limit) {
IstPrimzahl = true;
double WurzelDerAktuellenZahl = java.lang.Math.sqrt(AktuelleZahl);
for(int i = 1;i < PrimzahlNummer;i++) {
if(AktuelleZahl % Primzahlen[i] == 0) {
IstPrimzahl = false;
break;
}
if(Primzahlen[i] > WurzelDerAktuellenZahl) break;
}
if(IstPrimzahl) {
Primzahlen[PrimzahlNummer] = AktuelleZahl;
PrimzahlNummer++;
// System.out.println("" + AktuelleZahl);
}
AktuelleZahl = AktuelleZahl + 2;
}
for(long prim : Primzahlen) {
System.out.println("" + prim);
}
}
}
I have written this using python, as I just started learning it, and it works perfectly fine. The 10,000th prime generate by this code as same as mentioned in http://primes.utm.edu/lists/small/10000.txt. To check if n is prime or not, divide n by the numbers from 2 to sqrt(n). If any of this range of number perfectly divides n then it's not prime.
import math
print ("You want prime till which number??")
a = input()
a = int(a)
x = 0
x = int(x)
count = 1
print("2 is prime number")
for c in range(3,a+1):
b = math.sqrt(c)
b = int(b)
x = 0
for b in range(2,b+1):
e = c % b
e = int(e)
if (e == 0):
x = x+1
if (x == 0):
print("%d is prime number" % c)
count = count + 1
print("Total number of prime till %d is %d" % (a,count))
I have been working on find primes for about a year. This is what I found to be the fastest:
import static java.lang.Math.sqrt;
import java.io.PrintWriter;
import java.io.File;
public class finder {
public static void main(String[] args) {
primelist primes = new primelist();
primes.insert(3);
primes.insert(5);
File file = new File("C:/Users/Richard/Desktop/directory/file0024.txt");
file.getParentFile().mkdirs();
long time = System.nanoTime();
try{
PrintWriter printWriter = new PrintWriter ("file0024.txt");
int linenum = 0;
printWriter.print("2");
printWriter.print (" , ");
printWriter.print("3");
printWriter.print (" , ");
int up;
int down;
for(int i =1; i<357913941;i++){//
if(linenum%10000==0){
printWriter.println ("");
linenum++;
}
down = i*6-1;
if(primes.check(down)){
primes.insert(down);
//System.out.println(i*6-1);
printWriter.print ( down );
printWriter.print (" , ");
linenum++;
}
up = i*6+1;
if(primes.check(up)){
primes.insert(up);
//System.out.println(i*6+1);
printWriter.print ( up );
printWriter.print (" , ");
linenum++;
}
}
printWriter.println ("Time to execute");
printWriter.println (System.nanoTime()-time);
//System.out.println(primes.length);
printWriter.close ();
}catch(Exception e){}
}
}
class node{
node next;
int x;
public node (){
node next;
x = 3;
}
public node(int z) {
node next;
x = z;
}
}
class primelist{
node first;
int length =0;
node current;
public void insert(int x){
node y = new node(x);
if(current == null){
current = y;
first = y;
}else{
current.next = y;
current = y;
}
length++;
}
public boolean check(int x){
int p = (int)sqrt(x);
node y = first;
for(int i = 0;i<length;i++){
if(y.x>p){
return true;
}else if(x%y.x ==0){
return false;
}
y = y.next;
}
return true;
}
}
1902465190909 nano seconds to get to 2147483629 starting at 2.
Here the code that I made :
enter code here
#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
int main() {
/* Enter your code here. Read input from STDIN. Print output to STDOUT*/
unsigned long int n;
int prime(unsigned long int);
scanf("%ld",&n);
unsigned long int val;
for(unsigned long int i=0;i<n;i++)
{
int flag=0;
scanf("%ld",&val);
flag=prime(val);
if(flag==1)
printf("yes\n");
else
printf("no\n");
}
return 0;
}
int prime(unsigned long int n)
{
if(n==2) return 1;
else if (n == 1||n%2==0) return 0;
for (unsigned long int i=3; i<=sqrt(n); i+=2)
if (n%i == 0)
return 0;
return 1;
}
Using the Array.prototype.find() method in Javascript. 2214.486 ms
function isPrime (number) {
function prime(element) {
let start = 2;
while (start <= Math.sqrt(element)) {
if (element % start++ < 1) {
return false;
}
}
return element > 1;
}
return [number].find(prime)
}
function logPrimes (n) {
let count = 0
let nth = n
let i = 0
while (count < nth) {
if (isPrime(i)) {
count++
console.log('i', i) //NOTE: If this line is ommited time to find 10,000th prime is 121.157ms
if (count === nth) {
console.log('while i', i)
console.log('count', count)
}
}
i++
}
}
console.time(logPrimes)
logPrimes(10000)
console.timeEnd(logPrimes) // 2214.486ms
I can give you some tips, you have to implement it.
For each number, get the half of that number. E.g. for checking 21, only obtain the remainder by dividing it from range 2-10.
If its an odd number, only divide by odd number, and vice versa. Such as for 21, divide with 3, 5, 7, 9 only.
Most efficient method I got up to so far.
Since you want first 10000 primes only, rather than coding complex algorithm I'll suggest
the following
boolean isPrime(int n){
//even but is prime
if(n==2)
return true;
//even numbers filtered already
if(n==0 || n==1 || n%2==0)
return false;
// loop for checking only odd factors
// i*i <= n (same as i<=sqrt(n), avoiding floating point calculations)
for(int i=3 ; i*i <=n ; i+=2){
// if any odd factor divides n then its not a prime!
if(n%i==0)
return false;
}
// its prime now
return true;
}
now call is prime as you need it
for(int i=1 ; i<=1000 ; i++){
if(isPrime(i)){
//do something
}
}
This is an old question, but there's something here everyone's missing...
For primes this small, trial division isn't that slow... there are only 25 primes under 100. With so few primes to test, and such small primes, we can pull out a neat trick!
If a is coprime to b, then gcd a b = 1. Coprime. Fun word. Means it doesn't share any prime factors. We can thus test for divisibility by several primes with one GCD call. How many? Well, the product of the first 15 primes is less than 2^64. And the product of the next 10 is also less than 2^64. That's all 25 that we need. But is it worth it?
Let's see:
check x = null $ filter ((==0) . (x `mod`)) $ [<primes up to 101>]
Prelude> length $ filter check [101,103..85600]
>>> 9975
(0.30 secs, 125,865,152 bytes
a = 16294579238595022365 :: Word64
b = 14290787196698157718
pre = [2,3,5,7,11,13,17,19,23,29,31,37,41,43,47,53,59,61,67,71,73,79,83,89,97]
primes = (pre ++) $ filter ((==1) . gcd a) $ filter ((==1) . gcd b) [99,101..85600]
main = print $ length primes
Prelude> main
>>> 10000
(0.05 secs, 36,387,520 bytes)
A 6 fold improvement there.
(length is to force the list to be computed. By default Haskell prints things 1 Unicode character at a time and so actually printing the list will either dominate the time or dominate the amount of actual code used.)
Of course, this is running in GHCi - a repl running interpreted code - on an old laptop and it is not interpreting any of these numbers as int64s or even BigInts, nor will it even if you ask it to (well, you can force it, but it's ugly and doesn't really help). It is interpreting every single number there as generalized Integer-like things that can be specialized to some particular type via dictionary lookup, and it is traversing a linked list (which is not fused away here as it's not compiled) 3 times. Interestingly, hand fusing the two filters actually slows it down in the REPL.
Let's compile it:
...\Haskell\8.6\Testbed>Primes.exe +RTS -s
10000
606,280 bytes allocated in the heap
Total time 0.000s ( 0.004s elapsed)
Using the RTS report because Windows. Some lines trimmed because they aren't relevant - they were other GC data, or measurements of only part of the execution, and together add up to 0.004s (or less). It's also not constant folding, because Haskell doesn't actually do much of that. If we constant fold ourselves (main = print 10000), we get dramatically lower allocation:
...Haskell\8.6\Testbed>Primes.exe +RTS -s
10000
47,688 bytes allocated in the heap
Total time 0.000s ( 0.001s elapsed)
Literally just enough to load the runtime, then discover there's nothing to do but print a number and exit. Let's add wheel factorization:
wheel = scanl (+) 7 $ cycle [4, 2, 4, 2, 4, 6, 2, 6]
primes = (pre ++) $ filter ((==1) . gcd a) $ filter ((==1) . gcd b) $ takeWhile (<85600) wheel
Total time 0.000s ( 0.003s elapsed)
Cut down approximately 1/3rd relative to our reference of main = print 10000, but there's definitely room for more optimization. It actually stopped to perform a GC in there for example, while with tweaking there shouldn't be any heap use. For some reason, compiling for profiling here actually cuts the runtime down to 2 milliseconds:
Tue Nov 12 21:13 2019 Time and Allocation Profiling Report (Final)
Primes.exe +RTS -p -RTS
total time = 0.00 secs (2 ticks # 1000 us, 1 processor)
total alloc = 967,120 bytes (excludes profiling overheads)
I'm going to leave this as is for now, I'm pretty sure random jitter is starting to dominate.
def compute_primes(bound):
"""
Return a list of the prime numbers in range(2, bound)
Implement the Sieve of Eratosthenes
https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
"""
primeNumber = [True for i in range(bound + 1)]
start_prime_number = 2
primes = []
while start_prime_number * start_prime_number <=bound:
# If primeNumber[start_prime_number] is not changed, then it is a prime
if primeNumber[start_prime_number]:
# Update all multiples of start_prime_number
for i in range(start_prime_number * start_prime_number, bound + 1, start_prime_number):
primeNumber[i] = False
start_prime_number += 1
# Print all prime numbers
for start_prime_number in range(2, bound + 1):
if primeNumber[start_prime_number]:
primes.append(start_prime_number)
return primes
print(len(compute_primes(200)))
print(len(compute_primes(2000)))
This is a Python code that prints prime numbers between 1 to 1000000.
import math
k=0
factor=0
pl=[]
for i in range(1,1000000):
k=int(math.sqrt(i))
if i==2 or i==3:
pl.append(i)
for j in range(2,k+1):
if i%j==0:
factor=factor+1
elif factor==0 and j==k:
pl.append(i)
factor=0
print(pl)
print(len(pl))