I'm running through the problems on Project Euler to teach myself Ruby programming. I know there is a built-in function to do this, but I'm avoiding the built-in functions to help me learn.
So I have to write a method to determine if a number is a prime. The first method works, but the second doesn't. Can anyone explain why?
def is_prime n
for d in 2..(n - 1)
if (n % d) == 0
return false
end
end
true
end
def is_prime2 n
foundDivider = false
for d in 2..(n - 1)
foundDivider = ((n % d) == 0) or foundDivider
end
not foundDivider
end
It's because = is of higher precedence than or. See Ruby's operator precedence table below (highest to lowest precedence):
[ ] [ ]=
**
! ~ + -
* / %
+ -
>> <<
&
^ |
<= < > >=
<=> == === != =~ !~
&&
||
.. ...
? :
= %= { /= -= += |= &= >>= <<= *= &&= ||= **=
defined?
not
or and
if unless while until
begin/end
The problematic line is being parsed as...
(foundDivider = ((n % d) == 0)) or foundDivider
...which is certainly not what you mean. There are two possible solutions:
Force the precedence to be what you really mean...
foundDivider = (((n % d) == 0) or foundDivider)
...or use the || operator instead, which has higher precedence than =:
foundDivider = ((n % d) == 0) || foundDivider
Ruby comes with predefined classes such as Prime. All you have to do is to require that class into your project.
require 'prime'
Than, you can use some of the Prime methods such as first to get first x prime elements:
Prime.first(5) # Ret => [2, 3, 5, 6, 11]
Or you could do something like this:
Prime.each(100) do |prime|
p prime # Ret => [2, 3, 5, 7, 11, ..., 97]
end
I hope you find this useful.
def prime(n)
return false if n < 2
(2..n/2).none?{|i| n % i == 0}
end
A prime number is any number that has no positive divisors other than itself and 1.
def prime? n
(2..Math.sqrt(n)).none? {|f| n % f == 0}
end
The range of factors should start at 2 and end at the square root of n because every number is divisible by one and no number is divisible by two numbers greater than its square root.
Explanation: A non-prime number is the product of two numbers.
n = f1 * f2
n is always divisible by its square root so both f1 and f2 cannot be greater than the square root of n, otherwise f1 * f2 would be greater than n. Therefore, at least one factor is less than or at most equal to Math.sqrt(n). In the case of finding prime numbers its only necessary to find one factor so we should loop from 2 to the square root of n.
Find prime numbers from loop:
def get_prime_no_upto(number)
pre = [1]
start = 2
primes = (start..number).to_a
(start..number).each do |no|
(start..no).each do |num|
if ( no % num == 0) && num != no
primes.delete(no)
break
end
end
end
pre + primes
end
and use it as below:
puts get_prime_no_upto(100)
Cheers!
Here is code that will prompt you to enter a number for prime check:
puts "welcome to prime number check"
puts "enter number for check: "
n = gets
n = n.to_i
def prime(n)
puts "That's not an integer." unless n.is_a? Integer
is_prime = true
for i in 2..n-1
if n % i == 0
is_prime = false
end
end
if is_prime
puts "#{n} is prime!"
else
puts "#{n} is not prime."
end
end
prime(n)
Based on the answer by Darmouse but including edge cases
def prime? (n)
if n <= 1
false
elsif n == 2
true
else
(2..n/2).none? { |i| n % i == 0}
end
end
FYI - re: DarkMouses prime method above - I found it really helpful, but there are a few errors (I think!) that need explaining:
It should be parentheses rather than square brackets... Otherwise you get a TypeError
Range can't be coerced into Fixnum (TypeError)
Secondly, that first colon before 'false' would cause an error too. It's incorrect syntax, as far as I know. Get rid of it.
Lastly, I think you got it the wrong way round?? If you correct the errors I mentioned, it returns true if it ISN'T a prime, and false if it IS.
You can drop the ternary operator altogether I think, and just do:
def prime?(n)
(2..n/2).none?{|i| n % i == 0}
end
Obviously it doesn't cover the edge cases (0,1,2), but let's not split hairs.
...For those who enjoy hairsplitting, here is my full solution to this problem:
def prime?(n)
return false if n < 2
(2..Math.sqrt(n)).none? {|num| length % num == 0}
end
Hope I didn't miss anything :)
This is a little bit off topic according to the details, but correct for the title : using bash integration in ruby you could do :
def is_prime n
`factor #{n}`.split.count < 3
end
bash factor function returns a number plus all of his factors, so if the number is prime, there will be two words count.
This is usefull for code golf only.
I tried this and it worked:
def prime?(n)
return false if n < 2
return true if n == 3 || n == 2
if (2...n-1).any?{|i| n % i == 0}
false
else
true
end
end
def prime?(n)
if n <= 1
return false
else (2..n-1).to_a.all? do |integer|
n % integer != 0
end
end
end
From my prime? lab. Started with eliminating all integers less than or equal to 1.
def prime(n)
pn = [2]
if n < 2
return false
else
(2..n).each do |i|
not_prime = false
(2..Math.sqrt(i).ceil).each do |j|
not_prime = true if i % j == 0
end
pn.push(i) unless not_prime
end
end
return pn
end
p prime(30) gives
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
It will return true if the number is prime.
def prime_number(number)
(2..(number-1)).each do |value|
if (number % value) == 0
return false
end
return true
end
end
puts prime_number(4)
class Object
private
def prime? num
if (2..3).include? num
return true
else
!num.even? and num % 3 != 0 and num > 1
end
end
end
prime? 1
prime? 2
prime? 9
prime? 17
** FOR A SIMPLE SHORTED METHOD**
FIRST INSTALL PRIME GEM
require 'prime'
`p prime.first(20)`
Now save that file as your desired name, this will generate the first 20 prime numbers Automatically!! :-)
Related
I am new to Ruby and just read about methods and such I have been working on a program in order to get the user to type in a number and determine if it is prime or not. This is what I have so far.
print "Enter a number: "
num = gets.chomp
def prime(n)
factors = []
if num < 2
puts "#{num} is a prime number"
end
while n % num == 0 and num < n
factors.push(num)
num += 1
end
return factors
end
Not even sure if I'm on the right track. Really lost. Any help would be greatly appreciated!
require 'prime'
print "Enter a number: "
num = gets.chomp
Prime.prime?(n)
Or, from the scratch:
def prime?(n)
if n > 1
min_condition = Math.sqrt(n).to_i # The highest factor is necessary at most sqrt(n)
factors = 0
(1..min_condition).step(2).each do |i| # It's not necessary to check even numbers
factors += 1 if (n.to_f / i) % 1 == 0
break if factors > 1 # If the algorithm find 2 factors, the number isn't prime
end
!(factors > 1)
else
false
end
end
I know that you're doing this to learn Ruby, but keep in mind that you can just use Prime.prime? to determine whether or not a number is prime.
require 'prime'
Prime.prime?(3)
#=> true
Prime.prime?(4)
#=> false
As for your actual problem, you use both num and n inside your method, but num is defined outside, so won't be in scope. Also: you never seem to actually be calling prime.
There are some problems with your program:
You want to get the user to type in a number and determine if it is
prime or not but the output of your method prime doesn't answer
this question. It will return an array.
If a number is less then 2, it is not a prime number (wikipedia), so this piece of your code is wrong:
if num < 2
puts "#{num} is a prime number"
end
There are plenty of ways to check if a number is a prime number or not, this topic may help you to implement with Ruby.
If you don't want to use ruby base library than below code is useful for you
print "Enter a number: "
num = gets.chomp
def check_prime_number(num)
num = num.to_i
n = 1
factors = []
while (num >= n ) do
if (num % n == 0 )
factors << n unless factors.include?(n)
end
n += 1
end
if ( factors.size > 2)
puts "Factors of number #{num} :-> #{factors}"
elsif num > 0
puts "#{num} is prime"
end
end
check_prime_number(num)
Try if its helpful for you.
I have this code to return true if num is a power of 2.
def is_power_of_two?(num)
result = num.inject(0) {|n1, n2| n2 ** n1}
if result == num
true
else
false
end
end
p is_power_of_two?(16)
I keep getting an error though. How could I fix and simplify this code?
Clearly, n is a non-negative integer.
Code
def po2?(n)
n.to_s(2).count('1') == 1
end
Examples
po2? 0 #=> false
po2? 1 #=> true
po2? 32 #=> true
po2? 33 #=> false
Explanation
Fixnum#to_s provides the string representation of an integer (the receiver) for a given base. The method's argument, which defaults to 10, is the base. For example:
16.to_s #=> "16"
16.to_s(8) #=> "20"
16.to_s(16) #=> "10"
15.to_s(16) #=> "f"
It's base 2 we're interested in. For powers of 2:
1.to_s(2) #=> "1"
2.to_s(2) #=> "10"
4.to_s(2) #=> "100"
8.to_s(2) #=> "1000"
16.to_s(2) #=> "10000"
For a few natural numbers that are are not powers of 2:
3.to_s(2) #=> "11"
5.to_s(2) #=> "101"
11.to_s(2) #=> "1011"
We therefore wish to match binary strings that contain one 1.
Another Way
R = /
\A # match beginning of string ("anchor")
10* # match 1 followed by zero or more zeroes
\z # match end of string ("anchor")
/x # free-spacing regex definition mode
def po2?(n)
(n.to_s(2) =~ R) ? true : false
end
po2?(4) #=> true
po2?(5) #=> false
And one for the road
This uses Fixnum#bit_length and Fixnum#[]:
def po2?(n)
m = n.bit_length-1
n[m] == 1 and m.times.all? { |i| n[i].zero? }
end
po2? 0 #=> false
po2? 1 #=> true
po2? 32 #=> true
po2? 33 #=> false
Try:
def is_power_of_two?(num)
num != 0 && (num & (num - 1)) == 0
end
It is well explained here (for C#, but #GregHewgill's explanation applies here as well)
I would do something like this, using Ruby's Math module.
def power_of_two?(n)
Math.log2(n) % 1 == 0
end
Or, if you wanted to be really cool:
def power_of_two?(n)
(Math.log2(n) % 1).zero?
end
Some IRB output:
2.1.0 :004 > power_of_two?(2)
=> true
2.1.0 :005 > power_of_two?(32768)
=> true
2.1.0 :006 > power_of_two?(65536)
=> true
This method assumes that the input is a positive integer.
Source
Another way to solve this is to go the other way around than most of the answers here - we can use the number 1 to start and find out if the number is the power of two. Like this:
def power_of_two?(num)
product = 1
while product < num
product *= 2
end
product == num
end
We start with 1. Then we multiply the 1 by 2, and keep multiplying by 2 until the product is larger than num (product < num). Once we hit that condition, we stop, exit the loop, and check if it's equal to num (product == num). If it is, the num is the power of 2.
As was pointed out in the comments above, you were getting errors because you're trying to use the inject method on a non-iterable (an int). Here's a solution using the suggested log2
def is_power_of_two?(num)
result = Math.log2(num)
result == Integer(result)
end
Note: will fail with very big numbers close to binaries (like 2 ^ 64 - 1). A foolproof version (but slower) would be:
def is_power_of_two?(num)
while (num % 2 == 0 and num != 0)
num /= 2
end
num == 1
end
Please comment any improvements that any of you may find.
Here is another solution that uses recursion:
def power_of_2?(number)
return true if number == 1
return false if number == 0 || number % 2 != 0
power_of_2?(number / 2)
end
In my opinion, the easiest -- but maybe a little long -- way of doing what you need to do is just writing this recursive method like so:
def power_of_two?(number)
continue = true
if number == 1
return true
end
if number % 2 != 0
return false
else
while continue == true do
if number.to_f / 2.0 == 2.0
continue = false
return true
else
if number % 2 != 0
continue = false
return false
else
number /= 2
continue = true
end
end
end
end
end
One is a power of two (2^0), so it first checks if the number given is 1. If not, it checks if it is odd, because 1 is the only odd number that is a power of two.
If it is odd it returns false and moves on to the else statement. It will check if the number divided by 2 is two, because then it would obviously be a power of 2. It does this as a float, because 5/2 in Ruby would return 2.
If that is false, it then again checks if the number is odd -- unnecessary on the first round, necessary after that. If the number is not odd, it will divide the number by two and then do the loop another time.
This will continue until the program resolves itself by getting 2 or any odd number, and returns true or false, respectively.
I ran into this one in a bootcamp application prep. I'm not a math person and don't understand a few of these methods, so I wanted to submit a common sense approach for people like me. this requires little knowledge of math, except to know a number to the second power will be the result of some number multiplied by itself.
def is_power_of_two?(num)
num.times {|n| return true if (n+1) * (n+1) == num}
false
end
this method counts up to the num variable starting at 1 and returns true if (any of those numbers in the sequence multiplied by itself) is equal to num & if num is not 0 (more on that below).
example:
num = 9
1 * 1 == 9 #=> false
2 * 2 == 9 #=> false
3 * 3 == 9 #=> true
true is returned and method is finished running.
the #times method requires an integer > 0, so this edge case is "handled" by virtue of the fact that #times does nothing with "0" as the variable and returns false when outside of the #times iteration.
def power_of_two?(num)
num.to_s(2).scan(/1/).length == 1
end
This is my code. The code doesn't run at all which leads me to believe that the problem is an infinite loop or something of that sort.
# Question_7, Answer = 104743
# By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.
# What is the 10 001st prime number?
def prime_at(x)
primes = [2, 3]
n = 4
test = true
while primes.length <= x
primes.each do |i|
if n % i != 0
test = false
end
end
if test == true
primes.push(n)
end
n += 1
end
puts primes[-1]
end
prime_at(10001)
Yes you have an infinite loop. The bug is that once the variable test is set to false it never changes to true, so you are not adding elements into the primes array. That means that the condition of your loop primes.length <= x is going to be always true. Hence the infinite loop.
Your test is wrong. n%i != 0 means the number is not divisble by i, meaning the prime test should not fail. There's a lot of 'extra' syntax in here that makes your method a bit hard to understand. For example, test == true is redundant (if test is true or false or anything else, you can just say if test). n%i != 0 returns a boolean value already, so that whole if statement around it is unneeded.
It may seem a bit odd to drop so many constructs found in other languages, but Ruby lets you express your ideas very clearly and understandably. Here's what I would suggest for this method (I didn't change your algorithm at all, this is simply a rewrite):
def prime_at(x)
primes = [2,3]
n = 4
until primes.length == x
primes << n if primes.all? {|p| n % p > 0 }
n += 1
end
primes.last
end
Write a function that prints out all the factors for each of the numbers 1 through 100.
Really amateur coder but here's my attempt so far.
def factors_numbers(n1,n2)
(n1..n2).each do |n|
factors = []
factors << 1 ##every number has a factor of 1
factors << n ##every number is a factor of itself
i = 1
while i < n
new_number = n % (n-i)
if new_number == 0 #if 0, divisible and that means two numbers are factors
factors << new_number
factors << (n-i)
end
i += 1
end
return factors
end
end
Here is an improved version of your code:
def factors_numbers(n1,n2)
all_factors = {}
(n1..n2).each do |n|
factors = []
(1..Math.sqrt(n).floor).each do |i|
remainder = n % i
if remainder == 0 #if 0, divisible and that means two numbers are factors
factors << i
factors << n/i
end
end
factors = factors.sort.uniq
puts "Factors of #{n}: #{factors.join(',')}"
all_factors[n]=[factors]
end
return all_factors
end
Do you want unique factors? That is, in the range 1-100, should I get the number 1 a hundred times, or only once?
The easiest way to do this is by leveraging the "inject" Enumerable method.
def find_all_factors_between(n1,n2)
(n1..n2).inject([]) do |factors, num|
factors + (1..num).inject([]) { |arry, test| num % test == 0 ? arry + [test] : arry }
end
end
One final thing to note is that Ruby has implicit returns; that is, as long as the output of the last line of your method is your factors variable, you don't have to say return factors.
And my entry would be:
def find_all_factors_between(n1, n2)
factors = -> (n) { (1..n).select {|i| n % i == 0} }
(n1..n2).each { |n| puts "Factors of #{n}: #{factors.(n).join(', ')}" }
end
find_all_factors_between(1,100)
Well, if you wanted to do it with enumerables, there's always
def factor_numbers(rng)
factors = rng.map do |n|
(1..Math.sqrt(n).floor) # search numbers <= square root
.select { |d| n % d == 0 } # find factors <= square root
.flat_map { |x| [x, n / x] } # combine with factors >= square root
.sort # order from least to greatest
.uniq # remove dupes (basically the square root)
end
Hash[rng.zip(factors)] # rng = keys, factors = values
end
puts factor_numbers(1..100)
It's not the most efficient, but my point is just that many of the for/while constructs you'd see in languages like C or JavaScript can be expressed in other ways in Ruby.
(n1..n2).each{|x| print "#{x}: #{(1..x).select{|y| x % y == 0}}\n"}
That oughta do it :)
edit: Implemented Cary Swoveland's suggestion
def factor_nums(n1,n2)
all_factors = {}
(n1..n2).each do |n|
factors = []
(1..n).each do |i|
remainder = n % i
factors << i if remainder == 0
end
all_factors[n] = factors
end
return all_factors
end
I am learning Ruby and doing some math stuff. One of the things I want to do is generate prime numbers.
I want to generate the first ten prime numbers and the first ten only. I have no problem testing a number to see if it is a prime number or not, but was wondering what the best way is to do generate these numbers?
I am using the following method to determine if the number is prime:
class Integer < Numeric
def is_prime?
return false if self <= 1
2.upto(Math.sqrt(self).to_i) do |x|
return false if self%x == 0
end
true
end
end
In Ruby 1.9 there is a Prime class you can use to generate prime numbers, or to test if a number is prime:
require 'prime'
Prime.take(10) #=> [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
Prime.take_while {|p| p < 10 } #=> [2, 3, 5, 7]
Prime.prime?(19) #=> true
Prime implements the each method and includes the Enumerable module, so you can do all sorts of fun stuff like filtering, mapping, and so on.
If you'd like to do it yourself, then something like this could work:
class Integer < Numeric
def is_prime?
return false if self <= 1
2.upto(Math.sqrt(self).to_i) do |x|
return false if self%x == 0
end
true
end
def next_prime
n = self+1
n = n + 1 until n.is_prime?
n
end
end
Now to get the first 10 primes:
e = Enumerator.new do |y|
n = 2
loop do
y << n
n = n.next_prime
end
end
primes = e.take 10
require 'prime'
Prime.first(10) # => [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
Check out Sieve of Eratosthenes. This is not Ruby specific but it is an algorithm to generate prime numbers. The idea behind this algorithm is that you have a list/array of numbers say
2..1000
You grab the first number, 2. Go through the list and eliminate everything that is divisible by 2. You will be left with everything that is not divisible by 2 other than 2 itself (e.g. [2,3,5,7,9,11...999]
Go to the next number, 3. And again, eliminate everything that you can divide by 3. Keep going until you reach the last number and you will get an array of prime numbers. Hope that helps.
http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes
People already mentioned the Prime class, which definitely would be the way to go. Someone also showed you how to use an Enumerator and I wanted to contribute a version using a Fiber (it uses your Integer#is_prime? method):
primes = Fiber.new do
Fiber.yield 2
value = 3
loop do
Fiber.yield value if value.is_prime?
value += 2
end
end
10.times { p primes.resume }
# First 10 Prime Numbers
number = 2
count = 1
while count < 10
j = 2
while j <= number
break if number%j == 0
j += 1
end
if j == number
puts number
count += 1
end
number += 1
end
Implemented the Sieve of Eratosthene (more or less)
def primes(size)
arr=(0..size).to_a
arr[0]=nil
arr[1]=nil
max=size
(size/2+1).times do |n|
if(arr[n]!=nil) then
cnt=2*n
while cnt <= max do
arr[cnt]=nil
cnt+=n
end
end
end
arr.compact!
end
Moreover here is a one-liner I like a lot
def primes_c a
p=[];(2..a).each{|n| p.any?{|l|n%l==0}?nil:p.push(n)};p
end
Of course those will find the primes in the first n numbers, not the first n primes, but I think an adaptation won't require much effort.
Here is a way to generate the prime numbers up to a "max" argument from scratch, without using Prime or Math. Let me know what you think.
def prime_test max
primes = []
(1..max).each {|num|
if
(2..num-1).all? {|denom| num%denom >0}
then
primes.push(num)
end
}
puts primes
end
prime_test #enter max
I think this may be an expensive solution for very large max numbers but seems to work well otherwise:
def multiples array
target = array.shift
array.map{|item| item if target % item == 0}.compact
end
def prime? number
reversed_range_array = *(2..number).reverse_each
multiples_of_number = multiples(reversed_range_array)
multiples_of_number.size == 0 ? true : false
end
def primes_in_range max_number
range_array = *(2..max_number)
range_array.map{|number| number if prime?(number)}.compact
end
class Numeric
def prime?
return self == 2 if self % 2 == 0
(3..Math.sqrt(self)).step(2) do |x|
return false if self % x == 0
end
true
end
end
With this, now 3.prime? returns true, and 6.prime? returns false.
Without going to the efforts to implement the sieve algorithm, time can still be saved quickly by only verifying divisibility until the square root, and skipping the odd numbers. Then, iterate through the numbers, checking for primeness.
Remember: human time > machine time.
I did this for a coding kata and used the Sieve of Eratosthenes.
puts "Up to which number should I look for prime numbers?"
number = $stdin.gets.chomp
n = number.to_i
array = (1..n).to_a
i = 0
while array[i]**2 < n
i = i + 1
array = array.select do |element|
element % array[i] != 0 || element / array[i] == 1
end
end
puts array.drop(1)
Ruby: Print N prime Numbers
http://mishra-vishal.blogspot.in/2013/07/include-math-def-printnprimenumbernoofp.html
include Math
def print_n_prime_number(no_of_primes=nil)
no_of_primes = 100 if no_of_primes.nil?
puts "1 \n2"
count = 1
number = 3
while count < no_of_primes
sq_rt_of_num = Math.sqrt(number)
number_divisible_by = 2
while number_divisible_by <= sq_rt_of_num
break if(number % number_divisible_by == 0)
number_divisible_by = number_divisible_by + 1
end
if number_divisible_by > sq_rt_of_num
puts number
count = count+1
end
number = number + 2
end
end
print_n_prime_number
Not related at all with the question itself, but FYI:
if someone doesn't want to keep generating prime numbers again and again (a.k.a. greedy resource saver)
or maybe you already know that you must to work with subsequent prime numbers in some way
other unknown and wonderful cases
Try with this snippet:
require 'prime'
for p in Prime::Generator23.new
# `p` brings subsequent prime numbers until the end of the days (or until your computer explodes)
# so here put your fabulous code
break if #.. I don't know, I suppose in some moment it should stop the loop
end
fp
If you need it, you also could use another more complex generators as Prime::TrialDivisionGenerator or Prime::EratosthenesGenerator. More info
Here's a super compact method that generates an array of primes with a single line of code.
def get_prime(up_to)
(2..up_to).select { |num| (2...num).all? { |div| (num % div).positive? } }
end
def get_prime(number)
(2..number).each do |no|
if (2..no-1).all? {|num| no % num > 0}
puts no
end
end
end
get_prime(100)