I'm attempting to solve http://projecteuler.net/problem=1.
I want to create a method which takes in an integer and then creates an array of all the integers preceding it and the integer itself as values within the array.
Below is what I have so far. Code doesn't work.
def make_array(num)
numbers = Array.new num
count = 1
numbers.each do |number|
numbers << number = count
count = count + 1
end
return numbers
end
make_array(10)
(1..num).to_a is all you need to do in Ruby.
1..num will create a Range object with start at 1 and end at whatever value num is. Range objects have to_a method to blow them up into real Arrays by enumerating each element within the range.
For most purposes, you won't actually need the Array - Range will work fine. That includes iteration (which is what I assume you want, given the problem you're working on).
That said, knowing how to create such an Array "by hand" is valuable learning experience, so you might want to keep working on it a bit. Hint: you want to start with an empty array ([]) instead with Array.new num, then iterate something num.times, and add numbers into the Array. If you already start with an Array of size num, and then push num elements into it, you'll end up with twice num elements. If, as is your case, you're adding elements while you're iterating the array, the loop never exits, because for each element you process, you add another one. It's like chasing a metal ball with the repulsing side of a magnet.
To answer the Euler Question:
(1 ... 1000).to_a.select{|x| x%3==0 || x%5==0}.reduce(:+) # => 233168
Sometimes a one-liner is more readable than more detailed code i think.
Assuming you are learning Ruby by examples on ProjectEuler, i'll explain what the line does:
(1 ... 1000).to_a
will create an array with the numbers one to 999. Euler-Question wants numbers below 1000. Using three dots in a Range will create it without the boundary-value itself.
.select{|x| x%3==0 || x%5==0}
chooses only elements which are divideable by 3 or 5, and therefore multiples of 3 or 5. The other values are discarded. The result of this operation is a new Array with only multiples of 3 or 5.
.reduce(:+)
Finally this operation will sum up all the numbers in the array (or reduce it to) a single number: The sum you need for the solution.
What i want to illustrate: many methods you would write by hand everyday are already integrated in ruby, since it is a language from programmers for programmers. be pragmatic ;)
Related
I have an array containing multiple arrays (the number may vary) of significant dimensions (an array may contain up to 150 objects).
With the array, I need to find a combination (one element for each sub-array) that matches a condition.
Due to the dimensions, I tried to use Enumerator::Lazy as follows
catch :match do
array[0].product(*array[1..-1]).lazy.each do |combination|
throw :match if ConditionMatcher.match(combination)
end
end
However, I realize when I call each the enumerator is evaluated and it performs very slowly.
I have tried to replace each with methods included in Enumerator::Lazy such as take_while
array[0].product(*array[1..-1]).lazy.take_while do |combination|
return false if ConditionMatcher.match(combination)
end
But also, in this case, the product is evaluated with low performance.
For better performance, even id I don't really like it, I'm thinking to replace product with a nested each loop. Something like
catch :match do
array[0].each do |first|
array[1].each do |second|
array[2].each do |third|
throw :match if ConditionMatcher.match([first, second, third])
end
end
end
end
Due to the fact that the number of sub-arrays changes from time to time. I'm not sure how to implement it.
Moreover, is there a better way to loop through all the sub-arrays without loading the entire set of combinations?
Update 1 -
Each sub-array contains an ActiveRecord::Relation on a polymorphic association. Therefore, each element of each combination responds to the same 2 methods (start_time and end_time) each returning an instance of Time.
The matcher checks if all the objects in the combination don't have overlapping times.
The problem is that Array#product already returns a huge array containing all combinations. With 3 sub-arrays containing 150 items each, it returns a 150 × 150 × 150 = 3,375,000 element array. Calling lazy on that array won't speed up anything.
To make product calculate the Cartesian product lazily (i.e. one combination after the other), you simply have to (directly) pass a block to it:
first, *others = array
first.product(*others) do |combination|
# ...
end
I'm working through problem 8 on project Euler and have looked through a bunch of resources. Here is the problem:
"#8 - Find the greatest product of five consecutive digits in the 1000-digit number."
I split the 1000-digt number into an array of strings and converted that to an array of integers.
number = "73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450"
digits = number.split('').reject!{|i| (i=="\n")}
integer_digits = digits.map {|i| i.to_i}
From here, I want to take the first five values, multiple them, and take the resulting value and add it to a new array named "products". I'm trying to remove the first value of the integer_digit array with the .shift method, start the loop over with the second value of the array, and storing the next product of values [1..5] in the integer_digits array...and so on...
getproduct=1
products=[]
loop do
products << integer_digits[0..4].map {|x| (getproduct*=x) }.max
integer_digits.shift
break if integer_digits.length < 5
end
puts products.max
Once the loop went through all the digits, I hoped that I could display the greatest value using the .max method. The code I have returns an empty array...
My question: How do I keep adding the resulting value of the loop to the product array until there are less than five integer_digit values left? And will the .max method work once this is done?
This line:
products << integer_digits[0..4].map {|x| (getproduct*=x) }.max
makes very little sense. What you need is:
products << integer_digits.first(5).inject(:*)
However you shouldn't store all the results, you only need the biggest one:
max = 0
while integer_digits.length >= 5
product = integer_digits.first(5).inject(:*)
max = product if product > max
integer_digits.shift
end
puts max #=> 40824
UPDATE:
The reason why you are getting an empty string is most likely caused by running the loop twice without regenerating integer_digits array (which has 4 elements after the loop)
Also as suggested by #MarkThomas, you can use each_cons method:
integer_digits.each_cons(5).inject(0) {|max, ary| [max, ary.inject(:*)].max }
This has this advantage that it will not modify integer_digits, so you can run it mutliple times over the same set of digits.
I have a 250*2001 matrix. I want to find the location for the maximum value for a(:,i) where i takes 5 different values: i = i + 256
a(:,256)
a(:,512)
a(:,768)
a(:,1024)
a(:,1280)
I tried using MAXLOC, but since I'm new to fortran, I couldn't get it right.
Try this
maxloc(a(:,256:1280:256))
but be warned, this call will return a value in the range 1..5 for the second dimension. The call will return the index of the maxloc in the 2001*5 array section that you pass to it. So to get the column index of the location in the original array you'll have to do some multiplication. And note that since the argument in the call to maxloc is a rank-2 array section the call will return a 2-element vector.
Your question is a little unclear: it could be either of two things you want.
One value for the maximum over the entire 250-by-5 subarray;
One value for the maximum in each of the 5 250-by-1 subarrays.
Your comments suggest you want the latter, and there is already an answer for the former.
So, in case it is the latter:
b(1:5) = MAXLOC(a(:,256:1280:256), DIM=1)
I can't seem to find a definitive answer on this and I want to make sure I understand this to the "n'th level" :-)
a = { "a" => "Hello", "b" => "World" }
a.count # 2
a.size # 2
a.length # 2
a = [ 10, 20 ]
a.count # 2
a.size # 2
a.length # 2
So which to use? If I want to know if a has more than one element then it doesn't seem to matter but I want to make sure I understand the real difference. This applies to arrays too. I get the same results.
Also, I realize that count/size/length have different meanings with ActiveRecord. I'm mostly interested in pure Ruby (1.92) right now but if anyone wants to chime in on the difference AR makes that would be appreciated as well.
Thanks!
For arrays and hashes size is an alias for length. They are synonyms and do exactly the same thing.
count is more versatile - it can take an element or predicate and count only those items that match.
> [1,2,3].count{|x| x > 2 }
=> 1
In the case where you don't provide a parameter to count it has basically the same effect as calling length. There can be a performance difference though.
We can see from the source code for Array that they do almost exactly the same thing. Here is the C code for the implementation of array.length:
static VALUE
rb_ary_length(VALUE ary)
{
long len = RARRAY_LEN(ary);
return LONG2NUM(len);
}
And here is the relevant part from the implementation of array.count:
static VALUE
rb_ary_count(int argc, VALUE *argv, VALUE ary)
{
long n = 0;
if (argc == 0) {
VALUE *p, *pend;
if (!rb_block_given_p())
return LONG2NUM(RARRAY_LEN(ary));
// etc..
}
}
The code for array.count does a few extra checks but in the end calls the exact same code: LONG2NUM(RARRAY_LEN(ary)).
Hashes (source code) on the other hand don't seem to implement their own optimized version of count so the implementation from Enumerable (source code) is used, which iterates over all the elements and counts them one-by-one.
In general I'd advise using length (or its alias size) rather than count if you want to know how many elements there are altogether.
Regarding ActiveRecord, on the other hand, there are important differences. check out this post:
Counting ActiveRecord associations: count, size or length?
There is a crucial difference for applications which make use of database connections.
When you are using many ORMs (ActiveRecord, DataMapper, etc.) the general understanding is that .size will generate a query that requests all of the items from the database ('select * from mytable') and then give you the number of items resulting, whereas .count will generate a single query ('select count(*) from mytable') which is considerably faster.
Because these ORMs are so prevalent I following the principle of least astonishment. In general if I have something in memory already, then I use .size, and if my code will generate a request to a database (or external service via an API) I use .count.
In most cases (e.g. Array or String) size is an alias for length.
count normally comes from Enumerable and can take an optional predicate block. Thus enumerable.count {cond} is [roughly] (enumerable.select {cond}).length -- it can of course bypass the intermediate structure as it just needs the count of matching predicates.
Note: I am not sure if count forces an evaluation of the enumeration if the block is not specified or if it short-circuits to the length if possible.
Edit (and thanks to Mark's answer!): count without a block (at least for Arrays) does not force an evaluation. I suppose without formal behavior it's "open" for other implementations, if forcing an evaluation without a predicate ever even really makes sense anyway.
I found a good answare at http://blog.hasmanythrough.com/2008/2/27/count-length-size
In ActiveRecord, there are several ways to find out how many records
are in an association, and there are some subtle differences in how
they work.
post.comments.count - Determine the number of elements with an SQL
COUNT query. You can also specify conditions to count only a subset of
the associated elements (e.g. :conditions => {:author_name =>
"josh"}). If you set up a counter cache on the association, #count
will return that cached value instead of executing a new query.
post.comments.length - This always loads the contents of the
association into memory, then returns the number of elements loaded.
Note that this won't force an update if the association had been
previously loaded and then new comments were created through another
way (e.g. Comment.create(...) instead of post.comments.create(...)).
post.comments.size - This works as a combination of the two previous
options. If the collection has already been loaded, it will return its
length just like calling #length. If it hasn't been loaded yet, it's
like calling #count.
Also I have a personal experience:
<%= h(params.size.to_s) %> # works_like_that !
<%= h(params.count.to_s) %> # does_not_work_like_that !
We have a several ways to find out how many elements in an array like .length, .count and .size. However, It's better to use array.size rather than array.count. Because .size is better in performance.
Adding more to Mark Byers answer. In Ruby the method array.size is an alias to Array#length method. There is no technical difference in using any of these two methods. Possibly you won't see any difference in performance as well. However, the array.count also does the same job but with some extra functionalities Array#count
It can be used to get total no of elements based on some condition. Count can be called in three ways:
Array#count # Returns number of elements in Array
Array#count n # Returns number of elements having value n in Array
Array#count{|i| i.even?} Returns count based on condition invoked on each element array
array = [1,2,3,4,5,6,7,4,3,2,4,5,6,7,1,2,4]
array.size # => 17
array.length # => 17
array.count # => 17
Here all three methods do the same job. However here is where the count gets interesting.
Let us say, I want to find how many array elements does the array contains with value 2
array.count 2 # => 3
The array has a total of three elements with value as 2.
Now, I want to find all the array elements greater than 4
array.count{|i| i > 4} # =>6
The array has total 6 elements which are > than 4.
I hope it gives some info about count method.
I play around with arrays and hashes quite a lot in ruby and end up with some code that looks like this:
sum = two_dimensional_array.select{|i|
i.collect{|j|
j.to_i
}.sum > 5
}.collect{|i|
i.collect{|j|
j ** 2
}.average
}.sum
(Let's all pretend that the above code sample makes sense now...)
The problem is that even though TextMate (my editor of choice) picks up simple {...} or do...end blocks quite easily, it can't figure out (which is understandable since even I can't find a "correct" way to fold the above) where the above blocks start and end to fold them.
How would you fold the above code sample?
PS: considering that it could have 2 levels of folding, I only care about the outer consecutive ones (the blocks with the i)
To be honest, something that convoluted is probably confusing TextMate as much as anyone else who has to maintain it, and that includes you in the future.
Whenever you see something that rolls up into a single value, it's a good case for using Enumerable#inject.
sum = two_dimensional_array.inject(0) do |sum, row|
# Convert row to Fixnum equivalent
row_i = row.collect { |i| i.to_i }
if (row_i.sum > 5)
sum += row_i.collect { |i| i ** 2 }.average
end
sum # Carry through to next inject call
end
What's odd in your example is you're using select to return the full array, allegedly converted using to_i, but in fact Enumerable#select does no such thing, and instead rejects any for which the function returns nil. I'm presuming that's none of your values.
Also depending on how your .average method is implemented, you may want to seed the inject call with 0.0 instead of 0 to use a floating-point value.