Nesting loops on same array but skipping same element

Question

I'm having a bit of trouble trying to find a more Rubyist way to achieve the following. Essentially, I want to try and iterate over every element e and apply e.method(n) for every \$n \in \text{array}\,ドル \$n \ne e\$. In order to determine whether or not \$n = e\,ドル I'll have to use an index comparison (really just test for reference equality as opposed to functional equality).

arr = [413, 321, 654, 23, 11]
(0...arr.length).each do |outer_i|
 (0...arr.length).each do |inner_i|
 next if outer_i == inner_i
 arr[outer_i].apply arr[inner_i]
 end
end

This reeks of Java/C++ and I can tell that this is not the Ruby way, but I can't seem to find an alternative. Any ideas to improve its Ruby-ness? I was thinking of Array#product but I'm not sure where to go from there.

• Flambino: Since appears to be off-topic for Code Review in its current state, since the code you've posted is pseudo-code. See if you can reword it to be more on-topic
• Michael: @Flambino That's not pseudo code.. it will pass any Ruby interpreter (aside from the apply method)
• Flambino: The apply call is my complaint, since the code doesn't run, nor is the function of that method made clear. Hence, the code felt incomplete to me. Pseudo-code like if you'd had a do_stuff method. However, re-reading your question, and seeing tokland's answer, I've retracted my close-vote. I felt there wasn't enough to go on, and misunderstood a few things, but I was wrong.
• Michael: @Flambino Either way, thanks for pointing out the pseudo-code rule; wasn't aware of that! I'll try to steer clear of non-legal code in the future
• 200_success: Did you patch the Fixnum class to define an apply method? That may be the most questionable practice in this code.

Answer 1

Note that you are just doing a permutation of two elements from a set, and there is an abstraction in the core for that, Array#permutation(n):

arr.permutation(2).each { |x, y| x.apply(y) }

• Michael: Can't get much simpler than that!

Answer 2

Some remarks if you don't want to use the permutation-method.

The loop with a counter on an array is not rubyesk. In ruby an iterator like Array#each is preferred (I added the apply-method to the code to make it runnable):

class Fixnum
 def apply(i)
 puts "%i -- %i " % [self,i]
 end
end
arr = [413, 321, 654, 23, 11]
arr.each do |outer|
 arr.each do |inner|
 next if outer == inner
 outer.apply inner
 end
end

This works, unless the array contains an entry twice (e.g. arr = [413, 321, 654, 23, 11, 11]).

If you need the index you can use Array#.each_with_index:

arr.each_with_index do |outer, outer_i|
 arr.each_with_index do |inner, inner_i|
 next if outer_i == inner_i
 outer.apply inner
 end
end

You may also replace the inner loop with an array without the element of the outer loop:

arr.each_with_index do |outer, outer_i|
 arr2 = arr.dup #Without dup you would change the original array
 arr2.delete_at(outer_i)
 arr2.each do |inner|
 outer.apply inner
 end
end