Looping through an array with step

Question

I want to look at every n-th elements in an array. In C++, I'd do this:

for(int x = 0; x<cx; x+=n){
    value_i_care_about = array[x];
    //do something with the value I care about.  
}

I want to do the same in Ruby, but can't find a way to "step". A while loop could do the job, but I find it distasteful using it for a known size, and expect there to be a better (more Ruby) way of doing this.

Solution 1

Ranges have a step method which you can use to skip through the indexes:

(0..array.length - 1).step(2).each do |index|
  value_you_care_about = array[index]
end

Or if you are comfortable using ... with ranges the following is a bit more concise:

(0...array.length).step(2).each do |index|
  value_you_care_about = array[index]
end

Solution 2

array.each_slice(n) do |e, *_|
  value_i_care_about = e
end

Solution 3

Just use step() method from Range class which returns an enumerator

(1..10).step(2) {|x| puts x}

Solution 4

We can iterate while skipping over a range of numbers on every iteration e.g.:

1.step(10, 2) { |i| print "#{i} "}

http://www.skorks.com/2009/09/a-wealth-of-ruby-loops-and-iterators/

So something like:

array.step(n) do |element|
  # process element
end

View more solutions

Comments

[baash05]

Nothing about the step could be found :) each.. or for loop is easy to find.. I want to do every 5th or 10th or 9th element in the array

[Mischa]

Benjamin's answer is incorrect. You should look at Levi's or David's answer.

[Mischa]

Yes, it makes sense that 1.step(3, 2) gives 1 and 3. In this code the step is 2, so the second element gets skipped. Which leaves you with 1 and 3.

[baash05]

Yeah I see how it works now. Thanks all.

[mrbrdo]

By the way, this has nothing to do with Rails, it's a Ruby question.

[baash05]

You missed the bit about the step. That was the important bit.

[bdeonovic]

Just feeling a bit under the weather tonight =D

[Mischa]

This doesn't work. Array doesn't have a step method, so you will get "Undefined method for step". 1.step(10,2) does work, but that doesn't iterate over an array. Levi has the correct answer.

[baash05]

had to vote you down... if n is 1000 then and I've got 10000 elements I've just done 10000 if's I didn't need to to get 10 elements.

[baash05]

Yeah.. but in Ben's defence.. it was enough to figure it out. And Levi's doesn't iterate over the array either. Both excellent answers.

[doesterr]

I was just wondering why it got downvoted.. thanks for letting me know

[doesterr]

Of course you're absolutely right with that. The negative impact gets larger, the bigger your step size is. If you're as curious about it as I was just now, see the significant benchmark results in this gist: gist.github.com/3936015

[baash05]

excellent search dave. Anytime a compare is in the loop it's run each time.. if you can get if's out of your loop it runs faster. I like to watch my cycles and let the .... take care of them selves.

[doesterr]

then, I'll just replace my ifs with unless... jk ;)

[244an]

You can use (0...array.length) instead of (0..array.length - 1)

[baash05]

nice. Not in love with the idea of creating a range of a million where an int would do the job in c++.. but cool

[Mark Thomas]

Um, maybe, but in this case completely unnecessary. Just use the block form of select: [1, 2, 3, 4, 5, 6, 7].select(&:odd?)