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Possible Duplicate:
Ruby - What is the difference between map, each and collect?

I have looked in Ruby-Doc also but i cant understand the difference between

map
each

iterators.It would be great if you could give an example and explain.

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2 Answers2

76

each simply iterates over the given enumerable, running the block for each value. It discards the return value of the block, and each simply returns the original object it was called on:

[1, 2, 3].each do |x|
  x + 1
end  # => [1, 2, 3]

This is simply a nicer, more universal way of doing a traditional iterating for loop, and each is much preferred over for loops in Ruby (in fact, I don't think I've ever used a for loop in Ruby).

map, however, iterates over each element, using the return value of the block to populate a new array at each respective index and return that new array:

[1, 2, 3].map do |x|
  x + 1
end  # => [2, 3, 4]

So it “maps” each element to a new one using the block given, hence the name “map”. Note that neither each nor map themselves modify the original collection. This is a concise, functional alternative to creating an array and pushing to it in an iterative loop.

Andrew Marshall
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    It would be worth to emphasize that it doesn't set any values in the *original* collection; only in the returned copy of it. – SasQ Aug 23 '13 at 04:40
  • _"sets the current element being iterated over to the return value"_ sounds as if the current element is being replaced by the block's value. And _"with those changes"_ emphases that something is being changed. That would be a good explanation for `Array#map!` (with bang). `map` on the other hand (both, `Enumerable#map` and `Array#map`) returns a new array (it's always an array, regardless of given receiver) containing the values returned by the block without changing the receiver. – Stefan Sep 08 '17 at 08:41
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    @Stefan I can see the confusion, hopefully the update clarifies things a bit (this was written a *long* time ago!). – Andrew Marshall Sep 08 '17 at 13:16
8

each returns the original object. It's used to run an operation using each element of an array without collecting any of the results. For example, if you want to print a list of numbers, you might do something like this:

arr = [1, 2, 3, 4]
arr.each { |n| puts n }

Now, that puts method above actually returns nil. Some people don't know that, but it doesn't matter much anyway; there's no real reason to collect that value (if you wanted to convert arr to strings, you should be using arr.map(&:to_s) or arr.map { |n| n.to_s }.

map returns the results of the block you pass to it. It's a great way to run an operation on each element in an array and retrieve the results. If you wanted to multiple every element of an array by 2, this is the natural choice. As a bonus, you can modify the original object using map!. For example:

arr = [1, 2, 3, 4]
arr.map! { |n| n * 2}
# => [2, 4, 6, 8]
brymck
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