Question

I’m learning Ruby and RoR at the moment, and I came across this:

<% for post in @posts %>

in the Rails guide. I’d understood that the idiomatic way to do this in Ruby is with:

<% @posts.each do |post| %>

If there is a difference then what is it? And if there isn’t a difference then wouldn’t it be better for the Rails people to be pushing proper Ruby idioms (rather than this, which looks more pythonic to me)?

Edit: I’ve just found two conflicting explanations of this: Tutorials Point says they’re the same except “a for loop doesn’t create a new scope for local variables”, whereas CS.Auckland.ac.NZ says for is just syntactic sugar for the equivalent .each.

Edit2: The for ... in in question was for the index.html.erb generated in app/views/posts by script/generate scaffold. I’ve done a quick check and that now generates the .each syntax. I guess that part of the guide was written at an earlier stage of rails development when the scaffold generated for ... in.

Edit3: I can now confirm that for x in y was used in Rails 2.2.2, but by 2.3.8 it is using y.each do |x|. Just so you know.

Answer 1

The tutorialpoint page is correct, for is equivalent to each except for the scoping difference. Here’s a demonstration:

arr = [1,2,3]
arr.each do |x|
  last = x
end
last # NameError

vs.

arr = [1,2,3]
for x in arr
  last = x
end
last #=> 3

If you want to make it work using each, you need to do last = nil before the loop. This is, as the link pointed out, because blocks start a new scope while for does not.

Note however that this rarely makes a practical difference and few people are even aware of it.

When people use for in ruby it’s most often because that’s what they’re used to coming from other languages – not because of any differences between for and each.