difference between "each .. do" or "for .. in" loops in rails

Question

Possible Duplicate:
What is “for” in Ruby

Hey. My question is if these loops are the same when I iterate through an Array. Thx for your time!

<% for track in @tracks %>

or

<% @tracks.each do |track| %>

Duplicate: http://stackoverflow.com/questions/155462/what-is-for-in-ruby — ghoppe, Feb 24 '11 at 23:54

score 9 · Accepted Answer · answered Feb 24 '11 at 23:59

9

They are different (although it may not matter for your purposes).

for doesn't create a new scope:

blah = %w(foo bar baz)
for x in blah do
   z = x
end
puts z # "baz"
puts x # "baz"

.each creates a new scope for the block:

blah.each { |y| a = y }
puts a # NameError
puts y # NameError

answered Feb 24 '11 at 23:59

cam

What do you mean by new scope? – thenengah Feb 25 '11 at 00:03
2

@Sam: http://en.wikipedia.org/wiki/Scope_(programming) Notice how the `x` and `z` variables in my example can be accessed after the `for` loop, but `a` and `y` cannot be accessed outside of the block passed to `each`. – cam Feb 25 '11 at 00:09

score 3 · Answer 2 · answered Feb 25 '11 at 00:11

For the most part, you probably won't see any differences, but things are quite different in those two cases.

In one case, you have a loop directly in Ruby syntax, in the other case a data structure traversal is periodically yielding to a block.

return may work differently. If the outer code is in a lambda then return will return from the lambda within the loop but if executed within a block it will return from the enclosing method outside the lambda.
The lexical scope of identifiers created within the loop is different. (On general principles you really shouldn't create things inside a structured language feature and then use them later outside but this is possible in Ruby with the loop and not with the block.)

score 1 · Answer 3 · answered Feb 24 '11 at 23:54

There is no difference here. Just a difference in syntax.

What could make this different is adding elements to the block such as a counter, or more advanced features.

<% @tracks.each do |i, track| -%>
    <% #i is your counter... just and example of adding more information in the block %>
<% end -%>

3 Answers3