Ruby: What is the difference between a for loop and an each loop? [duplicate]

Question

Possible Duplicate:
for vs each in Ruby

Let's say that we have an array, like

sites = %w[stackoverflow stackexchange serverfault]

What's the difference between

for x in sites do
  puts x
end

and

sites.each do |x|
  puts x
end

?

They seem to do the same exact thing, to me, and the syntax of the for loop is clearer to me. Is there a difference? In what situations would this be a big deal?

Answer 1

There is a subtle difference regarding scoping, but I would recommend understanding it well as it reveals some of important aspects of Ruby.

for is a syntax construct, somewhat similar to if. Whatever you define in for block, will remain defined after for as well:

sites = %w[stackoverflow stackexchange serverfault]
#=> ["stackoverflow", "stackexchange", "serverfault"]

for x in sites do
  puts x
end
stackoverflow
stackexchange
serverfault
#=> ["stackoverflow", "stackexchange", "serverfault"]
x
#=> "serverfault"

On the other hand, each is a method which receives a block. Block introduces new lexical scope, so whatever variable you introduce in it, will not be there after the method finishes:

sites.each do |y|
  puts y
end
stackoverflow
stackexchange
serverfault
#=> ["stackoverflow", "stackexchange", "serverfault"]
y
NameError: undefined local variable or method `y' for #<Object:0x855f28 @hhh="hello">
    from (irb):9
    from /usr/bin/irb:12:in `<main>'

I would recommend forgetting about for completely, as using each is idiomatic in Ruby for traversing enumerables. It also recspects the paradigm of functional programming better, by decreasing chances of side-effects.