"for" vs "each" in Ruby

This is the only difference:

each:

irb> [1,2,3].each { |x| }
  => [1, 2, 3]
irb> x
NameError: undefined local variable or method `x' for main:Object
    from (irb):2
    from :0

for:

irb> for x in [1,2,3]; end
  => [1, 2, 3]
irb> x
  => 3

With the for loop, the iterator variable still lives after the block is done. With the each loop, it doesn't, unless it was already defined as a local variable before the loop started.

Other than that, for is just syntax sugar for the each method.

When @collection is nil both loops throw an exception:

Exception: undefined local variable or method `@collection' for main:Object

See "The Evils of the For Loop" for a good explanation (there's one small difference considering variable scoping).

Using each is considered more idiomatic use of Ruby.

Your first example,

@collection.each do |item|
  # do whatever
end

is more idiomatic. While Ruby supports looping constructs like for and while, the block syntax is generally preferred.

Another subtle difference is that any variable you declare within a for loop will be available outside the loop, whereas those within an iterator block are effectively private.

One more different..

number = ["one", "two", "three"]
 => ["one", "two", "three"] 

loop1 = []
loop2 = []

number.each do |c|
  loop1 << Proc.new { puts c }
end
 => ["one", "two", "three"] 

for c in number
  loop2 << Proc.new { puts c }
end
 => ["one", "two", "three"] 

loop1[1].call
two
 => nil 

loop2[1].call
three
 => nil

source: http://paulphilippov.com/articles/enumerable-each-vs-for-loops-in-ruby

for more clear: http://www.ruby-forum.com/topic/179264#784884

"for" vs "each" in Ruby

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