"for" vs "each" in Ruby

This is the only difference:

each:

irb> [1,2,3].each { |x| }  => [1, 2, 3]irb> xNameError: 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 whateverend

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.