“for” vs “each” in Ruby

Question

I just had a quick question regarding loops in Ruby. Is there a difference between these two ways of iterating through a collection?

# way 1
@collection.each do |item|
  # do whatever
end

# way 2
for item in @collection
  # do whatever
end

Just wondering if these are exactly the same or if maybe there's a subtle difference (possibly when @collection is nil).

Answer 1

I just want to make a specific point about the for in loop in Ruby. It might seem like a construct similar to other languages, but in fact it is an expression like every other looping construct in Ruby. In fact, the for in works with Enumerable objects just as the each iterator.

The collection passed to for in can be any object that has an each iterator method. Arrays and hashes define the each method, and many other Ruby objects do, too. The for/in loop calls the each method of the specified object. As that iterator yields values, the for loop assigns each value (or each set of values) to the specified variable (or variables) and then executes the code in body.

This is a silly example, but illustrates the point that the for in loop works with ANY object that has an each method, just like how the each iterator does:

class Apple
  TYPES = %w(red green yellow)
  def each
    yield TYPES.pop until TYPES.empty?
  end
end

a = Apple.new
for i in a do
  puts i
end
yellow
green
red
=> nil

And now the each iterator:

a = Apple.new
a.each do |i|
  puts i
end
yellow
green
red
=> nil

As you can see, both are responding to the each method which yields values back to the block. As everyone here stated, it is definitely preferable to use the each iterator over the for in loop. I just wanted to drive home the point that there is nothing magical about the for in loop. It is an expression that invokes the each method of a collection and then passes it to its block of code. Hence, it is a very rare case you would need to use for in. Use the each iterator almost always (with the added benefit of block scope).

Answer 2

As far as I know, using blocks instead of in-language control structures is more idiomatic.

Answer 3

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.

Answer 4

(1..4).each { |i| 


  a = 9 if i==3

  puts a 


}
#nil
#nil
#9
#nil

for i in 1..4

  a = 9 if i==3

  puts a

end
#nil
#nil
#9
#9

In 'for' loop, local variable is still lives after each loop. In 'each' loop, local variable refreshes after each loop.

Answer 5

Never ever use for it may cause bugs.

The difference is subtle but may cause tremendous bugs!

Don't be fooled, this is not about idiomatic code or style issues. This is a matter of avoiding almost untraceable bugs in production code. Ruby's implementation of for has a serious flaw and should not be used. Always use each loops, never ever use for loop.

Here is an example where for introduces a bug,

class Library
  def initialize
    @ary = []
  end
  def method_with_block(&block)
    @ary << block
  end
  def method_that_uses_these_blocks
    @ary.map(&:call)
  end
end

lib = Library.new

for n in %w{foo bar quz}
  lib.method_with_block { n }
end

puts lib.method_that_uses_these_blocks

Prints

quz
quz
quz

Using %w{foo bar quz}.each { |n| ... } prints

foo
bar
quz

Why?

In a for loop the variable n is defined once and only and then that one definition is use for all iterations. Hence each blocks refer to the same n which has a value of quz by the time the loop ends. Bug!

In an each loop a fresh variable n is defined for each iteration, for example above the variable n is defined three separate times. Hence each block refer to a separate n with the correct values.

Answer 6

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