This article is part of the “Meta Advent 2019” series. I’ve committed to writing a new blog post here every day until Christmas.

Another day of the advent, another short “Weird Ruby” installment. Today’s topic are for loops in Ruby. And no, we’re not going to be discussing how to emulate C-style for loops in Ruby. You know, the ones that look like:

for (int i = 0; i < array.length; i++) {
  // something
}

Instead we’re going to focus on Ruby’s own flavour of for loops. Did you even know about Ruby’s for loops? I won’t blame you if you didn’t, as almost no one uses those in practice. I’ve been doing Ruby programming for over a decade now and I’ve yet to see someone using for in the projects I’ve worked on. That’s how for loops look:

for element in array do
  # do something with element
end

That’s pretty similar to the much more common:

array.each do |element|
  # do something with element
end

If the two snippets are more or less the same then why is everyone using each? Yeah, each is a regular method in Enumerable and for is a special syntactic construct, but that’s not that big of a difference in practical terms, right?1 There must be something weird going on. Indeed, there is! The main difference between the two approaches is best illustrated with a short code example:

arr = [1, 2, 3]

arr.each { |elem| puts elem }

elem # => NameError: undefined local variable or method `last' for main:Object

for elem in arr
  puts elem
end

elem # => 3

As you can see for loops don’t introduce a new block scope and variables created inside them (including the binding for the current element) continue to exist after the for loop is done. This is reminiscent of the behavior of blocks before Ruby 1.9.

# Ruby 1.8
arr = [1, 2, 3]

arr.each { |elem| puts elem }

elem # => 3

arr.each { |arr| puts arr }

arr # => 3

# We have to recreate the array
arr = [1, 2, 3]

for arr in arr
  puts arr
end

arr # => 3

Thankfully blocks behave in a much saner manner today, unlike for loops. I’ve got no idea why someone decided that this behavior is desirable and I find it pretty weird that it wasn’t updated when the semantics where changed for regular blocks.

That’s all I have for you today! See you again tomorrow!

Articles in the Series

  1. In fact internally for is implemented in terms of each. ↩