Strange ruby for loop behavior (why does this work)

Question

def reverse(ary)
  result = []
  for result[0,0] in ary
  end
  result
end

assert_equal ["baz", "bar", "foo"], reverse(["foo", "bar", "baz"])

This works and I want to understand why. Any explanations?

Answer 1

If I were to rewrite this using each instead of for/in, it would look like this:

def reverse(ary)
  result = []

  # for result[0,0] in ary
  ary.each do |item|
    result[0, 0] = item
  end

  result
end

for a in b basically says, take each item in the array b and assign it to expression a. So some magic happens when its not a simple variable.

The array[index, length] = something syntax allows replacement of multiple items, even 0 items. So ary[0,0] = item says to insert item at index zero, replacing zero items. It's basically an unshift operation.

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But really, just use the each method with a block instead. A for loop with no body that changes state has to be one of the most obtuse and hard to read thing that doesn't do what you expect at first glance. each provides far fewer crazy surprises.