Question

I’ve been having a problem with a pesky little function in a class in a library that I did not create (and thus, cannot edit). Here’s a simple class with the annoying behavior isolated:

class Foo              # This is a class I cannot
  def setmyproc(&code) # safely edit.
    @prc = Proc.new    # Due to it being in a
  end                  # frustratingly complex
  def callmyproc()     # hierarchy, I don't
    @prc.call          # even know how to reopen
  end                  # it. Consider this class
end                    # set in stone.

I run into a problem when I try to iterate and generate an array of these objects. I expected a different value to be substituted for i into the Proc with each object, but what happens instead is that the variable i is shared between them.

$bar = []
for i in (0..15)
  $bar[i] = Foo.new
  $bar[i].setmyproc { puts i }
end

$bar[3].callmyproc # expected to print 3
$bar[6].callmyproc # expected to print 6

Output

  15
  15

What can I do inside the loop to preserve separate values of i for each object?

Answer 1

Use this:

$bar = []
(0..15).each do |i|
  $bar[i] = Foo.new
  $bar[i].setmyproc { puts i }
end

$bar[3].callmyproc # prints 3
$bar[6].callmyproc # prints 6

If you really need to make the change inside of the loop, use this (ruby 1.9 only):

$bar = []
for i in (0..15)
  ->(x) do
    $bar[x] = Foo.new
    $bar[x].setmyproc { puts x }
  end.(i)
end

$bar[3].callmyproc # prints 3
$bar[6].callmyproc # prints 6