Array#each Looping Gotcha

ruby-talk
1

I think I've found a gotcha. Really, it should be expected behavior, but
it wasn't immediately obvious to me, so I thought it's at least worth
mentioning.

Let's say you want to iterate through an array and delete any items in
that array that match a certain criteria. I thought it'd make sense to
do this:

items.each do |x|
  if x == ""
    items.delete(x)
  end
end

If items.each referred to items[] by reference, this would make sense.
But since it references by value, you're literally changing the array
live, as you're iterating through it. This means that, if you delete an
item in mid-iteration, you change the index of items[]. Since you
deleted something, you skip the next item.

1) Do I have this right?

2) Am I right in assuming that it's possible to create an infinite loop
this way by continually push()ing things on to items[]?

···

--
Posted via http://www.ruby-forum.com/.

2

Except the reference/value thing.

Look at Array#delete_if for your specific example.

-austin

···

On 4/19/06, Nathan Olberding <nathan.olberding@gmail.com> wrote:

I think I've found a gotcha. Really, it should be expected behavior, but
it wasn't immediately obvious to me, so I thought it's at least worth
mentioning.

Let's say you want to iterate through an array and delete any items in
that array that match a certain criteria. I thought it'd make sense to
do this:

items.each do |x|
        if x == ""
                items.delete(x)
        end
end

If items.each referred to items by reference, this would make sense.
But since it references by value, you're literally changing the array
live, as you're iterating through it. This means that, if you delete an
item in mid-iteration, you change the index of items. Since you
deleted something, you skip the next item.

1) Do I have this right?

2) Am I right in assuming that it's possible to create an infinite loop
this way by continually push()ing things on to items?

--
Austin Ziegler * halostatue@gmail.com
               * Alternate: austin@halostatue.ca

3

Hi,

items.each do |x|
if x == ""
  items.delete(x)
end
end

If items.each referred to items by reference, this would make sense.
But since it references by value, you're literally changing the array
live, as you're iterating through it. This means that, if you delete an
item in mid-iteration, you change the index of items. Since you
deleted something, you skip the next item.

1) Do I have this right?

I'm not sure what you mean by the word "right" here. It's ok for you
to write that kind of program, but I don't (can't) guarantee you will
get what you expect, although I try my best. It might cost you much
to guarantee loop safety. It shouldn't crash in any case though.

2) Am I right in assuming that it's possible to create an infinite loop
this way by continually push()ing things on to items?

Yes, Ruby obeys you if you command it to loop infinitely.

              matz.

···

In message "Re: Array#each Looping Gotcha" on Thu, 20 Apr 2006 07:43:46 +0900, Nathan Olberding <nathan.olberding@gmail.com> writes:

4

items.each do |x|
  if x == ""
    items.delete(x)
  end
end

Wow, in Java you this would throw ConcurrentModificationException (i.e. you can not iterate and modify a list at the same time, says Java) - AFAIK, in java there is no workaround for this. It is interesting that Ruby handles this as well, although i am not sure to which extent this is (black) magic :wink:

Cheers,
Peter

5

Yukihiro Matsumoto wrote:

I'm not sure what you mean by the word "right" here. It's ok for you
to write that kind of program, but I don't (can't) guarantee you will
get what you expect, although I try my best. It might cost you much
to guarantee loop safety. It shouldn't crash in any case though.

Though I was confused for half of the day today, I have to admit that
the current implementation is probably for the best. Where this:

arr.each do |x|
    if y; arr.delete(x); end
end

Can be made to work the way I personally thought it would, it's much
safer to do something like this:

delete_list =
arr.each do |x|
    if y; delete_list.push(x); end
end
arr -= delete_list

Or better yet, since Ruby includes the feature, use Array#delete_if.

Yes, Ruby obeys you if you command it to loop infinitely.

I was more asking if that's what would happen, but I suppose the wording
of my question made its own answer :slight_smile:

···

--
Posted via http://www.ruby-forum.com/\.

6

Not exactly true. This special scenario is well covered by Java:

Std. collections all support remove().

IMHO iterating and deleting like shown is a very bad idea. Since
there is a method for doing deletions that's exactly the way to go.

Kind regards

robert

···

2006/4/20, Peter Szinek <peter@rt.sk>:

> items.each do |x|
> if x == ""
> items.delete(x)
> end
> end
Wow, in Java you this would throw ConcurrentModificationException (i.e.
you can not iterate and modify a list at the same time, says Java) -
AFAIK, in java there is no workaround for this.

--
Have a look: Robert K. | Flickr

7

Hi --

Yukihiro Matsumoto wrote:

I'm not sure what you mean by the word "right" here. It's ok for you
to write that kind of program, but I don't (can't) guarantee you will
get what you expect, although I try my best. It might cost you much
to guarantee loop safety. It shouldn't crash in any case though.

Though I was confused for half of the day today, I have to admit that
the current implementation is probably for the best. Where this:

arr.each do |x|
   if y; arr.delete(x); end
end

Can be made to work the way I personally thought it would, it's much
safer to do something like this:

delete_list =
arr.each do |x|
   if y; delete_list.push(x); end
end
arr -= delete_list

Or better yet, since Ruby includes the feature, use Array#delete_if.

Also, don't forget (among reasons deleting from an array during an
iteration is bad) that delete deletes all instances:

irb(main):027:0> a = [1,2,3,1,4,5,1,6,7]
=> [1, 2, 3, 1, 4, 5, 1, 6, 7]
irb(main):028:0> a.delete(1)
=> 1
irb(main):029:0> a
=> [2, 3, 4, 5, 6, 7] # all the 1's deleted

Also, something like each_with_index would probably cease to make
sense....

David

···

On Thu, 20 Apr 2006, Nathan Olberding wrote:

--
David A. Black (dblack@wobblini.net)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

"Ruby for Rails" PDF now on sale! Ruby for Rails
Paper version coming in early May!

8

Robert Klemme wrote:

···

2006/4/20, Peter Szinek <peter@rt.sk>:

items.each do |x|
      if x == ""
              items.delete(x)
      end
end

Not exactly true. This special scenario is well covered by Java:

I stand corrected. I had so many problems with similar iterate-but-dont-touch scenarios in J that i threw remove() in the same bag automatically ;-).

Anyway, i agree that iterating and deleting is not a good idea...

Cheers,
Peter