From: Tim Pease <tim.pease@...>
Date: 2007-03-09T06:02:53+09:00
Subject: Re: Looping through array, deleting elements

On 3/8/07, dkmd_nielsen <donn@cmscms.com> wrote:
> My idea of looping through an array, interrogating elements that are
> of class instruction, and deleting the element does not appear to work
> as I thought.  I think I know why.  What is the best solution?
>
> block.each {|i| ab.delete(i) if i.parm == "FOO"}
>

Not a good idea!  You are modifying the array as you iterate over it
-- this will cause some unexpected behavior:

ary = [1,2,3,4]
ary.each {|x| ary.delete(x) if x > 2}

p ary  #=> [1, 2, 4]

oops!

> where block is a series of elements of class instruction(i).  Class
> instruction has two attributes, parm and arg.  If the instruction parm
> is FOO, then delete the instruction object from the array and keep
> looping.  Half of what should be deleted actually get's deleted.  I
> think the reason is: the loop is looking at element x foo, element x
> foo get's deleted, the array is shifted left, array pointer has not
> changed but is now referencing element y foo, loop advances the array
> pointer, and now array pointer is pointing at element z foo.
>

You are correct. What happens is that we delete the number 3 from the
array and this compacts the array -- 4 shifts into the position where
3 just was. Now the "each" method moves the index to the next position
thereby skipping the number 4.

> If what I think is correct, what is the best solution?  Do I set the
> current element to nil, then compact the array after the loop?  Do I
> delete the instruction and then do a redo?  Or is there a better
> solution than those?
>

Use the delete_if method on the array.

ary.delete_if {|x| x > 2}

p ary  #=> [1,2]

Blessings,
TwP