Re: Looping through array, deleting elements

On 3/8/07, dkmd_nielsen <donn@xxxxxxxxxx> 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

.

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