Re^2: Iterating through an array using multiple loops and removing array elements

Modifying an array while looping with foreach = not good.

What about while looping though it with while? Similarly, what about modifying a hash (using delete) while looping with for/foreach or while? From a quick skim through the doc it doesn't appear to be an issue.

I'll definitely keep in mind the shortcomings of this post in the future! Context is especially important when TIMTOWTDI!

Thanks for the help!

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Re^3: Iterating through an array using multiple loops and removing array elements by kcott (Archbishop) on Apr 25, 2014 at 07:10 UTC
"What about while looping though it with while?" `for (@array)` iterates over the list of values in `@array`. Changing that list in mid-iteration often has problems. We actually get quite a few questions like "Why doesn't my `for` loop work?" that are due to such a problem. So, as the doco says, "don't do that". `while (@array)` involves no iteration; it's a simple condition which basically says "Enter the loop if `@array` has any elements". Unless there's some other method for exiting that loop, you would expect elements to be removed from `@array` so that the loop can eventually terminate. "Similarly, what about modifying a hash (using delete) while looping with for/foreach or while? From a quick skim through the doc it doesn't appear to be an issue." If you're writing `for (%hash)` or `while (%hash)`, that's probably a mistake; perhaps you meant something else. You'll need to provide some code to show the scenario(s) you're considering here. By the way, in case you didn't know, `for` and `foreach` are synonymous. Save yourself four keystrokes by writing `for` instead of `foreach`: the code will run the same whichever you choose. -- Ken	[reply] [d/l] [select]

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Re^3: Iterating through an array using multiple loops and removing array elements
by kcott (Archbishop) on Apr 25, 2014 at 07:10 UTC

"What about while looping though it with while?"

for (@array) iterates over the list of values in @array. Changing that list in mid-iteration often has problems. We actually get quite a few questions like "Why doesn't my for loop work?" that are due to such a problem. So, as the doco says, "don't do that".

while (@array) involves no iteration; it's a simple condition which basically says "Enter the loop if @array has any elements". Unless there's some other method for exiting that loop, you would expect elements to be removed from @array so that the loop can eventually terminate.

"Similarly, what about modifying a hash (using delete) while looping with for/foreach or while? From a quick skim through the doc it doesn't appear to be an issue."

If you're writing for (%hash) or while (%hash), that's probably a mistake; perhaps you meant something else. You'll need to provide some code to show the scenario(s) you're considering here.

By the way, in case you didn't know, for and foreach are synonymous. Save yourself four keystrokes by writing for instead of foreach: the code will run the same whichever you choose.

-- Ken

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