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each
If you add or delete a hash's elements while iterating over it, entries may be skipped or duplicated--so don't do that

perlfaq4#What happens if I add or remove keys from a hash while iterating over it?

(contributed by brian d foy)

The easy answer is "Don't do that!"

If you iterate through the hash with each(), you can delete the key most recently returned without worrying about it. If you delete or add other keys, the iterator may skip or double up on them since perl may rearrange the hash table. See the entry for each() in perlfunc.


In reply to Re: Stepping through a hash while modifiying its content by Anonymous Monk
in thread Stepping through a hash while modifiying its content by Gorgarian

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    [Corion]: Hmmm. I feel a Meditation coming on. I wrote a module, DBIx::PivotQuery, which returns a table-like set of rows (AoA) but some columns are generated from column values, like in an (Excel) pivot table or a ROLLUP command
    [Corion]: My current approach for subtotals involves rerunning the given query, with the hint to the user that they should use a temporary table if they want better performance.
    [Corion]: But I could create that temporary table in the module and use it for the improved perfomance directly instead.
    [Corion]: And the question is, what would be better/preferred ;-)
    [Corion]: Hmm - not exactly like the ROLLUP command. Ah well.

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