Re: OT: clearing a hash (was: TIMTOWTDI doesn't mean invent an outlandish approach (usually))

The '%hash = undef' is incorrect:

$ perl -wle '%hash = undef; print scalar %hash'
Odd number of elements in hash assignment at -e line 1.
Use of uninitialized value in list assignment at -e line 1.
1/8
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It'll create a one element hash, whose key is the empty string, with an undefined value.

The last three solutions modify the hash while iterating over the keys, which should be a no-no. However, I just read that in the documentation of 'each' that deleting the item most recently returned by 'each()' is safe; this feature seems to have been added (or documented) in 5.6.1.

Abigail

Comment on Re: OT: clearing a hash (was: TIMTOWTDI doesn't mean invent an outlandish approach (usually)) Download Code

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Re^2: OT: clearing a hash by Aristotle (Chancellor) on Oct 13, 2003 at 11:04 UTC
Ah, so that's why I was puzzled - the hint on each was there the first I wanted to know about deleting keys while iterating. But you said three of the following will not necessarily work: `map { delete $hash{$_} } keys %hash;` `while (my ($k,$v) = each %hash) { delete $hash{$k} };` `for (keys %hash) { delete $hash{$_} };` `delete $hash{$_} for keys %hash;` With the each solution being valid that's one down, two to go, and now I'm more confused than before.. of the remaining three solutions, the `for` loops are identical except for syntactical details. So they must either both be valid or both invalid; then the map version must be valid and the `for` ones not. But I can't see any reason to conclude that any of them (all four) are not guaranteed to work. Makeshifts last the longest.	[reply] [d/l] [select]
Re: OT: clearing a hash (was: TIMTOWTDI doesn't mean invent an outlandish approach (usually)) by Abigail-II (Bishop) on Oct 13, 2003 at 11:36 UTC
Number 1) is safe because in this case, a list of keys will be produced before the deletes are called. For 3) and 4), I though that 'for (keys %hash)' was special cased to iterate over the hash instead of producing a list (just like 'for (@array)' is special cased), but I can't find it in the documentation right now. Anyway, regardless whether it's special cased or not, 3) and 4) are fine as well, due to the exception of modifying a hash while iterating over it by deleting the last element returned by 'each' (since 'keys' and 'values' uses the same iterator). Abigail	[reply]