Re: Using substr on Hash Keys

Replies are listed 'Best First'.
Re^2: Using substr on Hash Keys by jettero (Monsignor) on Sep 10, 2008 at 19:53 UTC
I was going to do something like this, but I couldn't reason out which of the delete vs the substr would happen first. It occurred to me, reading your post, that substr doesn't actually modify the string like splice modifies arrays -- so it doesn't matter. -Paul	[reply]
Re^3: Using substr on Hash Keys by ikegami (Patriarch) on Sep 10, 2008 at 23:37 UTC
I was going to do something like this, but I couldn't reason out which of the delete vs the substr would happen first. Doesn't matter, unless you expect a collision between a new key and an old key. If that's the case, other solutions have the problem too. (The solution is to use another hash.) What does matter is that all the `keys` are fetched before any insertion into and deletion from the hash. And they are. `for` loads the entire list over which to iterate into memory before starting to loop.	[reply] [d/l] [select]
Re^4: Using substr on Hash Keys by jettero (Monsignor) on Sep 11, 2008 at 01:45 UTC
I was picturing it like this (incorrectly thinking substring would modify $_), where I think the order does matter, and I can't predict what would happen: `$hash{ do{ $_ =~ s/(..)$//; $_ } } = delete $hash{ $_ } for keys % +hash;` [download] What appears to happen is that the lvalue is evaluated first, so I end up deleting the modified key into the new key position (leaving all the old keys in place and defining new keys to undef); but I couldn't have guessed until I tried it, so I'd tend to lean toward a for() loop like my first post. And I think that was jwkrahn's point too: `$hash{ substr $_, 1, 20, "" } = delete $hash{ $_ } for keys %hash;` [download] -Paul	[reply] [d/l] [select]
Re^3: Using substr on Hash Keys by jwkrahn (Abbot) on Sep 10, 2008 at 21:11 UTC
Actually, substr can modify the string if you use the fourth argument.	[reply]
Re^4: Using substr on Hash Keys by ikegami (Patriarch) on Sep 10, 2008 at 23:38 UTC
Yes, but hash keys aren't Perl strings, so `keys` actually returns a copy of the keys, so `substr` can only modify the copy, not the key itself.	[reply] [d/l] [select]
Re^5: Using substr on Hash Keys by jwkrahn (Abbot) on Sep 11, 2008 at 01:13 UTC
Re^6: Using substr on Hash Keys by ikegami (Patriarch) on Sep 11, 2008 at 01:31 UTC
Re^3: Using substr on Hash Keys by pc0019 (Acolyte) on Sep 10, 2008 at 20:28 UTC
Thanks everyone for the replies. I love the 'TMTOWTDI' factor about Perl, but are both methods as efficient as each other?	[reply]
Re^4: Using substr on Hash Keys by GrandFather (Saint) on Sep 10, 2008 at 21:04 UTC
Does it matter? Which do you find more readable? Which are you most likely to use again correctly in the future without looking it up? Which do you think least requires a comment? Perl reduces RSI - it saves typing	[reply]