Re: how do I efficiently remove one hash from another?

Whether the loop is explicit, or implicit, there's a loop. But one cool means is:

delete @hash1{ keys %hash2 };
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...which is pretty much the same thing as...

delete $hash1{$_} for keys %hash2;
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...but with an implicit loop (via the hash slice) rather than the explicit 'for' loop. Note in either case, there's no need to worry about checking exists: delete doesn't complain if the element doesn't already exist.

Dave

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Re^2: how do I efficiently remove one hash from another? by tobyink (Canon) on Nov 27, 2012 at 10:26 UTC
The former compiles to a much smaller op tree than the latter: `perl -MO=Concise -e'delete @hash1{keys %hash2}' perl -MO=Concise -e'delete $hash1{$_} for keys %hash2'` [download] On my machine, the slice performs about 20% faster than the loop. `perl -E'sub Monkey::do{say$_,for@_,do{($monkey=[caller(0)]->[3])=~s{::}{ }and$monkey}}"Monkey say"->Monkey::do'`	[reply] [d/l]
Re^3: how do I efficiently remove one hash from another? by ColonelPanic (Friar) on Nov 28, 2012 at 14:25 UTC
The slice option only calls `delete` once. It also doesn't have to go the trouble of assigning `$_` for each element. So that makes sense. When's the last time you used duct tape on a duct? --Larry Wall	[reply] [d/l] [select]
Re^2: how do I efficiently remove one hash from another? by perltux (Monk) on Nov 27, 2012 at 07:18 UTC
Thanks for the detailed explanation.	[reply]