Use of grep with arrays/hashes

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

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Re: Use of grep with arrays/hashes by merlyn (Sage) on May 24, 2001 at 01:59 UTC
The preferred syntax for this is: `$fields{$_}++ for @fields;` [download] Less typing, less wear-and-tear on the Perl. -- Randal L. Schwartz, Perl hacker	[reply] [d/l]
Re: Re: Use of grep with arrays/hashes by AidanLee (Chaplain) on May 24, 2001 at 02:57 UTC
The reason for this, if I may venture a theory, is that using grep to loop through an array is contrary to grep's desired use. grep is used to return an array of matches from a supplied array of candidates. The example posted uses grep for it's side effects (ie, executing code in the grep BLOCK that has nothing to do with testing the validity of the candidate elements in the array.	[reply]
Re: Re: Use of grep with arrays/hashes by tenya (Beadle) on May 24, 2001 at 07:50 UTC
but why do you need ++ since even undef does not remove a key?	[reply]
Re: Re: Re: Use of grep with arrays/hashes by merlyn (Sage) on May 24, 2001 at 09:12 UTC
The key doesn't exist until used as an lvalue. So we need to in fact do an assignment, which "++" does just fine. -- Randal L. Schwartz, Perl hacker	[reply]
Re: Use of grep with arrays/hashes by geektron (Curate) on May 24, 2001 at 01:54 UTC
the autoincrement adds one to the hash VALUE. also read: $hash{$field} = 1; since the original value of $hash{$field} is 0 ( or undef, i forget which ).	[reply]
Re: Re: Use of grep with arrays/hashes by petdance (Parson) on May 24, 2001 at 02:11 UTC
A hash value for a key that doesn't exist is referred to as being "undefined". Not surprisingly, the value returned by $hash{"non-existent"} is `undef`. It's just that undef+1 is 1. xoxo, Andy %_=split/;/,".;;n;u;e;ot;t;her;c; ". # Andy Lester 'Perl ;@; a;a;j;m;er;y;t;p;n;d;s;o;'. # http://petdance.com "hack";print map delete$_{$_},split//,q< andy@petdance.com >	[reply]
Re: Use of grep with arrays/hashes by Anonymous Monk on May 25, 2001 at 13:02 UTC
I will point out, that the grep form returns an array of the multiple values in the array.	[reply]
Re: Use of grep with arrays/hashes by DeaconBlues (Monk) on May 24, 2001 at 06:38 UTC
I prefer to use the map function for this type of hash population `map {$fields{$_}++} @fields;` [download] This is not much different than merlyn's; his is probably faster though. I wonder why ...	[reply] [d/l]
(ar0n) Re (2): Use of grep with arrays/hashes by ar0n (Priest) on May 24, 2001 at 06:49 UTC
his is probably faster though. I wonder why ... Because map takes a list and returns one, whereas for just iterates through one. This idiom is called "map in void context" (not using the list returned by map). It's considered bad form. See What's wrong with using grep or map in a void context?. [ ar0n ]	[reply]
Re (tilly) 2: Use of grep with arrays/hashes by tilly (Archbishop) on May 24, 2001 at 08:05 UTC
Of the two bad idioms, abusing map is worse because if your function returns multiple values per term and you are on any Perl before 5.6.1, the effort of constructing the return list is quadratic. But map and grep work out the same in this case, and the potential difference is largely irrelevant because both run more slowly than simple looping and waste more memory while they are at it. So use a loop.	[reply]