Conditional Map?

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

I'm just starting to get to grips with map and wondering what more I can do with it. Is it possible to do conditional mapping? To take the example in map, say I have an array

my @array = qw/
    foo
    bar
    baz
    bodkin
/;
[download]

and I want to make a hash using all but words beginning in 'f' as values,

my %hash2 = ();
foreach $_ (@array) {
    $hash2{getkey($_)} = $_ unless /^f/;
}
[download]

works, but is there an equivalent usage with map?

my %hash1 = map {getkey($_) => $_ unless /^f/} @array;
[download]

doesn't work.

Am I barking up the wrong tree? Which tree would you recommend?

§ George Sherston

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Re: Conditional Map? by broquaint (Abbot) on Oct 07, 2002 at 11:33 UTC
This sort of situation suits `for` just fine and doesn't really fit with a sole `map` as it's really designed towards applying a transform to a list, not iterating over one. You could however achieve your goal using `map` and `grep` in conjunction e.g `my %hash1 = map { getkey($_) => $_ } grep { !/^f/ } @array;` [download] HTH `_________ broquaint`	[reply] [d/l]
Re: Conditional Map? by blakem (Monsignor) on Oct 07, 2002 at 11:45 UTC
To "zero out" an element using map, simply return an empty list for that value... `my @evens = map { $_%2 ? () : $_ } 1..10;` [download] So, you'd need to do something like this: `my %hash = map { /^f/ ? () : (getkey($_) => $_) } @array;` [download] -Blake	[reply] [d/l] [select]
Re: Conditional Map? by seattlejohn (Deacon) on Oct 07, 2002 at 14:04 UTC
`map`'s best friend is `grep`, which returns the subset of list elements matching a condition. So you could start by selecting just the array elements you want: `grep {substr($_,0,1) ne 'f'} @array` This returns a list consisting of `qw(bar baz bodkin)`, which should be just what you need for your `map`: `my %hash1 = map {getkey($_) => $_} grep {substr($_,0,1) ne 'f'} @array;`	[reply] [d/l] [select]

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