In Perl, how can I transform an array of hashes in to an array of values?

Question

I have an array of hashes:

my @sports;
push @sports, { id=>1, name=>'Rugby' };
push @sports, { id=>2, name=>'Football' };

and I want to get an array of all the names (to display in a CGI popup menu). I tried using this grep statement:

my @names = grep { $_->{name} } @sports;

but it just returns an array of hash values...

So I am currently resorting to using a loop to iterate through all the values, pushing them into another array.

Is there a way to speed this up?

Sinan Ünür · Accepted Answer · 2012-05-16 11:18:53Z

7

You want map, not grep:

use strict;
use warnings;
my @sports;
push @sports, { id=>1, name=>'Rugby' };
push @sports, { id=>2, name=>'Football' };
my @names = map $_->{name}, @sports;
print for @names;

D:\temp> perl -l m.pl
Rugby
Football

But note this doesn't have anything to do with speed, at least not in a significant way, and speed shouldn't be your primary concern; readability should be it. Of course, efficiency is important, but don't let yourself be fooled into thinking that the more concise and cryptic your code the better your program's performance. This is not so. To assess performance, benchmark your programs.

edited May 16, 2012 at 11:18

Sinan Ünür

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answered May 16, 2012 at 9:29

Lumi

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3 Comments

brian d foy Over a year ago

I know that you are showing all the steps, but you can skip the temporary variable @names: print map $_->{name}, @sports.

Lumi Over a year ago

@briandfoy - Rather: print for map $_->{name}, @sports - else all sports will end up lumped together in one RugbyFootball. I use print for LIST a lot in combination with -l on the command-line, like: perl -lwe "print for sort keys %main::".

skeniver Over a year ago

Thanks! I understand your speed argument (in processing terms). I think I could have better described it as 'less typing'. Depending on where I retrieve the data from, means I have two loops in separate places just to create the names array. With the map I can do it in one place

Tom Melly · Accepted Answer · 2012-05-16 09:19:06Z

0

my @sports;
push @sports, { id=>1, name=>'Rugby' };
push @sports, { id=>2, name=>'Football' };
foreach(@sports){
  push @names, $_->{'name'};
}
foreach(@names){
  print "$_\n";
}

Ah, sorry - reread your post. TBH, however you write it, I suspect that internally a loop will be taking place anyway. Are you sure you're not trying to prematurely optimise? Is there a performance issue?

answered May 16, 2012 at 9:19

Tom Melly

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Comments

run · Accepted Answer · 2012-05-16 09:31:52Z

0

for my $i ( 0 .. $#sports) {  
     print "$sports[$i]{name} \n";    
}

infact you can also use map function to loop through

answered May 16, 2012 at 9:31

run

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Comments

flesk · Accepted Answer · 2012-05-16 10:59:59Z

0

The solution is trivial if you've really hard coded the populating of @sports as in your example:

my (@sports, @names);
push @sports, { id=>1, name=>'Rugby' };
push @names, 'Rugby';
push @sports, { id=>2, name=>'Football' };
push @names, 'Football';

This avoids the second loop altogether.

answered May 16, 2012 at 10:59

flesk

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