in reply to Re: Help understand why this grep does not work
in thread Help understand why this grep does not work

if speed matters you should better consider something like building a hash for array2 for simple lookups or at least a complex OR-ed regex looping just once over array2.

edit
  DB<102> @data1 = ( "a 1 a", "a 2 T", "a 3 C" );
 => ("a 1 a", "a 2 T", "a 3 C")

  DB<103> @data2 = ( "a 2 Y", "a 3 R", "a 4 Q", "b 5 R" );
 => ("a 2 Y", "a 3 R", "a 4 Q", "b 5 R")

  DB<104> $regex = join "|" , map {/^(\w\s+\d)/} @data1
 => "a 1|a 2|a 3"

  DB<105> grep { /^($regex)/ } @data2
 => ("a 2 Y", "a 3 R")

[download]

think I spotted (and corrected) a bug in your logic, you forgot to anchor to string start '^' in your grep regex.

Cheers Rolf

( addicted to the Perl Programming Language)

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Re^3: Help understand why this grep does not work
by drmrgd (Beadle) on Nov 02, 2013 at 16:15 UTC
    I wondered about building a hash for this. What would be the best way? Are you thinking something like this is better: 
    my %data_hash = map { /(\w\s+\d)/ => $_ } @data2;

for my $key ( %data_hash ) {
    print "$data_hash{$key}\n" if grep { /^$key/ } @data1;
}

    [download]

    I also wondered about anchoring that match. In earlier versions, I did anchor it. But, I wasn't sure if it was necessary given the data set. I think I'll add it back as it's probably safer.

    Thanks for the advice!

[reply]
[d/l]
      well kind of ... but your code only works if the first 2 chars combinations are unique otherwise you need to push into a HoA.

      I think the or-ed regex I've shown you is better, as long as @data1 is known from the beginning and doesn't change dynamically.

      Cheers Rolf

      ( addicted to the Perl Programming Language)

[reply]

        Yes, this is why I didn't do that in the first place. Although with the data I have listed I think this would work, if I had to expand it and there weren't unique values, you're right - this wouldn't work so well. I think your 'or-ed regex' is indeed better.

        For the sake of argument (I'm just playing around trying to get better a handling data in Perl and the map function), what's the best way to create a hash of arrays with this data? I can't seem to create the data structure with a map. I know this won't work as it overwrites the values on each pass:

        my %data_hash = map { /^(\w\s+\d)/ => [$_] } @data2;

        [download]

        But, I can't seem to figure out how to create a hash of arrays with the map function. Maybe it's not possible or not the best way?

[reply]
[d/l]

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