in reply to Nested loops?

If I'm understanding you correctly, exists to the rescue!

use warnings;
use strict;

my %one = (a => 1, b => 2, c => 3); # first hash
my %two = (a => 1, c => 3);         # second hash

for my $x (keys %two){
    if (exists $one{$x}){
        print "hash \$two key $x exists in hash \$one\n";
    }
}

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Output:

hash $two key c exists in hash $one
hash $two key a exists in hash $one

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In other words, it iterates over the second hash, checking if the "filter" key is in the first hash allowing you to perform some actions, otherwise the loop will just skip to the next iteration. A major benefit here is that it only iterates over a single hash... the smallest one, which completely avoids looping over one hash in its entirety, then a second entire hash for every key in the first.

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Re^2: Nested loops?
by Speed_Freak (Sexton) on Aug 17, 2017 at 21:05 UTC

    I'm currently trying out exists, thanks for that! In the example above, I would have to put the "filter" keys in the exists, because the $x becomes the sequence that gets augmented and compared to all $x's. So, if the key in %two exists as a key in %one, then the key from %one becomes the target. Which then gets assigned to a new variable with each option from the base list. Those are then checked against the $x keys for matches. 

    foreach my $sql1 (@{$sql1}) {
    $table1{$sql1->[1]}{$sql1->[0]}=undef;  #rearranges the sql pull -
+ large table of sequences and id's - These get used to create the alt
+s and house the entire list that needs to be searched
}

foreach my $sql2 (@{$sql2}) {                
    $table2{$sql2->[1]}{$sql2->[0]}=undef;  #rearranges the sql pull -
+ "filter" table of sequences and id's - Only used to dictate which se
+quences get used from the large table

my @bases = ('A','C','G','T');

foreach my $x (keys %table1){    

    if (exists $table2 ({$x})) {

my $found_alt = 0;

foreach my $bases (@bases) {
        my $alt = $x;
        substr($alt, 20, 1) = $opt;
        next if ($alt eq $x);
        if (exists $table1{$alt}) {
            $found_alt = 1;

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