Question

I have a hash table generated which I am then trying to add to a larger hash table, (if unique) for each of multiple files but I’m having trouble with the syntax and keep accidentally calling values or creating a hash of hash. All I want to do is turn:

(The actual $hash key) => $hash{$key};

into

 $compound_hash{$key} = $hash{$key};

Currently I have:

    if ($file_no == 0){
            while (my ($key, $value) = each %hash){
                    $compound_hash{$key} = $value;
            }       

    }else{
            while (my ($key, $value) = each %compound_hash){

                    if (exists $hash{$key}){
                            print "$key: exists\n";
                            $compound_hash{$key} .= ",$hash{$key}";
                    }else{
                          print "$key added\n";  
                          XXXXXXX
                    }

The end result is to concatenate the hash value on to the end of each line, making a .csv ie

     abc,0,32,45
     def,21,43,23
     ghi,1,49,54

Answer 1

Its hard to tell exactly, but I think what you are looking for is something like this:

for my $key (keys %hash) {  # for all new keys
     if (exists $compound_hash{$key}) {  # if we have seen this key
          $compound_hash{$key} .= ",$hash{$key}"  # append it to the csv
     }
     else {
          $compound_hash{$key} = $hash{$key}  # otherwise create a new entry
     }
}

In my own code, I might setup %compound_hash to be initially populated with array references, which are then joined down to strings once the data is filled.

for my $key (keys %hash) {
     push @{ $compound_hash{$key} }, $hash{$key}
}

and then later

for my $value (values %compound_hash) {
    $value = join ',' => @$value
}

Which will be more efficient than repeatedly appending data to the strings contained in the compound hash.