in reply to Not meaning to add values to a hash

You access complex structures with %flag_assignments as the root. Perl will helpfully bring intermediate elements into life automatically ("autovivify") if they don't exist.

In the following code, if $flag_assignments{$temp_key} does not exist, it will get autovivified due to its use a few lines later:

    ...
    if (defined $flag_assignments{$temp_key}) {
        print "KEY [$key] TK [$temp_key] TS [$temp_semester] FAS [$fla
+g_assignments{$temp_key}{starrez}] FAA [$flag_assignments{$temp_key}{
+abbreviation}]\n";
    }
    # Here it is possible that $flag_assignments{$temp_key} does not e
+xist
    
    
    # ONLY ONE FLAGE PER CATEGORY, AND GREEKS LIVING ON CAMPUS WILL AL
+SO GET NR ACCESS TO THE HOUSE OF THE FRAT/SORORITY

    if ($starrez{$key}{"dorm"} ne "None") {
        # Here, autovivification of a value for $flag_assignments{$tem
+p_key} happens
        # because it's part of $flag_assignments{$temp_key}{$temp_seme
+ster}
        if ($flag_assignments{$temp_key}{$temp_semester} ne "") {
...

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One approach to prevent autovivification is to use exists checks before accessing a complex data structure. Another could be to use Data::Diver. To prevent accidential autovivification, see autovivification.

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Re^2: Not meaning to add values to a hash ("autovivification")
by stu96art (Scribe) on Dec 21, 2012 at 16:44 UTC

    Thanks. I had used 'exists' before I tried 'defined' with the same results.

    What I am trying to accomplish is to only reference the element in the hash if it actually exists. I had also tried to do an if statement where the hash element would have to equal the correct key, but I received a multitude of "Uninitialized value...".

[reply]

      If one side of the comparison is uninitialized (due to not existing), then Perl warns.

[reply]
Re^2: Not meaning to add values to a hash ("autovivification")
by stu96art (Scribe) on Dec 21, 2012 at 17:29 UTC

    Thanks for all of the help. I have made adjustments and without using the autovivification module, I have the code working. I will admit, that it is not as elegant as I would like for it to be, but for now it works. Thanks. 

    for my $key (sort keys %starrez) {
    if (exists $starrez{$key}{"dorm"}) {
    } else {
        $starrez{$key}{"dorm"} = "None";
    }
    if (exists $starrez{$key}{"greek"}) {
    } else {
        $starrez{$key}{"greek"} = "None";
    }
    if (exists $starrez{$key}{"RA"}) {
    } else {
        $starrez{$key}{"RA"} = "None";
    }
    if (exists $explanation{$key}) {
    } else {
        $explanation{$key} = "None";
    }
    
    my $temp_key = $starrez{$key}{"dorm"};
    if ($starrez{$key}{"greek"} ne "None") {
        $temp_key = $temp_key . "\|$starrez{$key}{greek}";
    }
    
    my $temp_semester = $semester;
    if ($starrez{$key}{"RA"} ne "None") {
        $temp_semester = "RA\-" . $temp_semester;
    }
    
    if (exists $flag_assignments{$temp_key}{"starrez"}) {
        if (exists $flag_assignments{$temp_key}{"abbreviation"}) {
            if (($flag_assignments{$temp_key}{"starrez"} ne "") and ($
+flag_assignments{$temp_key}{"abbreviation"} ne "")) {
                #print "KEY [$key] TK [$temp_key] TS [$temp_semester] 
+FAS [$flag_assignments{$temp_key}{starrez}] FAA [$flag_assignments{$t
+emp_key}{abbreviation}]\n";
            }
        }
    }
    
    # ONLY ONE FLAG PER CATEGORY, AND GREEKS LIVING ON CAMPUS WILL ALS
+O GET NR ACCESS TO THE HOUSE OF THE FRAT/SORORITY

    if ($starrez{$key}{"dorm"} ne "None") {
        if (exists $flag_assignments{$temp_key}{$temp_semester}) {
            if ($flag_assignments{$temp_key}{$temp_semester} ne "") {
                if (($flag_assignments{$temp_key}{$temp_semester} ne "
+x") and ($flag_assignments{$temp_key}{$temp_semester} ne "None")) {
                    print "PIK [$key] TK [$temp_key] TS [$temp_semeste
+r] FA [$flag_assignments{$temp_key}{$temp_semester}]\n";
                    if ($starrez{$key}{"RA"} ne "None") {
                        # Patron is an RA or CRA so needs appropriate 
+access
                        if (exists $output{$key}) {
                            $output{$key} = $output{$key} . ",P$flag_a
+ssignments{$temp_key}{$temp_semester}";
                        } else {
                            $output{$key} = "C|$key|$flag_assignments{
+$temp_key}{abbreviation}|(P$flag_assignments{$temp_key}{$temp_semeste
+r}";
                        }
                    } else {
                        # Patron is not an RA or CRA so will just rece
+ive normal access
                        if (exists $output{$key}) {
                            $output{$key} = $output{$key} . ",P$flag_a
+ssignments{$temp_key}{$temp_semester}";
                        } else {
                            $output{$key} = "C|$key|$flag_assignments{
+$temp_key}{abbreviation},(P$flag_assignments{$temp_key}{$temp_semeste
+r}";
                        }
                    }
                }
            } else {
                #print "TK [$temp_key]\n";
                if ($temp_key =~ '|') {
                    my @temp_1 = split('|',$temp_key);
                    # Give access to their dorm
                    if (exists $output{$key}) {
                        $output{$key} = $output{$key} . ",P$flag_assig
+nments{$temp_1[0]}{$temp_semester}";
                    } else {
                        $output{$key} = "C|$key|$flag_assignments{$tem
+p_1[0]}{abbreviation}|(P$flag_assignments{$temp_1[0]}{$temp_semester}
+";
                    }
                    # Find which house their Greek affiliation is in a
+nd give them Non-Res (NR) access
                    for my $key2 (sort keys %flag_assignments) {
                        print "KEY2 [$key2]\n";
                        if (exists $flag_assignments{$key2}{"greek"}) 
+{
                            if ($flag_assignments{$key2}{"greek"} ne "
+") {
                                if ($temp_1[1] eq $flag_assignments{$k
+ey2}{"greek"}) {
                                    my $temp2semester = "NR-" . $semes
+ter;
                                    if ($flag_assignments{$key2}{"$tem
+p2semester"} ne "") {
                                        if (exists $output{$key}) {
                                            print "KEY [$key] NR-$seme
+ster [$flag_assignments{$key2}{$temp2semester}]\n";
                                            $output{$key} = $output{$k
+ey} . ",P$flag_assignments{$key2}{$temp2semester}";
                                        }
                                    } else {
                                        print BADOUTPUT "No NR flag,$k
+ey,$starrez{$key}{dorm},$starrez{$key}{greek},$starrez{$key}{RA}\n";
                                    }
                                }
                            }
                        } else {
                            #print "TEMP_KEY [$temp_key]\n";
                        }
                    }
                } else {
                    print BADOUTPUT "Not Greek and should be,$key,$sta
+rrez{$key}{dorm},$starrez{$key}{greek},$starrez{$key}{RA}\n";
                }
            }
        }
    }
}

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