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Re^2: Not meaning to add values to a hash

by stu96art (Scribe)
on Dec 21, 2012 at 16:56 UTC ( #1009937=note: print w/ replies, xml ) Need Help??


in reply to Re: Not meaning to add values to a hash
in thread Not meaning to add values to a hash

Yes, it looks like that is exactly what I am experiencing.

I guess that I am looking at this in a novice way, but I thought that in your code, if $empty_hash{foo}{bar} did not exist, then print "'foo'->'bar' exists\n"; would not be executed.

Would a way around this be to only check if the key in the hash existed and not the element itself? Example:

if (exists $empty_hash{foo}) {

Instead of:

if (exists $empty_hash{foo}{bar}) {


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Re^3: Not meaning to add values to a hash
by muba (Priest) on Jan 02, 2013 at 14:15 UTC
    I thought that in your code, if $empty_hash{foo}{bar} did not exist, then print "'foo'->'bar' exists\n"; would not be executed.

    Did you, in fact, try to run the code? You'll see that that print statement indeed will not be executed.

    As for the workaround you suggest, no. Checking whether a certain key exists in a hash is not a workaround for checking whether a subkey exists somewhere deeper down you structure. Obviously if the key doesn't exist, this implies that the subkey doesn't exist, but if the key existst this tells us nothing about the existance of the subkey. Proof:

    use strict; use warnings; use Data::Dumper; # Some arbitrary data my %earth = (); $earth{wind}{fire} = "water"; # Show me what we've got print Dumper \%earth; print "\n\n"; print "\nEarth has wind, wind has fire.\n" if exists $earth{wind}->{f +ire}; delete $earth{wind}{fire}; print "The fire is extinguised.\n" unless exists $earth{wind} +->{fire}; print "Yet earth still has wind.\n" if exists $earth{wind}; delete $earth{wind}; print "But the wind blows away.\n" unless exists $earth{wind} +; print Dumper \%earth; print "\n\n"; print "Relight that fire!\n" if exists $earth{wind}{fir +e}; # Autovivication happens here! print Dumper \%earth; print "\n\n";
    $VAR1 = { 'wind' => { 'fire' => 'water' } }; Earth has wind, wind has fire. The fire is extinguised. Yet earth still has wind. But the wind blows away. $VAR1 = {}; $VAR1 = { 'wind' => {} };

    The earliest moment where you autovivify elements in %flag_assignments is the print statement in this snippet:

    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"; }
    And you have many more statements like that where you autovivicate in string interpolation. This is not in itself a bad thing, but it explains the behaviour you are seeing.

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