Addition of hashes

the_dark_lord has asked for the wisdom of the Perl Monks concerning the following question:

Hi I was writing a program and instead of adding values in 2 different hashes, I summed 2 hashes themselves.

I caught my mistake but the value printed by the incorrect code made me curious.

Basically, if I add 2 hashrefs, I get some value which depends only on the first hashref added.

Example: If I try to run the code

my $a = { 
#    'key1' => 'valdsgdjgf`',
#    'key2' => 'val2',
};

my $b = { 
#    'key3' => 'val3',
#    'key4' => 'val4',
};

my $sum = $a + $b;
print "$sum\n";
[download]

I get some random value: 15401280

If I make a change to hashref $a, then this value changes. But if I make a change to hashref $b, then this value does not change.

Does anybody know what this value is?

Comment on Addition of hashes Download Code

Replies are listed 'Best First'.
Re: Addition of hashes by NetWallah (Canon) on Feb 03, 2017 at 07:25 UTC
You are adding memory addresses. If you change what $b points to, the add it to $a, the $sum will change. You state : I make a change to hashref $b Did you change the internal content of the hashref, or did you assign a new hashref to $b ? (Thereby changing the memory address) ...it is unhealthy to remain near things that are in the process of blowing up. man page for WARP, by Larry Wall	[reply]
Re: Addition of hashes by vinoth.ree (Monsignor) on Feb 03, 2017 at 07:19 UTC
Using a reference as a number produces an integer representing its storage location in memory. The only useful thing to be done with this is to compare two references numerically to see whether they refer to the same location *All is well. I learn by answering your questions...*	[reply] [d/l]
Re^2: Addition of hashes by morgon (Priest) on Feb 03, 2017 at 15:30 UTC
To expand a bit: The value of a hashref depends on the context - and not only scalar vs list, but it's different between numeric and string context, e.g. `perl -e 'print {}' # string-context HASH(0x114dcb8) perl -e 'print 0 + {}' # numeric context 18996408 perl -e '{} + 0' # does not compile syntax error at -e line 1, near "} +"` [download]	[reply] [d/l]
Re: Addition of hashes by BillKSmith (Monsignor) on Feb 03, 2017 at 14:14 UTC
Did you intend to 'merge' the hashes? use strict; use warnings; use Data::Dumper; my $a = { 'key1' => 'valdsgdjgf`', 'key2' => 'val2', }; my $b = { 'key3' => 'val3', 'key4' => 'val4', }; my $c = {%$a, %$b}; print Dumper($c); OUTPUT: $VAR1 = { 'key2' => 'val2', 'key4' => 'val4', 'key3' => 'val3', 'key1' => 'valdsgdjgf`' }; [download] Bill	[reply] [d/l]