How to pass a hash table by refrence Perl - perl

Question

I am currently trying to implement a suffix tree using Perl, however, when I attempt to set the reference for the tree function, the reference location is not set, if I pass the address via a string then check the text in the string vs the location of the hash table and they are different. Any help is appreciated!
use strict;
use warnings;
use Data::Dumper;
my $count = 0;
my $str; # holds the complete string
my %root;
# takes in all lines of code
open(IN, '<:encoding(UTF-8)', $ARGV[0]) or die "Could not open file '$ARGV[0]' $!\n";
while (<IN>) {
chomp;
# concatinates with string
$str .= $_;
}
# closes input
close(IN);
#length of input string
my $l_size = length($str) - 1;
#recursively makes
sub tree {
#recursive root
my %treeRoot;
#checking incomming data
print "1 ".Dumper(\#_)."\n";
#checking incomming data
print "2 ".Dumper(\%root)."\n";
#attempts to set tree's refrence
\%treeRoot, $count = #_;
#checking incomming data
print "3 ".Dumper(\%root)."\n";
#checking incomming data
print "4 ".$count."\n";
#leaf for each node
my %leaf;
for (my $i = 0; $i < $l_size; $i++) {
#creates alphabet tree
$treeRoot { substr($str, $i, 1) } = %leaf;
}
#checking incomming data
print "5 ".Dumper(\%root)."\n";
while ($count > 0) {
#checking incomming data
print "loop 6 ".Dumper(\%root)."\n";
$count--;
#checking incomming data
print "loop 7 ".$count."\n";
#recursion not implamented yet
#tree(\$treeRoot{'a'}, $count);
}
}
tree(\%root, 2);
#print Dumper(\%root);

Answer 1

You need parentheses to disambiguate. This:
\%treeRoot, $count = #_;
means this:
\%treeRoot;
$count = #_;
Because the assignment operator = has higher precedence than the comma operator ,. The warning that you got from running that code tells you this: Useless use of reference constructor in void context.
To pass the arguments correctly, you need parentheses:
(\%treeRoot, $count) = #_;
Unfortunately, this does not work, because you cannot assign to a reference this way. The following error tells you that: Can't modify reference constructor in list assignment.
So what you need is to pass the reference to a scalar:
my ($href, $count) = #_;
print $href->{'value'};
I think this method is a bit backwards, though. Passing variables by reference is likely to become a source of bugs. A more natural solution is to use the return value of the subroutine to assign values:
sub foo {
my %hash;
$hash{'value'} = ....
....
return \%hash;
}
my $hashref = foo();
print $hashref->{'value'};

Answer 2

Your question isn't actually how to pass a hash reference, but how to receive it, as the following will not work:
\%treeRoot, $count = #_;
Basically, you need to assign your reference to a scalar like so:
use strict;
use warnings;
sub example_sub {
my ($hashref, $count) = #_;
# Add two values to the hash:
$hashref->{newkey} = 'val';
$hashref->{newkey2} = 'val2';
}
my %root;
example_sub(\%root, 2);
use Data::Dump;
dd \%root;
Outputs:
{ newkey => "val", newkey2 => "val2" }
If you don't want to modify your original hash, you can assign the values to a new hash within the sub:
my %newhash = %$hashref;
For more info on working with references, check out: perlref - Perl references and nested data structures