How to distinguish between "0" and NULL in perl? - perl

Question

Here we are looking for the string "reftext" in the given file. The line next to this contains a string with 3 integers. So we are extracting them in #all_num. We are printing the value of #all_num[2] only if is not NULL. But the logic used here doesn't print #all_num[2] even if it has 0.
#!/usr/bin/perl
open( READFILE, "<myfile.txt" );
#list = <READFILE>;
$total_lines = scalar #list;
for ( $count = 0; $count < $total_lines; $count++ ) {
if (#list[ $count =~ /reftext/ )
{
#all_num = #list[ $count + 1 ] =~ /(\d+)/g;
if ( #all_num[2] != NULL ) {
print "#all_num[2]\n";
}
}
}

Answer 1

Hope this helps,
use strict;
use warnings;
my #fvals = (
[ i => undef ],
[ j => 0 ],
[ k => "" ],
);
for my $r (#fvals) {
my ($k, $v) = #$r;
if (!defined($v)) { print "$k is undef\n"; }
elsif (!length($v)) { print "$k is empty string\n"; }
# elsif (!$v) { print "$k is zero\n"; }
# recognizes zero value in "0.0" or "0E0" notation
elsif ($v == 0) { print "$k is zero\n"; }
}
output
i is undef
j is zero
k is empty string

Answer 2

Perl does not include a NULL, so the line
if(#all_num[2]!= NULL)
is nonsensical in Perl. (More accurately, it attempts to locate a sub named NULL and run it to get the value to compare against #all_num[2], but fails to do so because you (presumably) haven't defined such a sub.) Note that, if you had enabled use strict, this would cause a fatal error instead of pretending to work. This is one of the many reasons to always use strict.
Side note: When you pull a value out of an array, it's only a single value, so you should say $all_num[2] rather than #all_num[2] when referring to the third element of the array #all_num. (Yes, this is a little confusing to get used to. I hear that it's been changed in Perl 6, but I'm assuming you're using Perl 5 here.) Note that, if you had enabled use warnings, it would have told you that "Scalar value #all_num[2] better written as $all_num[2]". This is one of the many reasons to always use warnings.
If you want to test whether $all_num[2] contains a value, the proper way to express that in Perl is
if (defined $all_num[2])

Answer 3

This is how your program would look using best practices
You should
Always use strict and use warnings, and declare all your variables with my
Use the three-parameter form of open
Check that open calls succeeded, and include $! in the die string if not
Use a while loop to process a file one line at a time, in preference to reading the entire file into memory
#!/usr/bin/perl
use strict;
use warnings;
open my $fh, '<', 'myfile.txt' or die $!;
while ( <$fh> ) {
next unless /reftext/;
my $next_line = <$fh>;
my #all_num = $next_line =~ /\d+/g;
print "$all_num[2]\n" if defined $all_num[2];
}

Answer 4

Try this:
#!/usr/bin/perl
use warnings;
use strict;
open(READFILE, "<", "myfile.txt") or die $!;
my #list = <READFILE>;
my $total_lines = scalar #list;
close (READFILE);
for(my $count=0; $count<$total_lines; $count++)
{
if($list[$count] =~ /reftext/)
{
my #all_num = $list[$count+1] =~ /(\d+)/g;
if($all_num[2] ne '')
{
print "$all_num[2]\n";
}
}
}
To check a variable is null or not:
if ($str ne '')
{
print $str;
}
or better:
my ($str);
$str = "";
if (defined($str))
{
print "defined";
}
else
{
print "not defined";
}

Answer 5

If the other answers do not work, try treating the variable as a string:
if ( $all_num[2] == 'null' && length($all_num[2]) == 4 ){
# null
} else {
# not null
}
As with any code you write, be sure to test your code.