Perl - Use of uninitialized value in string - perl

Question

I started teaching myself Perl, and with the help of some Googling, I was able to throw together a script that would print out the file extensions in a given directory. The code works well, however, it will sometimes complain the following:
Use of uninitialized value $exts[xx] in string eq at get_file_exts.plx
I tried to correct this by initializing my array as follows: my #exts = (); but this did not work as expected.
#!/usr/bin/perl
use strict;
use warnings;
use File::Find;
#Check for correct number of arguments
if(#ARGV != 1) {
print "ERROR: Incorrect syntax...\n";
print "Usage: perl get_file_exts.plx <Directory>\n";
exit 0;
}
#Search through directory
find({ wanted => \&process_file, no_chdir => 1 }, #ARGV);
my #exts;
sub process_file {
if (-f $_) {
#print "File: $_\n";
#Get extension
my ($ext) = $_ =~ /(\.[^.]+)$/;
#Add first extension
if(scalar #exts == 0) {
push(#exts, $ext);
}
#Loop through array
foreach my $index (0..$#exts) {
#Check for match
if($exts[$index] eq $ext) {
last;
}
if($index == $#exts) {
push(#exts, $ext);
}
}
} else {
#print "Searching $_\n";
}
}
#Sort array
#exts = sort(#exts);
#Print contents
print ("#exts", "\n");

Answer 1

You need to test if you found an extension.
Also, you should not be indexing your array. You also do not need to manage 'push' just do it. It is not the Perl way. Your for loop should start like this:
sub process_file {
if (-f $_) {
#print "File: $_\n";
#Get extension
my ($ext) = $_ =~ /(\.[^.]+)$/;
# If we found an extension, and we have not seen it before, add it to #exts
if ($ext) {
#Loop through array to see if this is a new extension
my $newExt = 1;
for my $seenExt (#exts) {
#Check for match
if ($seenExt eq $ext) {
$newExt = 0
last;
}
}
if ($newExt) {
push #exts,$ext;
}
}
}
}
But what you really want to do is to use a hash table to record if you saw an extension
# Move this before find(...); if you want to initialize it or you will clobber the
# contents
my %sawExt;
sub process_file {
if (-f $_) {
#print "File: $_\n";
# Get extension
my ($ext) = $_ =~ /(\.[^.]+)$/;
# If we have an extension, mark that we've seen it
$sawExt{$ext} = 1
if $ext;
}
}
# Print the extensions we've seen in sorted order
print join(' ',sort keys %sawExt) . "\n";
Or even
sub process_file {
if (-f $_ && $_ =~ /(\.[^.]+)$/) {
$sawExt{$1} = 1;
}
}
Or
sub process_file {
$sawExt{$1} = 1
if -f && /(\.[^.]+)$/;
}
Once you start thinking in Perl this is the natural way to write it

Answer 2

The warning is complaining about a content of $exts[xx], not #exts itself.
Actually $ext can be undef, when the filename doesn't match to your regexp, for instance README.
Try like:
my ($ext) = $_ =~ /(\.[^.]+)$/ or return;

Answer 3

The main problem is that you aren't accounting for file names that don't contain a dot, so
my ($ext) = $_ =~ /(\.[^.]+)$/;
sets $ext to undef.
Despite the warning, processing continues by evaluating undef as the null string, failing to find that in #exts, and so percolating undef to the array as well.
The minimal change to get your code working is to replace
my ($ext) = $_ =~ /(\.[^.]+)$/;
with
return unless /(\.[^.]+)$/;
my $ext = $1;
But there is a couple of Perl lessons to be learned here. It used to be taught that good programs were well-commented programs. That was in the days of having to write efficient but incomprehensible code, but is no longer true. You should write code that is as clear as possible, and add comments only if you absolutely have to write something that isn't self-explanatory.
You should remember and use Perl idioms, and try to forget most C that you knew. For instance, Perl accepts the "here document" syntax, and it is common practice to use or and and as short-circuit operators. Your parameter check becomes
#ARGV or die <<END;
ERROR: Incorrect syntax...
Usage: perl get_file_exts.plx <Directory>
END
Perl allows for clear but concise programming. This is how I would have written your wanted subroutine
sub process_file {
return unless -f and /(\.[^.]+)$/;
my $ext = $1;
foreach my $index (0 .. $#exts) {
return if $exts[$index] eq $ext;
}
push #exts, $ext;
}

Answer 4

Use exists on $exts[xx] before accessing it.
exists is deprecated though as #chrsblck pointed out :
Be aware that calling exists on array values is deprecated and likely
to be removed in a future version of Perl.
But you should be able to check if it exists (and not 0 or "") simply with :
if($exts[index] && $exts[$index] eq $ext){
...
}