Perl input multiple text files from command line and print them

A few hints, and I'll let you get on your way...

Perl has what is called a diamond operator. This operator opens all files placed on the command line (which is read into the @ARGS array), and reads them line-by-line.

use strict;
use warnings;
use autodie;
use feature qw(say);

while my $line ( <> ) {
    chomp $line;
    say "The line read in is '$line'";
}

Try this program and run it as you would your program. See what happens.

Next, take a look at the Perl documentation for variables related to file handles. Especially take a look at the $/ variable. This variable is what used to break records. It's normally set to a new-line, so when you read in a file, you read it in line-by-line. You may want to try that. If not, you can fall back onto something like this:

use strict;
use warnings;
use autodie;
use feature qw(say);

while my $line ( <> ) {
    chomp $line;
    @words = split /\s+/, $line;
    for my $word ( @words ) {
        say "The word is '$word'";
    }
}

Now you can use a hash to track which words were in each file and how many times. You can also track the various types of words you've mentioned. However, please don't use variables such as $U. Use $first_letter_uppercase. This will have more meaning in your program and will be less confusing for you.

Your teacher is teaching you the way Perl was written almost 30 years ago. This was back before God created the Internet. (Well, not quite. The Internet was already 10 years old, but no one outside of a few academics had heard of it). Perl programming has greatly evolved since then. Get yourself a good book on Modern Perl (that is Perl 5.x).

The pragmas at the beginning of my program (the use statements) do the following:

• use strict - Use strict syntax. This does several things, but the main thing is to make sure you cannot use a variable unless you first declare it. (using most likely my). This prevents mistakes such as putting $name in one place, and referring to $Name in another place.
• use warnings - This warns you of basic errors such as you're attempting to use a variable that isn't defined. By default, Perl assumes the variable is a null string or equal to zero if you use it in an arithmetic context. When you attempt to print or check a variable that hasn't been assigned a value. It probably means you have a logic mistake.

The above two pragmas will catch 90% of your errors.

• use autodie - This will cause your program to automatically die in many circumstances. For example, you attempt to open a none existent file for reading. This way, you don't have to remember to check each instance of whether or not certain operations succeeded of failed.
• use feature qw(say) - This allows you to use say instead of print. The say command is just like print, but automatically adds a new line on the end. It can make your code way cleaner and easier to understand.

For example:

print "N=$N\n" ;

vs.

say "N=$N" ;

Here's how I'd write that program. But it won't get you many marks as it's a long way from the "fill in the blanks" approach that your teacher is using. But that's good, because your teacher's Perl is very dated.

#!/usr/bin/perl

use strict;
use warnings;
use 5.010;

my ($N, $P, $L, $F, $U);
my %words;

while (<>) {
  my @tokens = split /[^-\w]+/;
  foreach my $token (@tokens) {
    $N++ if $token =~ /^-\d+$/;
    $P++ if $token =~ /^\d+$/;
    next unless $token =~ /[a-z]/i;
    $L++ if $token eq lc $token;
    $U++ if $token eq uc $token;
    $F++ if $token eq ucfirst lc $token;
    push @{$words{lc $token}{$ARGV}}, $.;
  }
  close ARGV if eof;
}

say "N=$N";
say "P=$P";
say "L=$L";
say "F=$F";
say "U=$U";

for my $word (sort { $a cmp $b } keys %words) {
  print "$word ";
  for my $file (sort { $a cmp $b } keys %{$words{$word}} ) {
    print "$file (", join(' ', @{$words{$word}{$file}}), ') ';
  }
  print "\n";
}