Chapter 1: An Overview of Perl

1.3. An Average Example

Suppose you've been teaching a Perl class, and you're trying to figure out how to grade your students. You have a set of exam scores for each member of a class, in random order. You'd like a combined list of all the grades for each student, plus their average score. You have a text file (imaginatively named grades) that looks like this:

Noël 25
Ben 76
Clementine 49
Norm 66
Chris 92
Doug 42
Carol 25
Ben 12
Clementine 0
Norm 66
...

By the way, the line numbers are not part of the program, any other resemblances to BASIC notwithstanding.

 1  #!/usr/bin/perl
 2  
 3  open(GRADES, "grades") or die "Can't open grades: $!\n";
 4  while ($line = <GRADES>) {
 5      ($student, $grade) = split(" ", $line);
 6      $grades{$student} .= $grade . " ";
 7  }
 8 
 9  foreach $student (sort keys %grades) {
10      $scores = 0;
11      $total = 0;    
12      @grades = split(" ", $grades{$student});
13      foreach $grade (@grades) {
14          $total += $grade;
15          $scores++;
16      }
17      $average = $total / $scores;
18      print "$student: $grades{$student}\tAverage: $average\n";
19  }

We'd tell you to try running it, but you may not know how yet.

1.3.1. How to Do It

Gee, right about now you're probably wondering how to run a Perl program. The short answer is that you feed it to the Perl language interpreter program, which coincidentally happens to be named perl. The long answer starts out like this: There's More Than One Way To Do It.[10]

[10] That's the Perl Slogan, and you'll get tired of hearing it, unless you're the Local Expert, in which case you'll get tired of saying it. Sometimes it's shortened to TMTOWTDI, pronounced "tim-toady". But you can pronounce it however you like. After all, TMTOWTDI.

The first way to invoke perl (and the way most likely to work on any operating system) is to simply call perl explicitly from the command line.[11] If you are doing something fairly simple, you can use the -e switch (% in the following example represents a standard shell prompt, so don't type it). On Unix, you might type:

% perl -e 'print "Hello, world!\n";'

[11] Assuming that your operating system provides a command-line interface. If you're running an older Mac, you might need to upgrade to a version of BSD such as Mac OS X.

On other operating systems, you may have to fiddle with the quotes some. But the basic principle is the same: you're trying to cram everything Perl needs to know into 80 columns or so.[12]

[12] These types of scripts are often referred to as "one-liners". If you ever end up hanging out with other Perl programmers, you'll find that some of us are quite fond of creating intricate one-liners. Perl has occasionally been maligned as a write-only language because of these shenanigans.

For longer scripts, you can use your favorite text editor (or any other text editor) to put all your commands into a file and then, presuming you named the script gradation (not to be confused with graduation), you'd say:

% perl gradation

The most convenient way to invoke a script is just to name it directly (or click on it), and let the operating system find the interpreter for you. On some systems, there may be ways of associating various file extensions or directories with a particular application. On those systems, you should do whatever it is you do to associate the Perl script with the perl interpreter. On Unix systems that support the #! "shebang" notation (and most Unix systems do, nowadays), you can make the first line of your script be magical, so the operating system will know which program to run. Put a line resembling line 1 of our example into your program:

#!/usr/bin/perl

% gradation

% /home/sharon/bin/gradation

#!/bin/sh -- # perl, to stop looping
eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
    if 0;

Throughout this book, we'll just use #!/usr/bin/perl to represent all these notions and notations, but you'll know what we really mean by it.

A random clue: when you write a test script, don't call your script test. Unix systems have a built-in test command, which will likely be executed instead of your script. Try try instead.

A not-so-random clue: while learning Perl, and even after you think you know what you're doing, we suggest using the -w switch, especially during development. This option will turn on all sorts of useful and interesting warning messages, not necessarily in that order. You can put the -w switch on the shebang line, like this:

#!/usr/bin/perl -w

1.2. Natural and Artificial Languages		1.4. Filehandles

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