Intermediate Perl

Chapter 10. Building Larger Programs

This chapter looks at how to break up a program into pieces and includes some of the concerns that arise when we put those pieces back together again, or when many people work together on the same program.

The Cure for the Common Code

The Skipper writes many Perl programs to provide navigation for all the common ports of call for the Minnow. He finds himself cutting and pasting a very common routine into each program:

sub turn_toward_heading {
  my $new_heading = shift;
  my $current_heading = current_heading(  );
  print "Current heading is ", $current_heading, ".\n";
  print "Come about to $new_heading ";
  my $direction = 'right';
  my $turn = ($new_heading - $current_heading) % 360;
  if ($turn > 180) { # long way around
    $turn = 360 - $turn;
    $direction = 'left';
  }
  print "by turning $direction $turn degrees.\n";
}

This routine gives the shortest turn to make from the current heading (returned by the subroutine current_heading( )) to a new heading (given as the first parameter to the subroutine).

The first line of this subroutine might have read instead:

my ($new_heading) = @_;

This is mostly a style call: in both cases, the first parameter ends up in $new_heading. However, we’ve seen that removing the items from @_ as they are identified does have some advantages. So, we stick (mostly) with the “shifting” style of argument parsing. Now back to the matter at hand . . . .

After writing a dozen programs using this routine, the Skipper realizes that the output is excessively ...