Learning Perl, Fourth Edition

Variable-Length Parameter Lists

In real-world Perl code, subroutines are often given parameter lists of arbitrary length. That’s because of Perl’s “no unnecessary limits” philosophy. Of course, this is unlike many traditional programming languages, which require every subroutine to be strictly typed to permit only a certain, predefined number of parameters of predefined types. It’s nice that Perl is so flexible, but (as you saw with the &max routine earlier) that may cause problems when a subroutine is called with a different number of arguments than the author expected.

Of course, the subroutine can easily check that it has the right number of arguments by examining the @_ array. For example, we could have written &max to check its argument list like this:^[102]

    sub max {
      if (@_ != 2) {
        print "WARNING! &max should get exactly two arguments!\n";
      }
      # continue as before...
      .
      .
      .
    }

That if test uses the “name” of the array in a scalar context to find out the number of array elements, as you saw in Chapter 3.

But in real-world Perl programming, this sort of check is rarely used; it’s better to make the subroutine adapt to the parameters.

A Better &max Routine

So let’s rewrite &max to allow for any number of arguments:

    $maximum = &max(3, 5, 10, 4, 6);

    sub max {
      my($max_so_far) = shift @_;  # the first one is the largest yet seen
      foreach (@_) {               # look at the remaining arguments
        if ($_ > $max_so_far) {    # could this one be bigger yet?
          $max_so_far = $_;
        }
      }
      $max_so_far;
    }

This code uses what has ...