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4.8. 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 that we've already seen. Of course, this is unlike many traditional programming languages, which require every subroutine to be strictly typed; that is, to permit only a certain, predefined number of parameters of predefined types. It's nice that Perl is so flexible, but (as we 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:[113]

[113]As soon as you learn about warn (in Chapter 11, "Filehandles and File Tests"), you'll see that you can use it to turn improper usage like this into a proper warning. Or perhaps you'll decide that this case is severe enough to warrant using die, described in the same chapter.

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 we saw in Chapter 3, "Lists and Arrays ".

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

4.8.1. 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 often been called the "high-water mark" algorithm; after a flood, when the waters have surged and receded for the last time, the high-water mark shows where the highest water was seen. In this routine, $max_so_far keeps track of our high-water mark, the largest number yet seen.

The first line sets $max_so_far to 3 (the first parameter in the example code) by shifting that parameter from the parameter array, @_. So @_ now holds (5, 10, 4, 6), since the 3 has been shifted off. And the largest number yet seen is the only one yet seen: 3, the first parameter.

Now, the foreach loop will step through the remaining values in the parameter list, from @_. The control variable of the loop is, by default, $_. (But, remember, there's no automatic connection between @_ and $_; it's just a coincidence that they have such similar names.) The first time through the loop, $_ is 5. The if test sees that it is larger than $max_so_far, so $max_so_far is set to 5 -- the new high-water mark.

The next time through the loop, $_ is 10. That's a new record high, so it's stored in $max_so_far as well.

The next time, $_ is 4. The if test fails, since that's no larger than $max_so_far, which is 10, so the body of the if is skipped.

The next time, $_ is 6, and the body of the if is skipped again. And that was the last time through the loop, so the loop is done.

Now, $max_so_far becomes the return value. It's the largest number we've seen, and we've seen them all, so it must be the largest from the list: 10.



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