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 you’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 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:[*]
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 hardly ever 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 = $_; } } ...