Learning Perl, Fourth Edition

Answers to Chapter 4 Exercises

1. Here’s one way to do it:

       sub total {
         my $sum;  # private variable
         foreach (@_) {
           $sum += $_;
         }
         $sum;
       }

   This subroutine uses $sum to keep a running total. At the start of the subroutine, $sum is undef since it’s a new variable. Then, the foreach loop steps through the parameter list (from @_) using $_ as the control variable. (There’s no automatic connection among @_, the parameter array, and $_, the default variable for the foreach loop.)

   The first time through the foreach loop, the first number (in $_) is added to $sum. $sum is undef since nothing has been stored there. Since we’re using it as a number, which Perl sees because of the numeric operator +=, Perl acts as if it’s already initialized to 0. Perl thus adds the first parameter to 0, and puts the total back into $sum.

   Next time through the loop, the next parameter is added to $sum, which is no longer undef. The sum is placed back into $sum and on through the rest of the parameters. Finally, the last line returns $sum to the caller.

   There’s a potential bug in this subroutine depending on how you think of things. Suppose this subroutine was called with an empty parameter list as we considered with the rewritten subroutine &max in the chapter text. In that case, $sum would be undef which would be the return value. But in this subroutine, it would probably be more correct to return 0 as the sum of the empty list rather than undef. (If you wished to distinguish the sum of an empty list from the sum of, ...