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A.3. 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. (Note: once again, there's no automatic connection between @_, 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. Of course, $sum is undef, since nothing has been stored in there. But 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 upon how you think of things. Suppose that 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, and that 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. (Of course, if you wished to distinguish the sum of an empty list from the sum of, say, (3, -5, 2), returning undef would be the right thing to do.)

    If you don't want a possibly undefined return value, though, it's easy to remedy: simply initialize $sum to zero rather than using the default of undef:

    my $sum = 0;

    Now the subroutine will always return a number, even if the parameter list were empty.

  2. Here's one way to do it:

    # Remember to include &total from previous exercise!
    print "The numbers from 1 to 1000 add up to ", &total(1..1000), ".\n";

    Note that we can't call the subroutine from inside the double-quoted string,[383] so the subroutine call is another separate item being passed to print. The total should be 500500, a nice round number. And it shouldn't take any noticeable time at all to run this program; passing a parameter list of 1000 values is an everyday task for Perl.

    [383]We can't do this without advanced trickiness, that is. It's rare to find anything that you absolutely can't do in Perl.



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