Tinkering with Perl

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Subroutines

A subroutine is a miniature program that you can run from within your program. Earlier, we talked about a procedure that would tell a price for an amount of fruit. Here is how we write such a procedure. Before the procedure is used, we assume that there is some code that tells the price of the different kinds of fruit:

$PricePerApple = .25;
$PricePerOrange = .30;
$PricePerBanana = .20;

And here is the subroutine itself:

sub TellPrice()
    {
    $NumberOfApples = shift(@_);
    $NumberOfOranges = shift(@_);
    $NumberOfBananas = shift(@_);
    @RemainingArguments = @_;
    $PriceForApples = $NumberOfApples * $PricePerApple;
    $PriceForOranges = $NumberOfOranges * $PricePerOrange;
    $PriceForBananas = $NumberOfBananas * $PricePerBanana;
    $TotalPrice = $PriceForApples + $PriceForOranges + $PriceForBananas;
    print "The total price for the fruit is $TotalPrice.\n";
    }

Well, let's look at it piece by piece:

    $NumberOfApples = shift(@_);
    $NumberOfOranges = shift(@_);
    $NumberOfBananas = shift(@_);
    @RemainingArguments = @_;

This part finds out what the arguments to the procedure are, and stores them in the appropriate variables. This should go at the beginning of the subroutine; the syntax for using the subroutine is:

&TellPrice($NumberOfApples, $NumberOfOranges, $NumberOfBananas);

The part of the subroutine that gets the arguments should be of the following form:

    $first argument = shift(@_);
    $second argument = shift(@_);
    $third argument = shift(@_);
    ...
    @RemainingArguments = @_;

where first argument, second argument, and so on are the names of the arguments in order.

After firuging out what the arguments are, the subroutine then does some arithmetic to figure out the total price, and prints it out.

One more thing... At the beginning of the program, you need to declare your subroutine, by having a line like this:

sub &TellPrice();

Tinkering with Perl

Subroutines

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