150 Part III: Advanced Programming with Liberty BASIC

In this example, the main program instructions start with line one (NOMAINWIN) and stop with line nine (END). The subprogram, [hackeralert], starts on line 11 ([hacker alert]) and stops on line 18 (RETURN). The only time that the [hackeralert] subprogram runs is if you type an incorrect password in the first Prompt dialog box that asks, What is your password?

Ideally, you want to make all your subprograms small enough to fit on a single screen. The smaller your subprograms are, the easier they are to understand, debug, and modify.

If you use subprograms in a Liberty BASIC program, you’re essentially dividing a larger program into smaller parts, although you still save everything into a single file with a name such as PACMAN.BAS or BLACKJACK.BAS.

Be aware that programming languages often use similar terms to represent different items. If you divide a large program into smaller parts, for example, BASIC calls these smaller parts subprograms. But what BASIC calls a subprogram, the C language calls a function. So if you use a different language, make sure that you use the correct terms for that particular language; otherwise, you may get confused in talking to other programmers.

Using Subroutines

In Liberty BASIC, you can create subprograms, which are miniature programs that perform a specific task. Unfortunately, subprograms work exactly the same every time that they run.

For more flexibility, you may want to create subprograms that can accept data and calculate a new result, basing it on the data that it receives. You may, for example, want to create a subprogram that multiplies two numbers together. In most cases, you don’t want to multiply the same two numbers every time that the subprogram runs. Instead, the subprogram is more useful if you can feed it a different number each time and have the subprogram spit back an answer based on the number that it receives.

In most programming languages such as C/C++, Java, or Pascal, you can create the following two different types of subprograms:

Subroutines: These types of subprograms contain instructions that accept data and use that data for performing certain tasks, such as verifying that the user types a correct password or moving the cursor as the user moves the mouse.

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Functions: These types of subprograms calculate and return a single value, such as calculating the cube of a number or counting the number of words in a sentence. In Liberty BASIC, the COS(x) and SIN(x) commands are examples of a function.

Defining a subroutine

A subroutine consists of the following two or three parts:

The subroutine’s name

One or more instructions that you want the subroutine to follow

Any data that you want the subroutine to use (optional)

In Liberty BASIC, a subroutine looks as follows:

SUB SubroutineName Data

‘ One or more instructions

END SUB

If a subroutine doesn’t need to accept data from the main program or another subprogram, you can omit the Data portion of the subroutine, as in the following example:

SUB SubroutineName

‘ One or more instructions

END SUB

Passing data to a subroutine

A subroutine acts like a miniature program that performs one or more tasks. Although a subroutine is part of a bigger program, some subroutines can act independently from the rest of the program. You may, for example, have a subroutine that does nothing but display a message on-screen. If a subroutine can act independently from the rest of the program, you need to define only a name for your subroutine and any instructions that you want the subroutine to follow, as in the following example:

SUB SubroutineName

PRINT “This subroutine doesn’t use any data.” END SUB

152 Part III: Advanced Programming with Liberty BASIC

The preceding example of a subroutine is similar to a subprogram in Liberty BASIC. As a program runs the preceding subroutine, the subroutine merrily follows its instructions without using any data from any other part of the program.

But in many cases, a subroutine requires outside data to accomplish a given task, such as checking to see whether the user types a valid password. If your subroutine needs data from another part of your program, you need to create a variable to store this data, as in the following example:

SUB SubroutineName Variable

‘ One or more instructions here

END SUB

As another part of the program tells this subroutine to run, it “passes” data to this subroutine, in much the same way that a person may pass a baton to another person. After a subroutine receives data that another part of the program passes to it, the subroutine uses variables (which it traps inside parentheses) to “hold” any data that another part of the program sends to it.

The list of variables, which accepts outside data for a subroutine, is known as a parameter list.

Each variable inside a subroutine’s parameter list specifies one chunk of data, such as a number or string. If another part of the program passes your subroutine two chunks of data, the subroutine’s parameter list must contain exactly two variables to hold this data. The following subroutine example uses three variables in its parameter list:

SUB SubroutineName Variable1, Variable2, Variable3 ‘ One or more instructions here

END SUB

Make sure that you declare string variables by using the dollar sign ($) and number variables without the dollar sign. The following example uses a string and a number variable in its parameter list:

SUB SubroutineName Name$, Age

‘ One or more instructions here

END SUB

The first line of the preceding subroutine defines the name of the subroutine (which is SubroutineName) and creates two variables, Name$ and Age. The Name$ variable represents a string, and the Age variable represents an integer.

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Calling a subroutine

After you create a subroutine, the final step is to actually run the instructions inside the subroutine. Normally, the subroutine sits around and does absolutely nothing (much like a politician) until the computer specifically tells the subroutine to run its instructions. In technical terms, as you tell the subroutine to run its instructions, you’re calling a subroutine. Essentially, you’re saying, “Hey, stupid subroutine! Start running your instructions now!”

If you want to run the subroutine BurnOutMonitor, you must use the CALL command. Many versions of BASIC enable you to use one of the following two methods:

CALL BurnOutMonitor

If the BurnOutMonitor subroutine needs you to pass data to it for it to run, you just tack on the data to pass to the subroutine as follows:

CALL BurnOutMonitor 45

If you use the CALL command, make sure that you spell your subroutine correctly, including upper and lowercase letters. Liberty BASIC assumes that a subroutine by the name of DisplayMessage is completely different from a subroutine with the name displaymessage.

To help you understand subroutines better, try the following Liberty BASIC program. Notice that the subroutine appears after the END command. This program asks for a name (such as Bill McPherson) and then displays that name to a Notice dialog box by using either the string Bill McPherson must be a moron or Bill McPherson sounds like an idiot to me.

NOMAINWIN

PROMPT “Give me the name of someone you hate:”; enemy$

CALL DisplayMessage enemy$

END

SUB DisplayMessage stuff$

X = INT(RND(1) * 2) + 1

IF X = 1 THEN

NOTICE stuff$ + “ must be a moron.”

ELSE

NOTICE stuff$ + “ sounds like an idiot to me.”

END IF

END SUB