# Chapter 3
Reviewing and Practicing GW-BASIC

The practice sessions in this chapter will help you review what you have learned. If you have not done so, this is a good time to turn on your computer and load the GW-BASIC Interpreter.

## 3.1 Example for the Direct Mode

You can use your computer in the direct mode to perform fundamental arithmetic operations. GW-BASIC recognizes the following symbols as arithmetic operators:

Operation | GW-BASIC Operator
Addition | +
Subtraction | -
Multiplication | *
Division | /

To enter a problem, respond to the Ok prompt with a question mark (?), followed by the statement of the problem you want to solve, and press the RETURN key. In GW-BASIC, the question mark can be used interchangeably with the keyword PRINT. The answer is then displayed.

Type the following and press the RETURN key:

    ?2+2

GW-BASIC will display the answer on your screen:

    ?2+2
     4
    Ok

To practice other arithmetic operations, replace the + sign with the desired operator.

The GW-BASIC language is not restricted to arithmetic functions. You can also enter complex algebraic and trigonometric functions. The formats for these functions are provided in Chapter 6, "Constants, Variables, Expressions and Operators."

## 3.2 Examples for the Indirect Mode

The GW-BASIC language can be used for functions other than simple algebraic calculations. You can create a program that performs a series of operations and then displays the answer. To begin programming, you create lines of instructions called statements. Remember that there can be more than one statement on a line, and that each line is preceded by a number.

For example, to create the command PRINT 2+3 as a statement, type the following:

    10 print 2+3

When you press the RETURN key, the cursor shifts to the next line, but nothing else happens. To make the computer perform the calculation, type the following and press the RETURN key:

    run

Your screen should look like this:

    Ok
    10 print 2+3
    run
     5
    Ok

You have just written a program in GW-BASIC.

The computer reserves its calculation until specifically commanded to continue (with the RUN command). This allows you to enter more lines of instruction. When you type the RUN command, the computer does the addition and displays the answer.

The following program has two lines of instructions. Type it in:

    10 x=3
    20 print 2+x

Now use the RUN command to have the computer calculate the answer.

Your screen should look like this:

    Ok
    10 x=3
    20 print 2+x
    run
     5
    Ok

The two features that distinguish a program from a calculation are

1. The numbered lines
2. The use of the run command

These features let the computer know that all the statements have been typed and the computation can be carried out from beginning to end. It is the numbering of the lines that first signals the computer that this is a program, not a calculation, and that it must not do the actual computation until the RUN command is entered.

In other words, calculations are done under the direct mode. Programs are written under the indirect mode.

To display the entire program again, type the LIST command and press the RETURN key:

    list

Your screen should look like this:

    Ok
    10 x=3
    20 print 2+x
    run
    Ok
     5
    Ok
    list
    10 X=3
    20 PRINT 2+X
    Ok

You'll notice a slight change in the program. The lowercase letters you entered have been converted into uppercase letters. The LIST command makes this change automatically.

## 3.3 Function Keys

Function keys are keys that have been assigned to frequently-used commands. The ten function keys are located on the left side of your keyboard. A guide to these keys and their assigned commands appears on the bottom of the GW-BASIC screen. To save time and keystrokes, you can press a function key instead of typing a command name.

For example, to list your program again, you needn't type the LIST command; you can use the function key assign to it, instead:

- Press the F1 key.
- Press RETURN.

Your program should appear on the screen.

To run the program, simply press the F2 key, which is assigned to the RUN command.

As you learn more commands, you'll learn how to use keys F3 through F10. Chapter 4, "The GW-BASIC Screen Editor," contains more information about keys used in GW-BASIC.

## 3.4 Editing Lines

There are two basic ways to change lines. You can

- Delete and replace them
- Alter them with the EDIT command

To delete a line, simply type the line number and press the RETURN key. For example, if you type 12 and press the RETURN key, line number 12 is deleted from your program.

To use the EDIT command, type the command EDIT, followed by the number of the line you want to change. For example, type the following, and press the RETURN key:

    edit 10

You can then use the following keys to perform editing:

Key | Function
CURSOR UP, CURSOR DOWN, CURSOR LEFT, CURSOR RIGHT | Moves the cursor within the statement
BACKSPACE | Deletes the character to the left of the cursor
DELETE (DEL) | Deletes the current character
INSERT (INS) | Lets you insert characters to the left of the cursor.

For example, to modify statement (line) 10 to read x=4, use the CURSOR-RIGHT control key to move the cursor under the 3, and then type a 4. The number 4 replaces the number 3 in the statement.

Now press the RETURN key, and then the F2 key.

Your screen displays the following:

    Ok
    10 X=4
    RUN
     6
    Ok

## 3.5 Saving Your Program File

Creating a program is like creating a data file. The program is a file that contains specific instructions, or statements, for the computer. In order to use the program again, you must save it, just as you would a data file.

To save a file in GW-BASIC, use the following procedure:

1. Press the F4 key. The command word SAVE" appears on your screen.
2. Type a name for the program, and press the RETURN key. The file is saved under the name you specified.

To recall a saved file, use the following procedure:

1. Press the F3 key. The command load LOAD" appears on your screen.
2. Type the name of the file.
3. Press RETURN.

The file is loaded into memory, and ready for you to list, edit, or run.