Computer Decisions-The If-Then Statement - Commodore 128D User Manual

Computer
Decisions—
The IF-THEN
Statement
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This section describes how to use a number of powerful BASIC com
mands, functions and programming techniques that can be used in
both C128 and C64 modes.
These commands and functions allow you to program repeated
actions through looping and nesting techniques; handle tables of
values; branch or jump to another section of a program, and return
from that section; assign varying values to a quantity—and more.
Examples and sample programs show just how these BASIC con
cepts work and interact.
Now that you know how to change the values of variables, the next
step is to have the computer make decisions based on these
updated values. You do this with the IF-THEN statement. You tell the
computer to execute a command only IF a condition is true (e.g., IF
X = 5). The command you want the computer to execute when the
condition is true comes after the word THEN in the statement. Clear
your computer's memory by typing NEW and pressing RETURN,
then type this program:
10J = 0
20 ? J,"COMMODORE 128"
30 J = J + 1
40 IF J<>5 THEN GOTO 20
60 END
You no longer have to press the STOP key to break out of a looping
program. The IF-THEN statement tells the computer to keep printing
"COMMODORE 128" and incrementing (increasing) J until J = 5 is
true. When an IF condition is false, the computer jumps to the next
line of the program, no matter what comes after the word THEN.
Notice the END command in line 60. It is good practice to put an
END statement as the last line of your program. It tells the computer
where to stop executing statements.
51 USING C128 MODE—Advanced BASIC Programming