Commodore Plus 4 - Page 63

BASIC Version 3.5 Commands, Functions, and System Variables 81
Parameters: true-false condition THEN clause : ELSE clause
1. The conditions in the IF command can use comparison operators (=, <, >,
<>, <=, or >=) to compare values. The values can be any of the following:
• Numbers or text strings
• Any type of variable
• Variables on both sides of the comparison operator
• Mathematical formulas
2. The THEN clause contains commands that are executed only when the IF
command condition(s) is (are) true. The THEN clause, which is always a required
part of the IF command, can contain any legal commands. (If the THEN clause
contains more than one command, they must be separated by colons.) The THEN
clause must be typed on the same line as IF with no punctuation separating it
from the keyword THEN. If the THEN clause is a GOTO, the keyword GOTO
can be omitted.
Example: 50 IF X$ = "HALT" THEN END
The program ends when X$ does
equal HALT. The THEN clause is
not executed otherwise.
3. The ELSE clause contains a command that is executed only when the IF
condition(s) is (are) false. The ELSE clause can contain any legal command. If the
ELSE clause is a GOTO, the keyword GOTO can be omitted. The ELSE clause is
always optional, but the THEN clause is always required, so an IF command
cannot have an ELSE clause but no THEN clause.
ELSE must be separated from the THEN clause by a colon. ELSE is a clause,
not an independent command; type THEN and ELSE clauses on the same lines as
the IF command.
Note: IF commands can be "nested," but their ELSE commands will not be.
When an IF condition is found to be false, the next ELSE clause on the line is
always executed.
Examples:
30 IF A = B THEM" PRINT
"EQUALITY": ELSE GOTO 100
40 IF A = 0 THEN IF B = 0
THEN PRINT "BOTH 0":ELSE
PRINT "ONE NONZERO"
When A equals B, the THEN
clause executes and the ELSE
clause does not. When the condi
tion is false, the THEN clause
does not execute, and the ELSE
clause does.
Prints BOTH 0 if A and B are
zero. Prints ONE NONZERO if
either is not zero.