Commodore 128D - Page 267

DO/LOOP/WHILE/
UNTIL/EXIT
—Define and control a program loop
DO [UNTIL condition | WHILE condition] statements [EXIT]
LOOP [UNTIL condition | WHILE condition]
This loop structure performs the statements between the DO state
ment and the LOOP statement. If no UNTIL or WHILE modifies either
the DO or the LOOP statement, execution of the statements in
between continues indefinitely. If an EXIT statement is encountered
in the body of a DO loop, execution is transferred to the first state
ment following the LOOP statement. DO loops may be nested, follow
ing the rules defined by the FOR-NEXT structure. If the UNTIL
parameter is specified, the program continues looping until the con
dition is satisfied (becomes true). The WHILE parameter is basically
the opposite of the UNTIL parameter: the program continues looping
as long as the condition is TRUE. As soon as the condition is no
longer true, program control resumes with the statement immedi
ately following the LOOP statement. An example of a condition (bool
ean argument) is A = 1, or G>65.
EXAMPLE:
10X = 25
20 DO UNTIL X = 0
30X = X-1
40 PRINT "X = ";X
50 LOOP
This example performs the statements
X = X-1 and PRINT "X=";X until X = 0.
When X = 0 the program resumes with
the PRINT "End of Loop" statement
immediately following LOOP.
60 PRINT "END OF LOOP"
10 DO WHILE A$<> CHR$ (13):GETKEY A$:PRINT A$:LOOP
20 PRINT "THE RETURN KEY HAS BEEN PRESSED"
This DO loop waits for a key to be
pressed, receives input from the
keyboard one character at a time and
prints the letter of the key which was
pressed. If the RETURN key was
pressed, control is transferred out of
the loop and line 20 is executed.
10 DOPEN #8,"SEQFILE"This program opens file "SEQFILE"
20 DO
30 GET #8,A$
40 PRINT A$;
50 LOOP UNTIL ST
60 DCLOSE #8
and gets data until the ST system
variable indicates all data has been
input and printed.
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BASIC 7.0 ENCYCLOPEDIA—Basic Commands and Statements