ZX BASIC:DO

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DO ... LOOP

DO ... LOOP is a compound statement used to perform loops. The code within the DO ... LOOP statement will be repeated if the given condition is true. The loop is executed at less once when the loop condition is written at the end, even if the given condition is false at the first iteration.

Syntax

The DO ... LOOP construct is a very powerful sentence and can be used in up to 5 different ways:

Infinite loops

Sometimes we want a loop to repeat forever, no matter what, because we need to exit the loop when an external event happens. For example, we want to repeat forever waiting for a key press. Traditionally we use GOTO for this in Sinclair BASIC. Other languages use WHILE (1), etc. The best way to do this in ZX BASIC is this one:

DO
    [<sentences>]
LOOP: REM This loop repeats forever. 

This form loops forever. It's better to use this form instead of using STEP 0 in a FOR loop, or a WHILE 1 condition loop. The generated code is more efficient.

Looping UNTIL

DO
    [<sentences>]
LOOP UNTIL <condition>

This form repeats until the given condition is met. The loop is guaranteed to execute at least once regardless of loop exit condition - it is only evaluated at the end of the first loop.

You can also put the condition at the beginning, this way:

DO UNTIL <condition>
    [<sentences>]
LOOP

In this case, the condition is checked first, and the program won't enter to the inner sentences if the condition is not satisfied at first.

Example using UNTIL

Example: Loop until the user press a Key

REM in a single line!
DO LOOP UNTIL INKEY$ <> ""

Looping WHILE

DO
    [<sentences>]
LOOP WHILE <condition>

This form repeats while the given condition is true. The difference with the WHILE sentence is the latter won't execute the inner sentences if condition is false at the start. Remember: DO...LOOP will execute sentences at least once regardless of the condition upon entry to the loop - it is only evaluated at the end of the first loop.

You can also put the condition at the beginning, this way:

DO WHILE <condition>
    [<sentences>]
LOOP

In this case, the condition is checked first, and the program won't enter to the inner sentences if the condition is not satisfied at first.

Example using WHILE

Example: Loop while there is no key pressed

REM in a single line!
DO LOOP WHILE INKEY$ = ""

Remarks

  • This statement does not exist in Sinclair Basic.
  • WHILE can also be used with WHILE ... END WHILE loops.

See Also

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