From QB64 Wiki
The FOR statement creates a counter loop using specified start and stop numerical boundaries. The default increment is + 1.
- The FOR counterVariable name is required to define the counter span and may also be used after the NEXT keyword.
- The startValue TO stopValue can be any literal or variable numerical type. Both values are required.
- STEP can be used for a loop increment other than the default plus 1 and can be any positive or negative literal or variable numerical value as long as the STEP value corresponds to the loop's startValue and stopValue.
- NEXT ends the FOR loop code block and increments the counter to the next value even when it exceeds the stop limit.
- FOR...NEXT counter loops must be within the proper start, stop and increment values or the entire loop code block will not be executed.
- Avoid changing the FOR counterVariable's value inside of the loop. This obfuscates code and is a poor programming practice.
- Once the loop has been started, changing the variables holding the startValue, stopValue or increment value will not affect loop execution.
- If the STEP increment value does not match the startValue TO stopValue the FOR loop block will be ignored.
- Use EXIT FOR to leave a FOR loop early when a certain condition is met inside of the loop.
- The NEXT counter variable name is not required. NEXT loop increments can be separated by colons in nested FOR loops.
- NOTE: When the FOR loop is exited after the stopValue is reached, the counterVariable's value will be stopValue + 1 (or stopValue + increment)
- Beware of FOR loop counts that exceed the counterVariable type limits and may repeat without error in QB64.
- For example, if counterVariable is of type INTEGER and the stop limit exceeds 32767, the counterVariable will reset back to -32768 and loop endlessly.
Example 1: Adding all of the even numbers from 10 to 0.
FOR i = 10 TO 0 STEP -2 totaleven% = i + totaleven% PRINT totaleven%; NEXT PRINT "After loop, i ="; i
10 18 24 28 30 30 After loop, i = -2
- Explanation: The loop counts down from 10 to every even value below it. The counter keeps stepping down until the FOR stop limit is reached or exceeded. Note that the value of i is -2 after the loop is exited. NEXT always increments the counter one last time.
Example 2: How an entire FOR loop block is ignored when the start and stop limits do not match the default or STEP increment.