Mark Wickens

FOR ~ NEXT

Purpose

Executes the program lines between the FOR statement and NEXT statements and increments the control variable, starting with the initial value. Execution is terminated when value of the control variable exceeds the specified final value.

Format

FOR   control variable name     =      initial value
                                       └─────┬─────┘
                                     Numeric expression
                                     
               TO     final value       [ STEP      increment ]
                      └────┬────┘                   └───┬───┘
                   Numeric expression           Numeric expression
                   
NEXT  [ Control variable name ]   [ , Control variable name ]*

Example

FOR I = 1 TO 10 STEP 0.1

Parameters

1. control variable name: Array variables cannot be used.
2. initial value: Numeric expression
3. final value: Numeric expression
4. increment: Numeric expression (default value = 1)

Explanation

1. None of the statements between FOR and NEXT are executed and the program proceeds to the next executable statement after NEXT when the initial value is greater than the final value.
2. Each FOR requires a corresponding NEXT.
3. FOR ~ NEXT loops can be nested (a FOR ~ NEXT loop can be placed inside another FOR ~ NEXT~ loop). Nested loops must be structured as shown below with NEXT appearing in reverse sequence of the FOR (e.g. FOR A, FOR B, FOR C ~ NEXT C, NEXT B, NEXT A)`

    ┌────10  FOR I=1 TO 12 STEP 3
    │  ┌─20  FOR J=1 TO 4 STEP 0.5
    │  │ 30  PRINT I, J
    │  └─40  NEXT J
    └────50  NEXT I
         60  END  
   

4. FOR ~ NEXT loops can be nested up to 29 levels.
5. The control variable may be omitted from NEXT. However, use of the control variable in the NEXT statement is recommended when nesting loops.
6. NEXT statements can be chained by including them under one NEXT statement, separated by commas.

    ┌────10  FOR I=1 TO 12 STEP 3    ┌────10  FOR I=1 TO 12 STEP 3
    │  ┌─20  FOR J=1 TO 4 STEP 0.5   │  ┌─20  FOR J=1 TO 4 STEP 0.5
    │  │ 30  PRINT I, J              │  │ 30  PRINT I,J
    │  └─40  NEXT J                  └──┴─40  NEXT J,I
    └────50  NEXT I                      50  END
         60  END  
   

7. The control variable retains the value which exceeds the final value (and terminates the loop) when loop execution is complete. With the loop FOR I=3 TO 10 STEP 3, for example, the value of control variable I is 12 when execution of the loop is complete.
8. Jumping out of a FOR ~ NEXT loop is also possible. In this case, the current control variable value is retained in memory, and the loop can be resumed by returning with a GOTO statement.