FOR

PET VIC-20 C64 C16 Plus/4 C128 X16 M65

Syntax

FOR <varname> [AS <type>] = <start_value> TO <end_value> [STEP <step_value>]
  <statements>
NEXT [<varname>]

The FOR … NEXT loop initializes a counter variable and executes the statements in the block until the counter variable equals the value in the TO clause. After each iteration, the counter variable is incremented by the step value or 1 in case the step value was not specified. For example:

DIM i AS BYTE
FOR i = 1 TO 30 STEP 3
  PRINT i
NEXT i

As you can see above, the counter variable must be predefined. If the variable is not predefined, you can use the AS keyword to specify the type and this way the compiler will automatically define the variable before starting the loop. The following code is identical to the one above:

FOR i AS BYTE = 1 TO 30 STEP 3
  PRINT i
NEXT i

Warning

The counter variable, the start value, the end value and the step value must all be of the same numeric type and this type is concluded from the counter variable. All the other expressions will be converted to this type if possible, otherwise a compile-time error will be thrown.

Note

The variable name can be omitted in the NEXT statement.

Countdown of unsigned types

For performance and overflow-safety reasons, it is not possible to go downwards in a loop where the index variable is of an unsigned type (BYTE or WORD). You should rather use a DO loop for this purpose:

' This loop will never run because -1 is converted to 255
FOR b AS BYTE = 5 TO 1 STEP -1
  PRINT b
NEXT b

' Use this instead
DIM i AS BYTE
i = 5
DO WHILE i >= 1
  PRINT i
  i = i - 1
LOOP

' Or simply use a signed type
FOR x AS INT = 5 TO 1 STEP -1
  PRINT x
NEXT x

Continuing the loop

You can use the CONTINUE FOR command to skip the rest of the statements in the FOR … NEXT block and go to the next iteration, for example:

REM -- print numbers from 1 to 10, except 5 and 7
FOR num AS BYTE = 1 TO 10
  IF num = 5 OR num = 7 THEN CONTINUE FOR
  PRINT num
NEXT

Early exiting the loop

Finally, you can use the EXIT FOR statement to early exit a FOR … NEXT loop.

DIM a$ AS STRING * 1
FOR i AS BYTE = 1 TO 10
  PRINT "iteration #"; i : INPUT "do you want to continue? (y/n)"; a$
  IF a$ = "n" THEN EXIT FOR
  PRINT i
NEXT i

The counter is always STATIC

Despite variables in Subroutines or Functions can be defined either STATIC or dynamic, the counter of a FOR … NEXT loop must always be STATIC. For this reason,

If you try to use a previously defined dynamic variable as counter, compilation will fail
If you let the compiler automatically define the variable (using the AS keyword), the variable will be defined as STATIC.

SUB mydynamicsub ()
  DIM i AS BYTE
  FOR i = 0 TO 10 : PRINT i : NEXT ' Error, i can't be dynamic
END SUB

SUB mystaticsub () STATIC
  DIM i AS BYTE
  FOR i = 0 TO 10 : PRINT i : NEXT ' Works because i is now STATIC
END SUB

SUB mydynamicsub ()
  STATIC i AS BYTE
  FOR i = 0 TO 10 : PRINT i : NEXT ' This works, too
END SUB

SUB mydynamicsub ()
  FOR i AS BYTE = 0 TO 10 : PRINT i : NEXT ' Also works, i will be defined STATIC
END SUB