P4093¶

Code: P4093
Message: FOR step is zero

This error occurs when the step of a FOR loop, the value after BY, is zero.

A FOR loop assigns the start value to the control variable, runs its statements, adds the step to the control variable, and goes on while the control variable has not passed the end value. A step of zero never moves the control variable, so the loop never reaches the end value: unless a statement in its body leaves the loop with EXIT or RETURN, the loop runs forever and the scan never completes. That is a defect in the program, and it is known before the program runs, so the compiler reports it rather than compiling a loop that cannot end.

The step is judged when its value is known before the program runs: a step written as 0, as a typed literal such as INT#0, or as an expression of literals that folds to zero, such as 1 - 1, is reported, and so is a step that names a constant whose value is zero, whether the variable is declared CONSTANT or is never written. A step read from a variable that the program writes is not judged by this rule, since its value is only known when the program runs.

Example¶

The following code will generate error P4093:

PROGRAM main
VAR
    i : INT;
    total : INT;
END_VAR
    FOR i := 1 TO 5 BY 0 DO
        total := total + i;
    END_FOR;
END_PROGRAM

The step is 0, so i stays at 1 and the loop never ends.

To fix this error, step by a value that moves the control variable towards the end value. A step of 1 is the default, so the BY clause can be left out:

PROGRAM main
VAR
    i : INT;
    total : INT;
END_VAR
    FOR i := 1 TO 5 DO
        total := total + i;
    END_FOR;
END_PROGRAM

Counting down¶

A loop that counts down needs a negative step. A step of zero is often what is left when the sign is forgotten and the step is computed instead of written, as in the following code, which generates error P4093:

PROGRAM main
VAR
    i : INT;
    total : INT;
END_VAR
    FOR i := 5 TO 1 BY 1 - 1 DO
        total := total + i;
    END_FOR;
END_PROGRAM

The compiler folds 1 - 1 to 0 before judging the step, so the expression is reported where it was written.

To fix this error, write the step the loop needs. A negative step counts down from the start value to the end value:

PROGRAM main
VAR
    i : INT;
    total : INT;
END_VAR
    FOR i := 5 TO 1 BY -1 DO
        total := total + i;
    END_FOR;
END_PROGRAM

A constant step¶

A step that names a constant is judged by the constant’s value. The following code generates error P4093, since inc is 0 for the whole run:

PROGRAM main
VAR CONSTANT
    inc : INT := 0;
END_VAR
VAR
    i : INT;
    total : INT;
END_VAR
    FOR i := 1 TO 5 BY inc DO
        total := total + i;
    END_FOR;
END_PROGRAM

A variable the program never writes holds its initial value for the whole run too, so the same loop over inc : INT := 0 declared in a plain VAR block is reported as well. To fix this error, give the constant the step the loop needs:

PROGRAM main
VAR CONSTANT
    inc : INT := 1;
END_VAR
VAR
    i : INT;
    total : INT;
END_VAR
    FOR i := 1 TO 5 BY inc DO
        total := total + i;
    END_FOR;
END_PROGRAM

P4093¶

Example¶

Counting down¶

A constant step¶

See Also¶