Assignment and arithmetic in ST: := and operator precedence

Assignment is :=, not =

ST has two separate symbols:

Every assignment ends with a semicolon ;. A missing semicolon is the first compile error everyone meets.

Litres := Level * 25.0;      (* assignment *)
Lamp   := Level = 50.0;      (* assigning the result of a comparison, perfectly valid *)

The arithmetic operators

+ add, - subtract, * multiply, / divide, and MOD for the remainder.

Total     := A + B;
Remaining := Capacity - Total;
Volume    := Level * 25.0;
Average   := Total / 4.0;
Phase     := Counter MOD 3;   (* gives 0, 1, 2, 0, ... *)

MOD is more useful than it looks: splitting work across three scans, dividing down a scan counter, blinking a lamp.

Precedence

Operators are not evaluated left to right. They follow a fixed precedence, highest first:

That is why this line works without brackets around the comparison:

Pump := Start AND Level < 90.0;

< binds tighter than AND. Even so, add the parentheses. A line you have to read twice is a bad line, correct or not.

The trap: integer division

If both operands are integers, the result is an integer and the fraction is truncated, not rounded:

IntHalf  := 7 / 2;       (* 3, not 3.5 and not 4 *)
RealHalf := 7.0 / 2.0;   (* 3.5 *)

The classic version of this bug lives in scaling code:

Percent := Raw / 27648 * 100;      (* wrong: 0 for any Raw below 27648 *)
Percent := Raw * 100.0 / 27648.0;  (* right: multiply first, use reals *)

The rule: if the result is a ratio or a fraction, make at least one operand REAL, and multiply before you divide.

Division by zero

On most controllers a division by zero faults the program. Guard the denominator:

IF Span > 0.0 THEN
  Percent := (Raw - Zero) * 100.0 / Span;
END_IF