IF, ELSIF and ELSE: the rules and when to avoid them

The full construct

IF Level >= 90.0 THEN
  Band := 3;
ELSIF Level >= 60.0 THEN
  Band := 2;
ELSIF Level >= 20.0 THEN
  Band := 1;
ELSE
  Band := 0;
END_IF

ELSIF is one word, not ELSE IF. ELSE is optional. You can have as many ELSIF branches as you like.

The order is the logic

Branches are tested top to bottom, the first true condition wins, and execution jumps to END_IF. So the tightest test goes first. This code is broken:

IF Level >= 20.0 THEN
  Band := 1;              (* catches everything above 20% *)
ELSIF Level >= 90.0 THEN
  Band := 3;              (* can never run *)
END_IF

The semicolon rules

Three rules clear up 90% of compile errors:

1. Every statement inside a branch ends with ;.
2. No semicolon after THEN, and none after ELSE.
3. END_IF closes the construct. The standard needs no semicolon after it, but several vendors (Siemens SCL among them) write END_IF; and the lab here accepts that too.

IF Start THEN        (* no semicolon here *)
  Pump := TRUE;      (* semicolon here *)
END_IF

Nesting versus flattening

You can put an IF inside an IF:

IF Auto THEN
  IF NOT Stop THEN
    IF Level < 90.0 THEN
      Pump := TRUE;
    END_IF
  END_IF
END_IF

That is a slope. Three levels today become seven next year and nobody will read them. Flatten with AND:

IF Auto AND NOT Stop AND Level < 90.0 THEN
  Pump := TRUE;
END_IF

Rule of thumb: never go past three levels of nesting. If you need more, the logic is asking for a CASE or for a split into smaller pieces.

When a plain assignment beats IF

This is the most useful idea in the lesson. If the outcome is a single boolean, do not use IF at all:

(* four lines, with a trap on the way *)
IF Level >= 90.0 THEN
  HiAlm := TRUE;
ELSE
  HiAlm := FALSE;
END_IF

(* one line, and clearer *)
HiAlm := Level >= 90.0;

The real trap is the forgotten ELSE:

IF Level >= 90.0 THEN
  HiAlm := TRUE;
END_IF

The alarm goes up and never comes down, because nothing brings it down and the variable keeps its value between scans. Every IF that raises an output needs something that clears it.

IF earns its place when you pick one of several paths, run a group of statements, or guard a calculation against a division by zero.