Loop Statements Overview

FOR…DO…END_FOR;

Syntax

FOR Counter := Start_Value TO Limit_Value [BY Step_Value] DO
  Statements
END_FOR;

Rules

• BY is optional; if not specified, it defaults to 1.

• If BY equals 0, a runtime error occurs.

• Start_Value, Limit_Value, and Step_Value are evaluated once when the loop begins.

• Positive steps run ascending; negative steps run descending.

• The loop counter is read-only within the body.

• Only integer expressions are allowed for counter and step.

Example

// Ascending loop (default BY 1)
Sum_Result := 0;
FOR Counter := 1 TO 5 DO
  Sum_Result := Sum_Result + Counter;
END_FOR;
// Result: Sum_Result = 15

// Descending loop
Reverse_Sum := 0;
FOR Counter := 5 TO 1 BY -1 DO
  Reverse_Sum := Reverse_Sum + Counter;
END_FOR;
// Result: Reverse_Sum = 15

WHILE…DO…END_WHILE;

Syntax

WHILE Condition DO
  Statements
END_WHILE;

Rules

• The condition is checked before each iteration.

• The body executes only while the condition is TRUE.

• If the condition never changes, the loop never ends.

Example

Value_Now := 0;
WHILE Value_Now < 5 DO
  Value_Now := Value_Now + 1;
END_WHILE;
// Result: Value_Now = 5

REPEAT…UNTIL…END_REPEAT;

Syntax

REPEAT
  Statements
UNTIL Condition
END_REPEAT;

Rules

• The body runs once before checking the condition.

• The loop ends when the condition becomes TRUE.

Example

Cycle_Count := 0;
REPEAT
  Cycle_Count := Cycle_Count + 1;
UNTIL Cycle_Count >= 3
END_REPEAT;
// Result: Cycle_Count = 3

Boolean Conditions Must Be BOOL

Studio's Structured Text compiler requires loop and branch conditions to evaluate to BOOL. When working with numeric flags, compare them to an explicit value (or convert them) before using them in a condition.

Is_Active := 1; // Numeric flag
WHILE Is_Active <> 0 DO
  Is_Active := 0; // Clear the flag and exit next iteration
END_WHILE;

Early Termination with EXIT;

Exits the innermost loop immediately.

FOR Task_Index := 1 TO 5 DO
  IF Task_Index = 3 THEN
    EXIT;
  END_IF;
END_FOR;
// Result: Task_Index = 3 because EXIT stops before the BY step runs

Loop_Index := 0;
WHILE Loop_Index < 10 DO
  IF Loop_Index = 3 THEN
    EXIT; // Leave the loop once the target is reached
  END_IF;
  Loop_Index := Loop_Index + 1;
END_WHILE;
// Result: Loop_Index = 3

Summary Table

Construct	Condition Checked	One Iteration Guaranteed	Supports BY	Early Exit	Typical Use
FOR…DO…END_FOR;	Fixed range	No	Yes (default = 1)	Yes	Counted iteration or array processing
WHILE…DO…END_WHILE;	Before body	No	No	Yes	Repetition while condition is true
REPEAT…UNTIL…END_REPEAT;	After body	Yes	No	Yes	Do-once-then-check or retry logic