Programming > Structured Text > Concepts

Loops supported and specified by IEC-61131-3

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Loop - FOR

The FOR statement enables a sequence of instructions to be executed repeatedly while a progression of values is assigned to its control variable. The FOR statement increments or decrements the control variable from a starting value to an ending value determined by the value of an expression. If the BY construct is omitted, the increment value defaults to 1. Both the starting and ending values must be expressions of the same integer type (e.g., USINT, INT, WORD, or DINT).

FOR <counter> := <initial value> TO <end value> {BY <increment>} DO

END_FOR

The section inside the curly braces {} is optional

WPS executes <instructions> until <counter> reaches the <end value>. For positive increments, execution continues until <counter> exceeds the <end value>. For negative increments, execution continues until <counter> falls below the <end value>. This condition is checked before executing <instructions>.

Example

The example below shows one way to use the FOR loop to find the largest value in a global array of type USINT.

VAR

addr : WORD;

currentValue : USINT;

maxValue : USINT;

arr : ARRAY[0..5] OF USINT := [17, 9, 30, 25, 59, 41];

END_VAR

currentValue := 0;

maxValue := 0;

FOR addr := 0 TO 10 BY 1 DO

currentValue := arr[addr];

IF currentValue > maxValue THEN

maxValue := currentValue;

END_IF

END_FOR

At the end of the execution of the example code, the value of maxValue should be 59.

Loop - WHILE

In a WHILE statement, the sequence of statements enclosed up to the END_WHILE keyword is repeatedly executed as long as <boolean expression> evaluates to true. If <boolean expression> is false from the start, the statements are not executed at all.

WHILE <boolean expression> DO

END_WHILE

Example

The example below demonstrates a WHILE loop that executes ten times.

VAR

counter : USINT;

END_VAR

counter := 0;

WHILE counter < 100 DO

counter := counter + 10;

END_WHILE

At the end of the execution of the example code, the value of counter should be 100.

Loop - REPEAT

In the REPEAT statement, the sequence of instructions (<instructions>) grouped up to the UNTIL keyword is executed repeatedly at least once, until the boolean condition associated with it (<boolean expression>) is true.

REPEAT

UNTIL <boolean expression>

END_REPEAT

Example

The example below illustrates the use of a REPEAT loop to find the smallest multiple of 13 that is diferent than 13.

VAR

number : INT;

module : INT;

result : INT;

found : BOOL;

END_VAR

number := 1;

result := 1;

found := FALSE;

REPEAT

module := number MOD 13;

IF (module = 0) and (module <> 13) THEN

result := number;

found := TRUE;

END_IF

number := number + 1;

UNTIL found

END_REPEAT

At the end of the execution of the example code, the result value should be 26.

Statement - CONTINUE

The CONTINUE statement skips the remaining instructions in the current iteration of a loop, immediately proceeding to the next iteration. It applies from the CONTINUE keyword until the loop's termination keyword (END_FOR, END_WHILE, or END_REPEAT).

The example below shows one way to use the CONTINUE statement to find the sum of even numbers from 0 to 9.

VAR

number : INT;

module : INT;

result : INT;

END_VAR

number := 0;

module := 0;

result := 0;

WHILE number < 10 DO

module := number MOD 2;

IF module <> 0 THEN

number := number + 1;

CONTINUE

END_IF

result := result + number;

number := number + 1;

END_WHILE

At the end of the execution of the example code, the value of result should be 20.

Statement - EXIT

The EXIT statement is used in a FOR, WHILE, or REPEAT loop to terminate the loop regardless of other termination conditions.

When the EXIT statement is used within nested loops, it causes an exit from the innermost loop where the EXIT keyword appears. Control is then transferred to the statement following the nearest loop terminator (such as END_FOR, END_WHILE, or END_REPEAT) after the EXIT statement.

The example below demonstrates how to use the EXIT statement to exit the FOR loop when you find the array index that matches the desired number.

VAR

idx : WORD;

currentValue : USINT;

targetValue : USINT;

found : BOOL;

arr : ARRAY[0..5] OF USINT := [17, 9, 30, 25, 59, 41];

END_VAR

currentValue := 0;

targetValue := 25;

found := FALSE;

FOR idx := 0 TO 10 BY 1 DO

currentValue := arr[idx];

IF currentValue = targetValue THEN

found := TRUE;

EXIT

END_IF

END_FOR

At the end of the code execution, the value of idx should be 3, indicating the index where the number 25 is located in the array.