WHILE_DO
Use the WHILE_DO loop to continue performing an action while certain conditions are true.
Operands
WHILE bool_expression DO
<statement>;
Structured Text
Operand | Type | Format | Description |
|---|---|---|---|
bool_expression | BOOL | tag expression | BOOL tag or expression that evaluates to a BOOL value |
IMPORTANT:
Do not iterate within the loop too many times in a single scan.
The controller does not execute any other statements in the routine until it completes the loop.
A major fault occurs when completing the loop takes longer than the watchdog timer for the task.
Consider using a different construct, such as IF_THEN.
Description
The syntax is:
The following diagrams illustrate how a WHILE_DO loop executes, and how an EXIT statement leaves the loop early.
 |  |
While the bool_expression is true, the controller executes only the statements within the WHILE_DO loop. | To stop the loop before the conditions are true, use an EXIT statement. |
Affects Math Status Flags
No
Fault Conditions
A major fault will occur if | Fault type | Fault code |
the construct loops too long | 6 | 1 |
Example 1
If performing the following, | Enter this structured text | |
The WHILE_DO loop evaluates its conditions first. If the conditions are true, the controller then executes the statements within the loop. This differs from the REPEAT_UNTIL loop because the REPEAT_UNTIL loop executes the statements in the construct and then determines if the conditions are true before executing the statements again. The statements in a REPEAT_UNTIL loop are always executed at least once. The statements in a WHILE_DO loop might never be executed. | pos := 0; | |
While ((pos <= 100) & structarray[pos].value <> targetvalue)) do | ||
pos := pos + 2; | ||
String_tag.DATA[pos] := SINT_array[pos]; | ||
end_while; | ||
Example 2
If performing the following, | Enter this structured text |
Move ASCII characters from a SINT array into a string tag. (In a SINT array, each element holds one character.) Stop when you reach the carriage return. Initialize Element_number to 0. Count the number of elements in SINT_array (array that contains the ASCII characters) and store the result in SINT_array_size (DINT tag). If the character at SINT_array[element_number] = 13 (decimal value of the carriage return), then stop. Set String_tag[element_number] = the character at
SINT_array[element_number]. Add 1 to element_number. This lets the controller check the next character in SINT_array. Set the Length member of String_tag = element_number. (This records the number of characters in String_tag so far.) If element_number = SINT_array_size, then stop. (You are at the end of the array and it does not contain a carriage return.) | element_number := 0; |
SIZE(SINT_array, 0, SINT_array_size); | |
While SINT_array[element_number] <> 13 do | |
String_tag.DATA[element_number] := SINT_array[element_number]; | |
element_number := element_number + 1; | |
String_tag.LEN := element_number; | |
If element_number = SINT_array_size then | |
exit; | |
end_if; | |
end_while; |
Provide Feedback