- Vytvoril/a D2000 Dev Team, naposledy zmenené mar 26, 2024
Function
Assignment (or conditional assignment) of one value.
Declaration
dstIdent := expression [TIME timeExpression] dstIdent :?= expression
Parameters
| dstIdent | in | Values destination (row identifier or whole structure identifier). |
| expression | in | Expression defining a value. |
| TIME timeExpression | in | Expression (AbsTime) defining the time of the value occurrence (optional parameter). |
Description
The time of a value created by an assignment is the current time. If the parameter TIME is defined, then the value of the parameter
timeExpression will be used for the time of when the value is generated.dstIdent is the identifier of one value of: - object or local variable of INT, BOOL, REAL, TIME, and TEXT types.
The parameterexpressionmust be given type. If not, the script will attempt to convert the value into the correct type. If it is not successful, then the result value will be invalid.
- a local variable of RECORD type or object of Structured variable type.
You can carry out an assignment just to one item of the particular structure row. Other assignment rules are compliant with simple type.
RECORD (SD.RecordDef) _lArr REDIM _lArr[10] _lArr[2]^Int := 1
- local variable of ALIAS type (untyped).
If it is not assigned to a D2000 system object, the error _ERR_NO_ASSIGNED_ALIAS will occur. Otherwise, the expression is evaluated and the result value is assigned to the object with the local variable assigned.
ALIAS _a SET _a AS U.Int _a := 1 WAIT
- local variable of ALIAS type (typed)
You can carry out an assignment just to one item of the particular structure row. If the local variable is not linked to an D2000 system object, the error _ERR_NO_ASSIGNED_ALIAS will occur. Otherwise, the expression is evaluated and the result value is assigned to the object item.
ALIAS (SD.RecordDef) _aArr SET _aArr AS SV.Structure _aArr[2]^Int := 1 WAIT
Conditional assignment :?=
In a conditional assignment, the expression is evaluated first, and then the resulting value is compared to the current dstIden value.
If the values are different, the assignment is performed. If the values are not different, the assignment will not be performed.
When comparing values (if both values are valid), only the value without other attributes (such as the time of value creation, limits, user flags) is taken into account,
The comparison considers two invalid values to be identical.
Conditional assignment makes sense especially when changing the values of objects within DODM, which prevents unnecessary activities that follow the assignment.
For example:
- when writing to the output I/O tag, communication takes place at the protocol level
- when writing to a user variable or to an item of a structured variable that has "Save start value" anabled, storing the new value to the configuration database is performed
A conditional assignment is the equivalent of the following code
IF M.Output\VLD THEN
IF _newValue\VLD THEN ; old valid, new valid
IF M.Output # _newValue THEN
M.Output := _newValue
ENDIF
ELSE ; old valid, new invalid
M.Output := _newValue
ENDIF
ELSE
IF _newValue\VLD THEN ; old invalid, new valid
M.Output := _newValue
ENDIF ; old invalid, new invalid
ENDIF
Note
If an assignment changes the value of a D2000 system object, in principle it is just a ''request'' for changing the value. After the assignment is executed, the script continues in the execution of the next actions and there is no warranty, that the object value has been changed (it depends on the current situation - the system load).
Example:
INT _prevValue _prevValue := U.Int ; assign previous value U.Int := U.Int + 1 ; add the value of 1 to object value IF U.Int = _prevValue THEN ; check the value ; the assignment has not been executed ELSE ; the assignment has been executed ENDIF
The case that assignment is not still executed, is more probably. WAIT action allows waiting for the assignment execution (its completion).
Indexed variable in an active picture
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