Using local variables allows to effectively implement loops, subprograms and other control algorithms.
The identifier of a local variable is a sequence of characters, which can be used for the name of D2000
system (rules for object name definition). It must begin
with the character "_" (underscore). For example: _i, _i1, _locVar.1. Maximum: 64 characters.
Value of a local variable is characterized by all value attributes as well as objects in D2000 system
(time, limits, process alarm etc.).
Available types of local variables:
The objects SD.RecordDef and SV.Structure are used for a demonstration of some features of local variables.
Local variable definition is denoted by its type (in case of RECORD and ALIAS type, by the type of
the structure), which is followed by the local variable identifiers of that type, separated with commas,
e.g.:
INT _a, _b, _c,
or
RECORD NOALIAS (SD.RecordDef) _rec1, _rec2.
TIME represents also a key word in assignment.
Variables of INT, BOOL, REAL, TIME a TEXT types represent simple values of given type.
Variables of TEXT type are internally encoded in UTF-8.
A variable of ALIAS type represents a reference to a D2000 system object. After running the script, the
reference is empty and its value is not defined. It is not possible to use such a variable and the error
_ERR_NO_ASSIGNED_ALIAS, which causes interruption of the script execution, will be generated, if
you attempt to do it. After initialization of such a variable (setting the reference to an object), it
acquires the value of the linked object by means of particular action (SET
AS). It is possible to use such a variable in an expressions, or to assign a value to it in the same
way as if a linked object was used.
Example:
ALIAS _obj
INT _i
_obj := 1 ; action will cause
ERR_NO_ASSIGNED_ALIAS error
ALIAS _obj
INT _i
SET _obj AS U.Int
_obj := 1 ; assigns the value of 1
the user variable U.Int
WAIT ;
wait for execution of the assignment
_i := _obj ; assigns
the value of U.Int to the local variable _i
If an ALIAS type local variable is linked with an object of Value array type, it allows to use indexing in the same way as for the object of Value array type:
Example:
ALIAS _obj
INT _i
SET _obj AS X.Array
_i := _obj[2] ; assigns the
value of the second item of X.array to local variable _i
There are two different types of ALIAS type local variables:
When you declare a typed ALIAS it is necessary to specify a type (in our case, it is an object of Structure definition type), which limits the set of possible objects (of Structured variable type), to which the local variable (typed ALIAS) may be linked. For a local variable declared in this way, it is able, in expressions (of course, after an association using SET AS action), to use indexing and to access items in the same way as for object of Structured variable type.
Example:
ALIAS (SD.RecordDef) _recAlias
INT _i
_i := 2
ALIAS _recAlias AS SV.Structure
_recAlias[_i]^Int := 1
The variable of RECORD type allows to create a local variable of Record type. The structure type is
given by the object of Structure definition type, which is declared.
If RECORD variable is a formal parameter, its structure type - non-typed RECORD
need not be set.
Example:
RECORD (SD.RecordDef) _recLocal
After running the script, the array size of such a local variable defined in this way is one (1) and the values of all the items are invalid.
Example: Assigning a value
RECORD (SD.RecordDef) _recLocal
_recLocal[1]^Text := "text value"
_recLocal[1]^Int := 8
Change of the array size of a local variable may be executed by using REDIM action.
Example: Change the array size to ten (10) items
REDIM _recLocal [10]
Minimum array size is zero (0). An array size change doesn't lose values of items contained in the array
before the action. Potential new items created are initialized to invalid value.
An item of local variable of Record type (or object of Structured
variable type) may be Object type (see the topic Structure definition).
Such an item is interpreted in the script as an untyped ALIAS. Consequence:
Assignment:
_recLocal[1]^Object := 1
may cause the error _ERR_NO_ASSIGNED_ALIAS if an object is not linked to an item. Assignment of an object to item is identical with assignment of variables of ALIAS type:
SET _recLocal[1]^Object AS U.Int
Note: Analogous to untyped ALIAS, the access to an item of a local variable that is not linked with will generate the error _ERR_NO_ASSIGNED_ALIAS.
To avoid this behaviour use the keyword NOALIAS when you declare the local variable.
RECORD NOALIAS (SD.RecordDef) _recLocal
For a local variable declared this way, all items of Object type are interpreted as standard values of any type (INT, BOOL, REAL, TEXT, TIME). Therefore the following declaration is correct:
_recLocal[1]^Object := 1
_recLocal[1]^Object := "TEXT"
The item Object changes its own type together with individual assignments.
After assigning a value to an Object type item of
Structured variable type object, this assignment is automatically
redirected to the linked object (likewise untyped ALIAS). If the item is not linked to an object, an
error will occur only if the action WAIT follows.
ESL allows to assign either a whole structure row or a whole structure at once. This assignment is executed by SET WITH action.
Action syntax:
SET identDst_Rec WITH identSrc_Rec
where
Both identifiers must represent a structure row or a whole structure. Structure types must be the same (the error: Incompatible types). In case of a whole structure, their number of rows must be identical (the error: _ERR_RANGE_ERROR). The action will assign all the values from the source (identSrc_Rec) to the destination (identDst_Rec).
!!! IT DOES NOT CHANGE THE ASSOCIATED (LINKED) OBJECTS IN INDIVIDUAL ITEMS !!!.
SET AS action permits to assign and change linked objects (for items of Object type) in a whole structure row, or in a whole structure together.
Action syntax:SET identDst_Rec AS identSrc
The same limits as listed for the action SET WITH are also available
for identSrc and identDst, but they may not be local variables of Record type declared
as NOALIAS.
Permitted combinations of identSrc and identDst parameters for
SET WITH a SET AS actions.
SET BIND action links a row of a local variable of Record type to a row of an object of
Structured variable type.
Action syntax:
SET _recLocal_Row BIND struct_Row
where
SET _recLocal[2] BIND SV.Structure[1]
After the action execution, the values of all the items of the row 2 of the local variable (_recLocal[2])
are the same the values of the row 1 of the object SV.Structure. For example: the expression
_recLocal[2]^Int returns the same value as the expression SV.Structure[1]^Int.
The feature of row binding is also allowed for other actions:
Example: Assigning a value
_recLocal[2]^Int := 1
will assign the value of 1 to the item SV.Structure[1]^Int. Since the value of the item _recLocal[2]^Int
copies the value of the item SV.Structure[1]^Int, any change of the local variable is executed after a change
in SV... object (it is possible to use WAIT action). Behaviour of rows of
a local variable
Record type, which are not linked (SET BIND) to
another row, is unchanged
(described above).
Each row of the local variable _recLocal may be linked to another (or the same) row of an object of
Structured variable type. The only limitation is, that local variable and object
must be of the same structured type (structure definition).
Unlinking a row (cancelling the action SET BIND) is allowed by the action:
SET _recLocal[2] BIND NONE
All values of items will acquire undefined state.