Difference between revisions of "Error Messages/40616"

Latest revision as of 17:59, 8 July 2016

Error message examples

You cannot assign to a slice or subscript of a global variable object.

Cause

Analytica won't let you assign a value to a slice (subscripted element) of a global variable, even in an OnClick or OnChange attribute, or Function called from one of those, where you would normally be able to assign to a global variable. It will let you assign to a slice of a local variable.

Here the OnClick attribute of Button Change_Xtries to assign to a slice of the global variable X, and so triggers this error message:

 Index I := 1..2
 Variable X := Table(I)(10, 20)
 Button <code>Change_X</code>    
        OnClick: X[I = 2] := 30

Remedies

Analytica does let you assign to a slice of a local variable (in any Definition), and it does let you assign to a global variable (not a slice) in an OnClick or OnChange attribute, or Function called from one of those attributes. So you can work around this problem.

Copy the global array, X, into a local variable, local_x,
Assign a new value to a slice of local_x,
Assign the value of local_x back to the global, X,

For example:

 Index I := 1..2
 Variable X := Table(I)(10, 20)
 Button <code>Change_X</code>    
   OnClick: 
      VAR local_X := X; 
      local_X[I = 2] := 30; 
      X := local_X

This does what you need.

@@ Line 1: / Line 1: @@
-= Error Message Examples  =
+[[Category: Error messages]]
-  You cannot assign to a slice or subscript of a global variable object.
+== Error message examples  ==
-= Cause  =
+:<code>''You cannot assign to a slice or subscript of a global variable object.''</code>
-You are trying to assign a value to a slice (subscripted element) of a global variable. This is not permitted, since it would contravene Analytica's general principle of ''no side-effects''.  You may assign to a slice of a local variable.
+== Cause  ==
-This example triggers this error. Here BadDef tries to assign to a slice of the global variable X:
+Analytica won't let you assign a value to a [[slice]] ([[subscript]]ed element) of a global variable, even in an [[OnClick]] or [[OnChange]] attribute, or Function called from one of those, where you would normally be able to [[Assignment Operator :=|assign]] to a global variable. It will let you assign to a [[slice]] of a [[Local Variables|local variable]].
-<code>Index I := 1..9
-Variable X := Table(I)(7, 8, 3, 2, 5, 4, 9, 1, 5)
+Here the [[OnClick]] attribute of Button <code>Change_X</code>tries to assign to a slice of the global variable <code>X</code>, and so triggers this error message:
-Variable BadDef := X[I = 5] := 6
+<pre style="background:white; border:white; margin-left: 1em;">
-</code>
+ Index I := 1..2
+ Variable X := Table(I)(10, 20)
+ Button <code>Change_X</code>
+        OnClick: X[I = 2] := 30
+</pre>
-= Remedies  =
+== Remedies  ==
-You may assign to a global variable in a Button script or in a Function called from a Button script [[Scripting|Scripts and Buttons]], directly or indirectly. The rationale is that this does not violate the No-side effects principle because clicking a button means the user is causing the effect directly.
+Analytica ''does'' let you assign to a slice of a local variable (in any Definition), and it ''does'' let you assign to a global variable (not a slice) in an [[OnClick]] or [[OnChange]] attribute, or Function called from one of those attributes. So you can work around this problem.
+# Copy the global array, <code>X</code>, into a local variable, <code>local_x</code>,
+# Assign a new value to a slice of <code>local_x</code>,
+# Assign the value of <code>local_x</code> back to the global, <code>X</code>,
-You can often work around this issue by copying the global array into a local, assigning to a slice of the array, and then assigning the local back to the global within a function called from a Button script. For example, suppose you want to assign 999 to the 10th element over I of global variable X.  If you define a variable
+For example:
-   Variable TrytoAssignX := X[I=10] := 999
-it will give the error above.
+<pre style="background:white; border:white; margin-left: 1em;">
-So instead define
+ Index I := 1..2
-    Function AssigntoX(v)
+  Variable X := Table(I)(10, 20)
-    Definition: Variable local_X:= X;
+ Button <code>Change_X</code>
-        local_X[i=10] := v;
+    OnClick:
-        X := local_X
+      VAR local_X := X;
-   Button Assign_to_X
+      local_X[I = 2] := 30;
-   Script:  Assign_to_X(999)
+      X := local_X
+</pre>
-When you press the button, it will call Assign_to_X(999) and make the assignment.
+This does what you need.
-<br><comments />
+==See Also==
+* [[User-Defined Function]]
+* [[Local Variables]]
+* [[Assignment Operator :=]]
+* [[Slice]]
+* [[Subscript]]
+* [[Script]]
+* [[Button]]