Error Messages/40616

Error Message Examples

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

Cause

You may not assign a value to a slice (subscripted element) of a global variable. You may assign to a slice of a local variable. And in a Script or Function called from the Script of a Button or other object, you may assign a new value to a global variable (but not a slice of it.)

This example triggers this error. Here BadDef tries to assign to a slice of the global variable X:

Index I := 1..9
Variable X := Table(I)(7, 8, 3, 2, 5, 4, 9, 1, 5)
Variable BadDef := (X[I = 5] := 99)

Remedies

While you may not normally assign to a global variable, either directly or to a slice of the variable, in the definition of a variable, you may assign to a global variable (but not to a slice) in a Button script or in a Function called from a Button script Scripts and Buttons. The latter does not violate the No-side effects principle because by clicking a button the user causes the effect directly.

You can often work around the prohibition against slice assignment by writing a function, F, that

1. copies the global array, X, into a local variable, local_x,
2. assigns to a slice of the array, local_x, and then
3. assigns the value of local_x back to the global, X,
4. and finally, cal function F from a Button script.

For example, suppose you want to assign 999 to the 5th element over index I of global variable X:

  Function Assign_to_X(v)
    Definition: 
       Variable local_x := X;
       local_x[I=5] := v;
       X := local_x

  Button Do_Assign
    Script:  Assign_to_X(999)

When you press the button, Do_Assign, it calls Assign_to_X(999) and completes the slice assignment.

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