Error Messages/41107

Message variants

Encountered Subscript(*,Time,*) referencing 'future' Time. 
Encountered Slice(*,Time,*) referencing 'future' Time.
 
Consider using a conditional in the third parameter to ensure the third parameter is not greater than Time.

Cause & Remedy

In a Dynamic function, you may express the value at Time=t in terms of earlier values. If, however, you attempt to define the value at Time=t based on future value, the error results. A trivial case would be:

Dynamic(1,Self[Time=Time+1])

When the error indicates that Subscript is to blame, this may be due to an explicit call to the Subscript function, or it may be resulting from a subscript expression of the form expr[Time=x]. Similarly, if it reads Slice, it may either be an explicit call to the Slice function, or a slice operator of the form expr[@Time=n].

The error may occur in less obvious situations when position parameter is computed, or is an array. For example, suppose you have an index named T defined to be have the same index values as Time. If you attempted a re-indexing operation such as:

Dynamic(1,A[Time=T])

you would encounter this error, since when Dynamic is evaluating at Time=2, T contains the value 3, etc. Because a "hard error" occurs in this case, rather than just a warning, you can't simply ignore warnings as is possible in other subscript-out-of-range instances. Hence, you may need to do include a conditional (or other expression) in the third parameter, as well as a conditional outside, as follows:

Dynamic(1,If T>=Time Then Null Else A[ Time=(If T>=Time Then 1 Else T) ] )

The conditional inside the third parameter serves only to ensure that it never references a Time value beyond the computed range. The following variation is often more syntactically convenient:

Dynamic(1,If T>=Time Then Null Else A[ Time=Min([T,Time-1]) ] )