In VBA I can do the following:
A = B + IIF(C>0, C, 0)
so that if C>0 I get A=B+C and C<=0 I get A=B
How about simply using the fact that MATLAB automatically converts variable types when required by the operation? E.g., logical to double.
If your variables are scalar double, your code, I believe, can be replaced by
a = b + (c > 0) * c;
In this case, the operator (c > 0) values 1 (logical type) whenever c > 0 and values to 0 otherwise.