These kind of situations are best handled by storing the original data in a variable type that is designed to map strings to data. The simplest option is to use a structure which supports indexing with a string variable:
s.a = 1;
s.b = 2;
s.c = 3;
s.d = 4;
V = 'b';
s.(V) = 3000;
In some situations, you might want to look at the containers.Map object.