Mapping - Field XREF Values

I assume noobish question, but I’m trying to design a map to populate different descriptions in an output field based on different codes in an input field. For example $input.code to $output.description. If code=A then description=Alpha, if code=B then description=Beta, etc…

So it’s one input field mapped to one output field, but not sure how to do it. Right now I’m doing a goofy string replace type of thing for every possibility in one big rule, but I’m sure there has to be a more straight forward way.