Function to have one field automatically fill in another.

Function to have one field automatically fill in another.

Hello! 

So i'm writing a function in the Leads module, in which i want to have the zip code fill in another field called SPA depending on what it is. I'm aware I can do this over a workflow, but i have approximately 50 different possibilities, and it's to big for a workflow to handle. 

Just as an example, i want the zip codes 93532, 93534, 93535, and 93543 to all trigger the other field to be SPA 1 - Antelope Valley.

Thank you!