When I got into professional programming compilers were a round trip to the machine room away. So one thing we did to save time was to 'desk check' our code. This usually meant a simple simulation using a piece of paper to hold the variables, crossing them out when they were overwritten. At some point I could do it for limited pieces of code by just simulating the code in my head.

Simulating code in your head goes a long way towards writing solid code, you'll be able to hold a model of the data and your code together and see whether it will work or not and fix bugs before your first compile.