The semantics of arithmatic operations is the same as integer operations in C (so 5/3 = 1). The extra amount of coding should be small. Once you have the code generator complete, you can make another copy of your code and instead of generating code for <program>, you let the calculator to evaluate <expression>. The new code needs to evaluate the expression in the left-associative order, same as you do in code generation.
Submit your calculator with code generator. The Makefile should
produce both executables.