The most important reason to use functions is to aid in the conceptual organization of a program
. Dividing a program into functions is, as we discussed in Chapter 1, “The Big Picture,” one of the major principles of structured programming. (However, object-oriented programming provides additional, more powerful ways to organize programs.)
Another reason to use functions (and the reason they were invented, long ago) is to reduce program size. Any sequence of instructions that appears in a program more than once is a candidate for being made into a function. The function’s code is stored in only one place in memory, even though the function is executed many times in the course of the program. Figure 5.1 shows how a function is invoked from different sections of a program.