Learn to Program with Small Basic

Subroutines are basic building blocks for creating large programs (see
Figure 10-1). In this chapter, you’ll delve into the wild world of subroutines,
learn how to move data in and out of them, and use them to build large
programs and fun games!
Figure 10-1: Subroutines are the
building blocks of larger programs
Why Use Subroutines?
Let’s say you run a construction company. Your job is to coordinate the work
among your contractors and build houses. As a manager, you don’t have to
know all the nitty-gritty details of building a home: the plumber handles the
plumbing, the roofer shingles the roof, and the electrician runs all the wires.
Each contractor knows their job and is always ready to work when they
receive your call.
That’s very similar to how subroutines
work! Each subroutine has its own name, like
how the plumber’s name is Mario. Each subroutine
does something different, just like how
the plumber and the roofer have different jobs,
but all are needed to build the house. As the
programmer, you’re the manager, and your job
is to solve problems as you build your program.
You call your contractors (that is, your subroutines)
and let them know when you need them
to work (see Figure 10-2). You start writing the
program by typing statements in the editor. When you need to perform a
job that a subroutine handles, you just call that subroutine and wait. When
the subroutine completes its task, you move on to the next step in your
program.
There’s nothing new about this call-and-wait strategy; you’ve been
doing it since Chapter 1. When you call an object’s method, you’re actually
giving the work to that object in the Small Basic library. Subroutines are
like methods, but you have to write all the statements in the subroutines.
Subroutines help you organize your thinking process and make it easier
to fix errors.
Boss
Hello, Bob.
Please work
on the roof.
Bob
Sure, boss.
I’ll let you know
when I’m done.
Figure 10-2: The boss (main
program) calling the Bob
subroutine
Writing Subroutines
Let’s use a fun example to learn how to write a subroutine: in his travels,
Gulliver had dinner with the king and queen of Lilliput (the land of tiny
people). During dinner, the king explained that he was 8.5 glum-gluffs tall.
130 Chapter 10