Section 1.4
Fundamental Building Blocks of Programs
THERE ARE TWO BASIC ASPECTS of programming:
data and instructions. To work with data, you need to understand
variables and
types; to work with instructions,
you need to understand control structures
and subroutines. You'll spend a
large part of the course becoming familiar with these concepts.
A variable is just a memory location
(or several locations treated as a unit) that has been given a
name so that it can be easily referred to and used in a program.
The programmer only has to worry about the name; it is the compiler's
responsibility to keep track of the memory location. The programmer
does need to keep in mind that the name refers to a kind of "box"
in memory that can hold data, even if the programmer doesn't have to
know where in memory that box is located.
In Java and most other languages, a variable has a
type that indicates what sort of
data it can hold. One type of variable might hold integers
-- whole numbers such as 3, -7, and 0 -- while another holds
floating point numbers -- numbers with decimal points such
as 3.14, -2.7, or 17.0. (Yes, the computer does make a distinction
between the integer 17 and the floating-point number 17.0;
they actually look quite different inside the computer.) There
could also be types for individual characters ('A', ';', etc.),
strings ("Hello", "A string can include many characters",
etc.), and less common types such as dates, colors, sounds, or
any other type of data that a program might need to store.
Programming languages always have commands for getting
data into and out of variables and for doing computations
with data. For example, the following "assignment statement,"
which might appear in a Java program, tells the computer to
take the number stored in the variable named "principal",
multiply that number by 0.07, and then store the result in the
variable named "interest":
interest = principal * 0.07;
There are also "input commands" for getting
data from the user or from files on the computer's disks
and "output commands" for sending data in the
other direction.
These basic commands -- for moving data from place to place
and for performing computations -- are the building blocks
for all programs. These building blocks are combined into
complex programs using control structures and subroutines.
A program is a sequence of instructions. In the ordinary "flow
of control," the computer executes the instructions in the
sequence in which they appear, one after the other. However, this
is obviously very limited: the computer would soon run out of
instructions to execute. Control structures
are special instructions that can change the flow of control. There
are two basic types of control structure: loops,
which allow a sequence of instructions to be repeated over and over,
and branches, which allow the computer
to decide between two or more different courses of action by
testing conditions that occur as the program is running.
For example, it might be that if the value of the variable
"principal" is greater than 10000, then
the "interest" should be computed by multiplying the
principal by 0.05; if not, then the interest should be computed
by multiplying the principal by 0.04. A program needs some way
of expressing this type of decision. In Java, it could be
expressed using the following "if statement":
if (principal > 10000)
interest = principal * 0.05;
else
interest = principal * 0.04;
(Don't worry about the details for now. Just remember that the
computer can test a condition and decide what to do next on the
basis of that test.)
Loops are used when the same task has to be performed
more than once. For example, if you want to print out a
mailing label for each name on a mailing list, you might say,
"Get the first name and address and print the label;
get the second name and address and print the label;
get the third name and address and print the label..."
But this quickly becomes ridiculous -- and might not work at
all if you don't know in advance how many names there are.
What you would like to say is something like "While
there are more names to process, get the next name and address,
and print the label." A loop can be used in a program
to express such repetition.
Large programs are so complex that it would be almost impossible
to write them if there were not some way to break them up into
manageable "chunks." Subroutines provide one way to
do this. A subroutine consists of
the instructions for performing some task, grouped together
as a unit and given a name. That name can then be used as a
substitute for the whole set of instructions. For example,
suppose that one of the tasks that your program needs to
perform is to draw a house on the screen. You can take the
necessary instructions, make them into a subroutine, and
give that subroutine some appropriate name -- say,
"drawHouse()". Then anyplace in your
program where you need to draw a house, you can do so with
the single command:
drawHouse();
This will have the same effect as repeating all the house-drawing
instructions in each place.
The advantage here is not just that you save typing. Organizing
your program into subroutines also helps you organize your thinking
and your program design effort. While writing the house-drawing
subroutine, you can concentrate on the problem of drawing a house
without worrying for the moment about the rest of the program.
And once the subroutine is written, you can forget about the
details of drawing houses -- that problem is solved, since you
have a subroutine to do it for you. A subroutine becomes
just like a built-in part of the language which you can use
without thinking about the details of what goes on "inside"
the subroutine.
Variables, types, loops, branches, and subroutines are the
basis of what might be called "traditional programming."
However, as programs become larger, additional structure is
needed to help deal with their complexity. One of the most
effective tools that has been found is object-oriented programming,
which is discussed in the next section.
