Chapter 3. Data and Expressions
Really useful programs take in data, perform actions on it and
output it somewhere. In C, you use named pieces of memory called
variables to store data. C programs can change
the data stored in a variable at any time, hence the name. Every
variable has an identifier which you can use to refer to it's data
when you want to use or change it's value. An
expression is anything that can be evaluated
i.e. 1 + 1 is an expression of the value
2. In this expression, the plus sign is a
binary operator; it operates on two values to
create a single value.
The rules for naming a variable are the same as for naming a
function, you can use letters, numbers, and the underscore character
and the first character must not be a number. Also like functions,
variables must be declared before they can be used. The identifier
you give to a variable should say what the the variable will be used
for, this makes you code much easier to read. You can define your own
variables or you can use one of the types
already defined for you. Before we get bogged down in terminology
let's look at a quick code example to show how simple it all is. In
this example we will use two variables of the pre-defined type
int.
Example 3-1. bicycles.c
#include <stdio.h>
int
main()
{
int number_of_bicycles;
int number_of_wheels;
number_of_bicycles = 6;
number_of_wheels = number_of_bicycles * 2;
printf("I have %d bicycles\n", number_of_bicycles);
printf("So I have %d wheels\n", number_of_wheels);
return 0;
}
3.1. Bicycle Dissection
There are a few new things to look at here, we'll break the
program into chunks to explain them.
These two lines each declare a variable. int
is one of the built-in data types of the C language. Variables of
type int can store positive or negative whole
numbers.
This line stores the value 6 in the variable
number_of_bicycles. The equals sign is known as
"the assignment operator", it assigns the value on the
right hand side of it to the variable on the left hand side.
Again, this line uses the assignment operator but it also uses
the multiplication operator. The asterisk is another binary operator,
it multiplies two values to create a single value. In this case it
creates the value 12 which is then stored in
number_of_wheels.
Here we see printf() again but it's being
used unlike we have seen before. Here it is taking two
arguments which are separated by a comma. The
first argument to printf() is known as the
format string. When a %d is
encountered in the format string printf() knows to
expect an extra argument. The %d is replaced by
the value of this extra argument. One addition argument is expected
for each %d encountered.
With this new knowledge it should be no surprise that when we
compile and run this piece of code we get the following:
I have 6 bicycles
So I have 12 wheels
As always, don't worry if you are unsure about certain parts. We'll
do plenty more examples.
