A variable is used to hold data within your program.
A variable represents a location in your computer's memory.
You can put data into this location and retrieve data out
of it. Every variable has two parts, a name and a data type.
Variable Names
Variable names must start with a letter or an underscore.
The remaining characters can be letters, numbers or the
underscore, no other characters can be used - those are
the rules guys!. A variable name must not
be a C keyword such as if, for, else, or while. Variable
names are case sensitive. So, Age, AGE, aGE and AgE could
be names for different variables, although this is not recommended
since it would probably cause confusion and errors in your
programs. Let's look at some variable declarations to better
understand these rules. Note that int, float and double
are built in C data types as explained latter in this lesson.
Which of the following are valid variable names?
int idnumber;
int transaction_number;
int __my_phone_number__;
float 4myfriend;
float its4me;
double VeRyStRaNgE;
float while;
float myCash;
int CaseNo;
int CASENO;
int caseno;
Remember that variables must start with an underscore
or a letter. The remaining characters must be letters, numbers
or the underscore. No other characters can be used.
Data Types
C provides built in data types for character, float and
integer data. You can even define your own data types in
C. In C you may assign a value to a variable when you are
declaring it.
Integer variables are used to store whole numbers (e.g.
1, 100, 500, 37 etc.). There are several keywords used to
declare integer variables, including int, short, long, unsigned
short, unsigned long. The difference deals with the number
of bytes used to store the variable in memory (how large
the number is), long vs. short, or whether negative and
positive numbers may be stored, signed vs. unsigned. These
differences will be explained in more advanced tutorials.
For now, use int to declare integer variables.
Example 1:
int count;
int number_of_students = 30;
float variables are used to store floating point
numbers. Floating point numbers may contain both a whole
and fractional part, for example, 52.7 or 3.33333333. There
are several keywords used to declare floating point numbers
in C including float, double and long double. The difference
here is the number of bytes used to store the variable in
memory (how many decimal places the number has). Double
allows larger values than float. Long double allows even
larger values. These differences will be explained in more
advanced tutorials. For now, use float to declare floating
point variables.
Example 2:
float owned = 0.0;
float owed = 1234567.89;
Character variables are used to store character values.
The use of characters and strings will be covered in a latter
tutorial. Character variables are declared with the keyword
char.
Example 3:
char firstInitial = 'J';
char secondInitial = 'K';
Note that when declaring a single character (e.g. A or
b or X or 3) you should enclose the character in a single
quotation mark ' ' (e.g. 'A', 'b' etc..). If you are assigning
more than one character you should use the full double quotation
marks " " (e.g. "string", "do"
etc.). This will be explained later in a further tutorial.
Remember:
int x;
x = 5; /*
This is fine. */
5 = x; /*
This is illegal. */
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