2.5 VARIABLES

§  A variable is a data name that may be used to store a data value.

§  Unlike constants that remain unchanged, during the execution of a program, a variable may take different values at different times during execution.

§  A variable name can be chosen by the programmer in a meaningful way so as to reflect its function or nature in the program.

Rules for writing variables:

a)    Variable name can be composed of letters (both uppercase and lowercase letters), digits and underscore '_' only.

b)    The first letter of a variable should be either a letter or an underscore. But, it is discouraged to start variable name with an underscore though it is legal. It is because, variable name that starts with underscore can conflict with system names and compiler may complain.

c)    There is no rule for the length of a variable. However, the first 31 characters of a variable are discriminated by the compiler. So, the first 31 letters of two variables in a program should be different.

d)    Keyword cannot be used as a variable.

Declaration of Variables:

After deciding suitable variable names, we must declare them to the compiler. This declaration does two things:

i.                    It tells the compiler what the variable name is.

ii.                  It specifies what type of data the variable will hold and also the memory associated with it.

The syntax for declaring a variable is as follows:

                        datatype v1, v2, …., vn;

      where v1, v2, …., vn are the names of variables.

ü  Variables are separated by commas.

ü  A declaration statement must end with a semicolon.

e.g.

            int count;

            char grade;

            float ratio;

Assigning values to Variables:

1)      Using Assignment Operator ( = )

Values can be assigned to variables using the assignment operator = as follows:

            variable_name = constant_value;

e.g.

            count = 0;

            grade = ‘A’;

            ratio = 6.234;

The assignment statement implies that the value of the variable on the left of the ‘equal sign’ is set equal to the value of the quantity on the right.

year = year + 1;

means that the ‘new value’ of year is equal to ‘old value’ of year plus 1.

It is also possible to assign a value to a variable at the time the variable is declared.

            datatype variable_name = constant_value;

e.g.

            int sum = 0;

NOTE: The process of giving initial values to variables is called initialization.

2)      Reading data from Keyboard

Another way of giving values to variables is to input data through keyboard using the scanf() function.

The general format of scanf() is as follows:

            scanf(“Format String”, &v1, &v2,…,&vn);

the format string contains the format of data being received. The ampersand (&) before each variable name is an operator that specifies the variable name’s address.

e.g.

            int number;     

scanf(“%d”, &number);

NOTE: When the scanf() statement is encountered by the compiler, the execution stops and waits for the value of the variable to be typed in.

Thus, the use of scanf() function provides an interactive feature and makes the program “user friendly”.