Storing and Retrieving Values
Variables are given values by an input statement (value comes from outside the program) or an assignment statement (value is computed in the program).
Values are retrieved by writing the variable name in an expression, such as on the right side of an assignment or in an output statement.
During the execution of a program a variable may assume many different values and be used many times.
A variable must be given a value (by input or assignment) before being used in an expression.