Data Variables and Constants

A computer, under the guidance of a program, can do many things. It can add numbers, sort names, command the obedience of a speaker or video screen, calculate cometary orbits, prepare a mailing list, dial phone numbers, draw stick figures, draw conclusions, or anything else your imagination can create.
 
To do these tasks, the program needs to work with data, the numbers and characters that bear the information you use. Some types of data are preset before a program is used and keep their values unchanged throughout the life of the program. These are constants. Other types of data may change or be assigned values as the program runs; these are variables.
 
Primary data types could be of three varieties—char, int, and float. Not only this, the primary data types themselves could be of several types. For example, a char could be an unsigned char or a signed char. Or an int could be a short int or a long int.