In basic terms, a computer is an electronic device that processes data, converting it into information that is useful to people. Any computer—regardless of its type—is controlled by programmed instructions, which give the machine a purpose and tell it what to do.
Which are everywhere around you— arc digital computers (see Figure 1A.1). “Processing Data," digital computers are so called because they work “by the numbers." That is, they break all types of information into tiny units, and use numbers to represent those pieces of information. Digital computers also work in very strict sequences of steps, processing each unit of information individually, according to the highly organized instructions they must follow.
A lesser-known type of computer is the analog computer which works in a very different way from digital computers. The earliest computers were analog systems, and today's digital systems owe a great deal to their analog ancestors. Analog and digital computers differ in many respects, but the most important distinction is the way they represent data. Digital systems represent data as having one distinct value or another; with no other possibilities. Analog systems, however represent data as variable points along a continuous spectrum of values. This makes analog computers somewhat more flexible than digital ones, but not necessarily more precise or reliable. Early analog computers were mechanical devices, weighing several tons and using motors and gears to perform calculations (see Figure 1 A.2).
A more manageable type of analog computer is the old-fashioned slide rule (see Figure 1A3). Computers can be categorized in several ways. For example, some computers are designed for use by one person, some arc meant to be used by groups of people, and some are not used by people at all. They also can be categorized by their power, which means the speed at which they operate and the types of tasks they can handle. Within a single category, computers may be subcategorized by price, the types of hardware they contain, the kinds of software they can run, and so on.