Digital computers are very proficient at storing and recalling numbers; unfortunately, this process isn't without error. For example, you instruct your computer to store the number: 1.41421356. The computer does its best, storing the closest number it can represent: 1.41421354. In some cases this error is quite insignificant, while in other cases it is disastrous. As another illustration, a classic computational error results from the addition of two numbers with very different values, for example, 1 and 0.00000001. We would like the answer to be 1.00000001, but the computer replies with 1. An understanding of how computers store and manipulate numbers allows you to anticipate and correct these problems before your program spits out meaningless data.
These problems arise because a fixed number of bits are allocated to store each number, usually 8, 16, 32 or 64. For example, consider the case where eight bits are used to store the value of a variable. Since there are 28 = 256 possible bit patterns, the variable can only take on 256 different values. This is a fundamental limitation of the situation, and there is nothing we can do about it. The part we can control is what value we declare each bit pattern to represent. In the simplest cases, the 256 bit patterns might represent the integers from 0 to 255, 1 to 256, -127 to 128, etc. In a more unusual scheme, the 256 bit patterns might represent 256 exponentially related numbers: 1, 10, 100, 1000, …, 10254, 10255. Everyone accessing the data must understand what value each bit pattern represents. This is usually provided by an algorithm or formula for converting between the represented value and the corresponding bit pattern, and back again.
While many encoding schemes are possible, only two general formats have become common, fixed point (also called integer numbers) and floating point (also called real numbers). In this book's BASIC programs, fixed point variables are indicated by the % symbol as the last character in the name, such as: I%, N%, SUM%, etc. All other variables are floating point, for example: X, Y, MEAN, etc. When you evaluate the formats presented in the next few pages, try to understand them in terms of their range (the largest and smallest numbers they can represent) and their precision (the size of the gaps between numbers).