INTRODUCTION TO BASIC
Fundamental Concepts and Language Structure
BASIC is different from other programming languages in its concern for the inexperienced user. Although it is a general-purpose programming language, it is designed primarily to be easy to learn, easy to use, and easy to remember. BASIC is oriented toward, but not restricted to, interactive use. Its statement structure is kept simple, and special rules are kept to a minimum. A BASIC program is meant to be simple so that even a novice is able to deter-mine what the program is expected to do on the basis of examination. Only a little knowledge of BASIC is required to solve simple problems.
Simple BASIC programs can be used much like a small calculator. You might want to ask the computer to multiply 4 times 25 and print the results. The following example shows you how to accomplish this:
You keyed three lines into the computer. In the first line, line number 10, you told the computer to calculate 4 times 25 and print the answer (* in BASIC means multiply). In the second line, line number 20, you told the computer there were no more instructions in the program. The last command you gave the computer wasRUN, a system command which tells the computer to execute the instructions and give the results. The computer multiplied 4 times 25 and gave the answer, which is 100.
As simple as it may seem, this is a computer program. The same function which took only one statement in BASIC, would take several lines of coding in other high-level programming languages such as COBOL or FORTRAN.
The instructions which are preceded by line numbers are called statements. A complete set of statements to solve a problem is called a program. The very last statement in each program must be theEND statement.
Program statements in the BASIC language can be constructed in free form. The various parts must all appear, however, and be given in a definite order, as shown below:
STATEMENT NUMBER.—(Frequently referred to as line number). This number has two vital functions: (1) to identify the statement itself (statement label); and (2) to indicate to the BASIC compiler (interpreter) where you want this statement placed in the program sequence. The statement number must be an integer (a whole number—no decimal parts or fractions). For the pur-pose of this text, the range for statement numbers is from 1 to 99999. However, the number of digits may vary depending on the computer you are using. Table 2-1 shows examples of statements with valid and invalid statement numbers.
Table 2-1.—BASIC Statement Numbers
BASIC LANGUAGE KEYWORD.—Keywords are used to tell the computer what function is to be performed by this statement. For example, LET, PRINT, INPUT, and END as shown in Table 2-1. These and other keywords will be introduced and discussed as appropriate throughout the remaining chapters. The spelling of each keyword must be exact, or the compiler (interpreter) will tell you this is an invalid statement.
DESCRIPTIVE INFORMATION.—This information completes the description of the function to be performed and varies with the keyword used. See the first three examples in Table 2-1. There area few instances where no additional information is required or allowed. See the last example in Table 2-1. In order to write these statements, you must first know the syntax; that is, the characters and symbols used to construct the statements, as well as the rules, conventions, and special features of the language. This includes the character set and the methods used to represent numbers and predefined functions.