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1
Preliminaries
A computer is a machine that can perform basic arithmetic operations at a very high speed. It can take logical decisions and select alternative paths depending upon the program logic. It can communicate with the external world via its input/output devices. To get a job done, the computer needs to be instructed through a computer program. A computer program is a set of instructions through which one instructs a computer to perform a specific job. The computerās processor understands only one language, called the machine language. The machine language is machine dependent and is difficult for humans to learn. To circumvent this difficulty, several artificial languages (sometimes called high-level languages) have been developed. These artificial languages are very easy to learn and are practically machine independent. However, this requires translation to the machine language of the particular machine. The translation is done by the computer itself through a system program called a compiler. The compiler, while translating, checks the grammar of the language; if the source program is free from grammatical errors, it generates the machine language version of the source program, called the object program for the machine, which is subsequently linked (using a system program called the linker) to various libraries of the system. The resultant code, called the executable code, is executed by the machine. As different machines use different machine languages, the compilers are naturally machine dependent. Therefore, that a particular machine can execute a program written in a high-level language implies that the compiler for that high-level language is available to the computer system.
Fortranāone such programming languageāis the abbreviation of Formula translation. It is widely used in solving scientific and engineering problems that require a lot of numerical computation. In this book, Fortran stands for Fortran 2018, the current version of Fortran.
It must be mentioned at this point that no computer can directly execute any program written in Fortran or any other, so-called high-level, language like Fortran. The compiler for the corresponding language must be available to the computer so that the translated version of the program written in a high-level language may be executed by the computer. As this translationāFortran to machine language of a particular machineāis transparent to the programmer, one may assume that the computer is executing the Fortran program.
The compiler generates an object program only when the source is free from grammatical errors. In case of any grammatical error being flagged by the compiler, the programmer has to go back to the source, make the necessary correction(s) to the source and recompile the source to get the object program. The object program will not be generated until all grammatical errors are removed from the source program.
A program, free from grammatical errors, may not give a correct result. The program must be free from logical errors. A logical error is an error in the program logic at the source level. For example, a particular program requires addition of two numbers, but the programmer, by mistake, has performed multiplication instead of addition. A runtime error may occur during the execution of the program. Suppose a program has to divide two numbers. The division process is valid so long as the second number (denominator) is not zero. Division by zero is not a valid arithmetic operation. This error will show up during the execution of the program should the denominator become zero. The program behaves normally so long as the denominator remains nonzero.
Therefore, to obtain a correct result from a program, the following three conditions must be satisfied:
ā¢ The program must be free from grammatical errors.
ā¢ The program must be free from logical errors.
ā¢ There should not be any runtime error.
In this book, we frequently use the term processor. According to the Fortran report, a processor is a combined object, consisting of software (compiler, operating system, etc.) and hardware, that converts a Fortran program into its machine language equivalent and executes the same.
1.1 Character Set
The programming language and its syntax are described by a set of characters. The character set that is available to a Fortran programmer consists of (a) all letters of the English alphabet, both uppercase (AāZ) and lowercase (aāz); (b) underscore character (_); (c) all digits (0ā9); (d) several special characters, such as brackets, colon and full stop; and (e) several unprintable characters, such as tab, linefeed and newline characters.
Some characters may appear only within comments, character constants, input/output records and edit descriptors. The English letters, numerals and the underscore character are collectively called alphanumeric characters.
Normally, Fortran is case insensitive; that is, it does not distinguish between the uppercase and lowercase letters. There are, however, exceptions (character strings and input/output). In addition, specifiers like file names in open and inquire statements (Chapter 9) may make it necessary to distinguish between lowercase and uppercase letters. This is processor dependent and will be discussed at appropriate places in the text. Table 1.1 shows the list of special characters.
In this book, we consider only ASCII set of characters. ASCII, abbreviation of American Standard Code for Information Interchange, is an industry standard for electronic communication. The processor may also support other types of character sets.
TABLE...
