Computer Programming Languages in Practice. Made Simple Computerbooks

254 Pages·1985·9.723 MB·

by  C. A. Hofeditz (Auth.)

IN THE SAME SERIES
Basic
Computer Programming Languages in Practice
Hardware Design
Systems Analysis
Word Processing for the Professions
Computer Programming
Languages in Practice
Made Simple Computerbooks
C. A. Hofeditz
Made Simple Books
HEINEMANN : London
Copyright © 1985 William Heinemann Ltd
All rights reserved, including the right
of reproduction in whole or in part
in any form whatsoever
Phototypeset by Wilmaset, Birkenhead
Printed and bound in Great Britain
by Richard Clay (The Chaucer Press), Ltd, Bungay, Suffolk
for the publishers, William Heinemann Ltd,
10 Upper Grosvenor Street, London W1X 9PA
This book is sold subject to the
condition that it shall not, by
way of trade or otherwise, be lent,
re-sold, hired out, or otherwise
circulated without the publisher's
prior consent in any form of binding
or cover other than that in which it is
published and without a similar condition
including this condition being imposed
on the subsequent purchaser
British Library Cataloguing in Publication Data
Hofeditz, Calvin A.
Computer programming languages in practice.—(Made simple
computer books)—(Made simple books)
1. Programming languages (Electronic computers)
2. Electronic digital computers—Programming
I. Title II. Series III. Series
001.64'24 QA76.7
ISBN 0 434 98400 0
Introduction
WHY LEARN TO PROGRAM?
Despite the simplification of computer systems in the past few
years, they are still complex tools. And there is a great difference
between operating a machine and programming one. A user often
has a tutorial display shown on the screen to guide him through a
task. Even if such instructions are not provided, there is always an
operator's manual to rely on for assistance.
On the other hand, the programmer has to create the displays on
the screen, determine how the computer performs, and write the
operating instructions. Quite a difference, indeed. Somewhat like
the difference between using a lawn mower and building one.
It's not difficult to write simple programs in BASIC. Within a
week and using no more than ten of the instructions in either of
these computer languages, the beginner can have his computer
performing many calculations and other tasks.
Unfortunately, these tasks alone do not justify the cost of a
computer and its software for they could be done with a calculator
and basic instructions. Thus, the beginner is not likely to be
satisfied with having developed only a rudimentary programming
skill.
There are, of course, many reasons why people would want to
develop the ability to use computers and the programming
languages available with them. All are related to self-interest.
The Competition Will Use the New Tools
The computer is a new tool that is rapidly becoming available to
millions of people who have never used sophisticated equipment
before. With respect to small businesses, computers properly
xii Computer Programming Languages in Practice
applied can greatly reduce operating costs. Small machines can be
purchased or leased, and time on larger machines can be rented.
These new tools and methods are available, and are being used by
a great many small businesses now. Use of computers will spread
even more quickly as prices continue to come down. The small
businessman has no choice. He must learn to use these new tools
to his advantage because the competition will.
To Organize Jobs for the Computer
But why does the computer user need to know anything about
programming and programming languages? To organize jobs for
the computer to process is one answer. There is much more
involved in processing a stack of bills to be paid than sitting in
front of a display screen and typing the information at the
keyboard. Actually, the data entry at the machine is the last step in
the organizing process. Before the power switch is turned on
for the first time, the user must have determined which records
are to be maintained and in what form, how the bill-paying
process is to affect other records, and what the outputs are to
be. The organizing of inputs, processing, and outputs for each
task will always be required. The more the computer user
knows about programming, the better he will be able to organize
his jobs.
To Understand the Products Offered
Another good reason is to understand what you're buying.
Differences in programs and programming languages have a
dramatic effect on the performance of a computer. The sales
sheets available to the prospective computer user aren't much help
unless he is able to understand the material listed.
Many programs are often provided only as extra-cost options.
Even the lowest-priced retailer is offering programming language
packages at £50 to £100 each. A poor choice of machine and
programs can thus be very expensive to the owner of a small
computer; not only in terms of actual cost but also in disappointing
performance. It pays off, therefore, to understand the products
being offered before buying them.
Introduction xiii
Do-It-Yourself Projects
The fourth reason for learning to program may be the most
important. It is the need to do it yourself. Although there are
hundreds of programs available for the most popular computers, it
may be difficult to find one that fits your task perfectly. The
alternatives in this case are to change your methods to match the
capacity of the program, to hire a programmer to prepare a
customized program, or finally to do it yourself.
As a Hobby and for Entertainment
We haven't said much about personal computers so far, having
concentrated on business use. But let's assume that you are
interested in a personal computer.
It's hard to define exactly what a personal computer is. We can
say, however, that it is a small desktop unit, and if it has
supporting equipment it is likely to be limited to a small printer
and a disk drive. What does it do? Generally anything that a big
machine does, only in smaller volumes and more slowly.
As of this writing, personal computers are offered for sale at
prices as low as £25. These are incomplete machines, however.
They rely on using a TV screen for display and a cassette tape
recorder for storage, but they are unable to communicate with
other machines without optional adapters. A complete personal
computer with diskette storage and a display screen still costs £500
and up. At these prices, it is an expensive 'toy,' so we have to
assume that the buyer uses it for tasks other than entertainment.
A wide variety of programs are available for the most popular
personal computers. Most are optional, and they're not cheap.
Therefore, one good way to get the most use out of a personal
computer is to program it yourself. It should be an interesting
hobby for those liking mathematics and electronics.
Most readers interested in programming their own machines are
likely to prefer BASIC, particularly a modern version. This is a
good selection because it is widely offered with personal compu-
ters, and the new versions have been developed fully to handle
graphics and provide other features the novice programmer should
find attractive.
xiv Computer Programming Languages in Practice
And as a Profession
Skilled programmers are in great demand in the job market, and
this should remain so for the foreseeable future. Many people in
the computer industry believe that equipment development has
outpaced the programs and considerable time will elapse before
the programmers catch up.
In general, an applicant for a programmer's job should have a
degree in a field of learning that provides considerable training
in mathematics. A good deal of the work involved in pro-
gramming is similar to that necessary to lay out and solve a
complex equation. In fact, the word 'algorithm,' which is seen in
mathematics, is also frequently used to mean 'the programming
solution to a problem.'
WHY ARE THERE SO MANY LANGUAGES?
The answer to this question is fairly simple. Computers have been
used in business for over thirty years, while some models were
used in engineering and scientific applications even before that. A
great many models were developed during this time. As one would
expect, programming groups were also busy providing languages
for the various models, always trying to make the languages more
powerful and easy to use. Logically then, many languages were
produced.
Among the most commonly used high-level languages are
BASIC, COBOL, FORTRAN, Pascal, and RPG. We cover all
but Pascal in this book but devote less attention to RPG, a
language with a limited purpose and whose instructions are not in
the same class as the others mentioned, and to FORTRAN, which
is primarily intended for engineering and scientific applications.
Any serious reader will have no difficulty in finding descriptions of
at least five more high-level languages on a computer science
bookshelf in a large bookstore.
WHAT YOU'LL FIND IN EACH CHAPTER
When one reaches the computer science section in a bookstore,
one finds that most of the reference books dealing with program-
Introduction xv
ming languages are fairly large. A typical reference book on
BASIC alone has as many pages as this one. How then can we
hope to discuss such complex subjects within these covers? We'll
answer that question in the following paragraphs.
The largest portion of this book is devoted to two very popular
languages—BASIC and COBOL. They have a good deal in
common. Records, files, arithmetic operators, coding forms, and
flowcharts, for example, are more alike than different from one
language to another. Therefore, one general discussion of these
subjects common to all languages is given in Chapter 2.
Next, we have concentrated on the 'core' of each language,
eliminating discussions of rarely used instructions and those that
apply to only one brand of computer. Knowledge of uncommon
instructions is of very little use to anyone other than a programmer
of a specific model.
We've also limited the number of sample programs to one per
language. A great many programming books have an excess of
sample programs and only the most dedicated reader ever uses
them.
These are some of the ways in which we've managed to 'boil
down' a great deal of material to a usable size. Now let's examine
how the material is organized and what you can expect to find in
each chapter.
There are five chapters, which are supplemented by a Glossary
of Terms. Each chapter is followed by a self-test. The five chapters
are:
Chapter 1. What Programming Is
2. Components of a Programming Language
3. BASIC, Beginner's All-purpose Symbolic /nstruc-
tion Code
4. COBOL, Common Business Oriented Language
5. Other Languages, Old and New
Chapter 1, What Programming Is, provides the fundamentals:
What programs are, how they are planned and organized, what
elements of the computer the programmer controls, flowcharting,
and how computer data is organized.
Chapter 2, Components of a Programming Language, covers
material common to all languages. It shows that each language is
really a system, consisting of a set of words the programmer may
use and a group of complex programs that interpret and apply
those words. The entry program, the compiler, the run-time
xvi Computer Programming Languages in Practice
system are described, and their relationship to the programmer is
shown.
This chapter also discusses syntax diagrams and coding forms,
which establish the sentences that may be used and the form in
which they are provided. Next covered are topics common to all
languages, including character sets, operators, variables, con-
stants, expressions, arrays, functions, and procedures.
Chapters 3 and 4, each deal with a specific programming
language, covering BASIC and COBOL, respectively. Each
chapter provides the following information:
• A brief history of the language's development and use.
• A description of how the programming system is organized. Its
major components, divisions of instructions, and a description
of its instruction set (instruction-by-instruction).
• How a program is written, including a sample program.
• A self-test, including exercises in which programming state-
ments must be written.
Chapter 5, Other Languages, Old and New, discusses those
languages which the reader is less likely to use but should know
about. Included are descriptions of FORTRAN and RPG II. This
chapter does not provide programming examples but does show
the instruction set and the appearance of a program. Also included
in Chapter 5 are descriptions of recent additions to BASIC.
1
What Programming Is
THE PURPOSE OF A PROGRAM
Many people think of a computer as an intelligent machine; it is
not. All the 'intelligence' is entered into the machine by man,
either as part of the machine design or in the form of a program.
The question to be asked then is: What is a program? And the
answer is a very simple one: It is a set of instructions that make the
computer perform a specific task. The instructions chosen must all
be within the capacity of the computer to perform them, and they
must be arranged in the proper order.
Computer programmers originally had to choose from a set of
instructions each of which executed a very small step. A great
many instructions were necessary to perform even small jobs, and
arranging the steps in order was a complex task, requiring a great
deal of time and skill.
Fortunately, high-level programming languages have been
available for many years. A high-level language is one in which
each instruction performs a recognizable operation rather than
one small step. For example, a PRINT instruction in the high-level
language called BASIC causes information to be shown on the
screen. Actually, the computer must execute many small steps to
produce the display, but a programmer using the high-level
language does not have to be concerned about them.
WHAT IS SOFTWARE?
For many years the term 'software' has meant the programs that a
computer executes. 'Hardware' has become the commonly used
term for the equipment—it being 'hard' in the sense that it is not