Pascal for Students (including Turbo Pascal)

Ray Kemp, Brian H. Hahn, Brian Hahn

• Publisher: Butterworth-Heinemann
• Year: 1995
• Print ISBN: 9780340645888
• eBook ISBN: 9780080928708
• Edition: 3
• Topic: Computer Science
• Subtopic: Programming Languages
• Index: Computer Science

The third edition of this best-selling text has been revised to present a more problem oriented approach to learning Pascal, without substantially changing the original popular style of previous editions. With additional material on Turbo Pascal extensions to the standard Pascal, including binary files and graphics, it continues to provide an introduction which is as suitable for the programming novice as for those familiar with other computer languages.

Section 1

Pre-defined simple types and control structures

1

First steps

Introduction

The very simplest kinds of computer program are introduced in this chapter. Even so there is much to learn—the basic structure of Pascal programs, how to feed in data and layout results, but most of all how to get the computer to do what you want it to and find out what’s going wrong when it doesn’t.

1.1 Some simple programs

Computers, like the Delphic Oracle, are used to answer questions, but, as with the Oracle, you have to be careful how you phrase your request or you may get a misleading answer. In fact the computer may give you no answer at all if it doesn’t like the question.

The first question we shall ask the computer is: ‘given that a stone takes 3 seconds to reach the bottom of a well how deep is the well?’ It would be nice if we could phrase the question exactly in this way and expect the computer to get the right answer. It will be a few years before that becomes possible. In the meantime the best way to get the computer to provide an answer is to write a computer program to do the job. Program 1.1 does just this. Before we examine the program in detail have a preliminary look at it to see if you can work out what’s happening.

Program 1.1

Ignoring much of the punctuation for now let’s look at the program line by line. (Note that the line numbers that appear on the left-hand side are for reference only and are not part of the program.)

On the first line the program is given a title. The user chooses an appropriate name but, as with all Pascal names, there must be no break in the middle—Well depth would not work. The name output in parentheses, signifies that the answer that the computer gives should be transmitted to the device called output. This might be a printer*, vdu* or teletype*. Lines 3 to 10 contain a description of the purpose of the program. This description enclosed between braces ‘{’ and ‘}’ is known as a comment. In all programs, whatever the language, it is important to have a piece of text at or near the beginning describing the purpose of the program and, possibly, giving other information such as the name of the writer and the date of completion. In Pascal, any information enclosed between ‘{’ and ‘}’ is disregarded by the compiler* (even if it extends over several lines) and so explanatory text may be inserted at appropriate points^#. In this example the rectangle of asterisks has no significance for the computer but helps to highlight the enclosed introductory description for the reader.

A blank line follows, and then, on lines 13 and 14, the constants to be used in the program are given values. In order to work out the depth of the well the computer needs to know how long a stone takes to reach the bottom and also how fast it is moving. The time taken was 3 seconds and a name timetaken has been invented to represent this. Note, again, that it would not be acceptable to have a name in two parts (time taken). There must be no spaces in the middle. The second constant 9.8 is given the name gravity. The stone actually falls because of the gravitational pull of the earth and if we are to determine the depth of the well this will be needed too.

After the next blank line, the word var (short for ‘variable’) indicates that the name following it denotes a variable in the program. The word that follows is depth and represents the depth of the well. At this point you may be wondering why depth is called a variable giving the impression that it is something that alters. Surely the depth remains unchanged in the same way that the time taken and the gravitational constant do? The distinction occurs because we know the values of the constants before we start but the value of the depth is to be calculated in the program itself. In Pascal any names used to denote values that are computed during the running of a program are designated as variables.

The fact that the depth will be a number which will probably contain a fractional part is indicated by placing the word real after the variable name.

None of the code up to line 17 actually commands the computer to carry out the task we are interested in. However, the directions on lines 12 to 17 are important to the organisation of information within the computer and to the execution of the code contained between the words begin and end in the ‘statement part’.

The code on line 21 computes the depth from ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface to the first edition
• Preface to the second edition
• Preface to the third edition
• Introduction
• Section 1: Pre-defined simple types and control structures
• Section 2: Structured and enumerated data types
• Section 3: Turbo Pascal
• Section 4: Mathematical applications of Turbo Pascal
• Appendix A: Syntax diagrams for Pascal
• Appendix B: Reserved words and required identifiers
• Appendix C: Required functions and procedures
• Appendix D: Turbo Pascal functions and procedures
• Appendix E: Common codes
• Appendix F: Portability
• Glossary
• Index