Programming is a skill requiring knowledge of both the basic constructs of computer languages and the techniques employing these constructs. How these are used in any application is often determined by experience of existing programs. This book provides such experience by describing the techniques and implementing them in one useful program: an advanced drawing package. The book also shows the development of a large program. The program is implemented using TopSpeed Modula-2 running on IBM PCs and compatibles. Modula-2 is ideally suited for the task. A knowledge of the language is assumed. The program, including its source, is available on disk.

Modula-2 is a simple yet powerful programming language that is suitable for a wide variety of applications. It is based on Pascal, a successful programming language that was introduced in 1970 by Niklaus Wirth. During the 1970's Pascal became the most widely taught programming language and it gained acceptance in science and industry. In 1980 Dr. Wirth released the Modula-2 program ming language. Modula-2 is an evolution of Pascal. It improves on the successes of Pascal while adding the MODULE - a tool for ex pressing the relations between the major parts of programs. In ad dition Modula-2 contains low-level features for systems program ming and coroutines for concurrent programming. Programming languages are important because they are used to express ideas. Some programming languages are so limited that certain ideas can't be easily expressed. For example languages that lac k floating point arithmetic are inappropriate for scientific com putations. Languages such as Basic and Fortran that lack recur sion are unsuitable for text processing or systems programming. Sometimes a programming language is useable for a certain appli cation but it is far from ideal. A good example is the difficulty of writing large programs in pure Pascal. Pascal is a poor language for large jobs because it lacks facilities for partitioning a program viii Preface 6

This text is an introduction to programming in general, and a manual for programming with the language Modula-2 in particular. It is oriented primarily towards people who have already acquired some basic knowledge of programming and would like to deepen their understanding in a more structured way. Neveltheless, an introductory chapter is included for the benefit of the beginner, displaying in a concise form some of the fundamental concepts of computers and their programming. The text is therefore also suitable as a self-contained tutorial. The notation used is Modula-2, which lends itself well for a structured approach and leads the student to a working style that has generally become known under the title of structured programming. As a manual for programming in Modula-2, the text covers practically all facilities of that language. Part 1 covers the basic notions of the variable, expression, assignment, conditional and repetitive statement, and array data structure. Together with Palt 2 which introduces the important concept of the procedure or subroutine, it contains essentially the material commonly discussed in introductory programming courses. Part 3 concerns data types and structures and constitutes the essence of an advanced course on programming. Palt 4 introduces the notion of the module, a concept that is fundamental to the design of larger programmed systems and to programming as team work. The most commonly used utility programs for input and output are presented as examples of modules.

This book is intended for the novice as well as for the experienced programmer who wants to learn Modula-2. We do not limit ourselves to just a description of Modula-2. Instead, we seek to familiarize the reader with the concept of algorithms and to show him/her how to implement algorithms in Modula-2. The programming language Modula-2 was developed by Niklaus Wirth (also the father of world-famous Pascal) and made public in 1978. Compared to other programming languages such as Ada, COBOL or PL/!, Modula-2 is a compact language, which makes it easy to learn. Nevertheless, Modula-2 contains all important language elements necessary for formulating complicated algorithms and for implementing the modern concepts of software engineering. Modula-2 is distinguished by a systematic structure that makes it possible to write easily readable programs. The language supports many of the principles of modern software engineering. All this makes Modula-2 a useful instrument for an introduction to the basics of programming. This textbook strives to establish a solid foundation in the techniques of programming with up-to-date methods of program development. Use of the programming language Modula-2 is reinforced with numerous hands-on exercises. This book does not presuppose any knowledge of programming, but it does require a certain ability in the realm of abstract thinking, some pleasure in problem solving, and a desire to come to terms with complex interrelationships.

MODULA-2 is a new programming language which was created by Niklaus Wirth of the Swiss Federal Institute of Technology (ETH) in Zurich. The lan guage is derived from PASCAL: it includes all aspects of PASCAL and some times improves on them. Moreover, MODULA-2 includes the important "mod ule" concept, as well as multiprogramming capabilities and a way of implemen ting low-level software in an elegant manner. In summary, MODULA-2 may be used equally well as a general-purpose programming language and as a system implementation language. MODULA-2 provides the programmer with a good way of writing high quality software. In particular, modules are powerful tools for achieving modularity, reliability, readability, extensibility, reusability and ma chine-independence. This book presents the complete MODULA-21anguage from the beginning. Each topic is presented by means of numerous examples and each concept is justified. The syntax of the language is explained using syntactic diagrams. This book is not a reference manual for MODULA-2, but a textbook from which the student can learn the language progressively. The most important con cepts (i.e. procedures, modules and data structures) are explained in great detail and methodological aspects are also emphasized. Beginning in the first chapter, the student may execute his/her own pro grams. Program examples in this book have been executed on several machines (APPLE II, IBM PC and VAX 11/780) and they may be taken as a basis for stu dents.