Chapter 2.3 Modula 2 An Introduction

Summary First example Terminal symbols Comments Data –Constants –Variables Actions –Assignment statement –Control statements

A first example MODULE FirstExample; (* This program computes the area of a circle *) FROM InOut IMPORT WriteString, WriteLn; FROM RealInOut IMPORT WriteReal, ReadReal; CONST pi = ; VAR radius, area : REAL; BEGIN WriteLn; WriteLn; WriteString("Hello, I compute the area of a circle for you"); WriteLn; WriteString("Give me the radius in meters "); ReadReal(radius); area := pi * radius * radius; WriteString("The area of this circle is "); WriteReal(area,0); WriteString(" square meters "); WriteLn; END FirstExample.

Terminal Symbols Upper- and Lower-case letters a b c d e... A B C D E... Digits Special signs ;,. | + - * / = # > < ( ) [ ] { } Composed signs (* *) := >= <= Reserved Words MODULE BEGIN END IMPORT FROM

Source BSource CSource DSource A Reloc. DReloc. AReloc. BReloc. C Compiler XCompiler YAssembler LINKER Object Code A+B+C+D Role of a Linker FROM InOut IMPORT WriteString, WriteLn; FROM RealInOut IMPORT WriteReal, ReadReal;

Comments (* This program computes the area of a circle *) (* Text not containing (* nor *) *) Comment

Summary First example Terminal symbols Comments Data –Constants –Variables Actions –Assignment statement –Control statements

Constants Data whose value can not change during program execution Literal Constants Only a value: Named Constants A name and a value: The value can be a constant expression "The area of this circle is " pi = twopi = pi * 2.0

Syntax of Constants = Constant ExpressionIdentifier Constant Declaration Whole Number Literal Literal Real Literal String Literal Pointer Literal

Whole Number Literal + - Decimal Literal Octal Literal Hexadecimal Literal

Decimal Literal DecimalDigit

Octal Literal OctalDigit B 10B 8B

Hexadecimal Literal HexadecimalDigit H ABCDE F DecimalDigit HexadecimalDigit 10FFFH 0FFFFFH 1000F FFFFFH

Real Literal E E E-4 3,1416

String Literal "The area of this circle is " 'The area of this circle is ' "This string contains a ' "'but no " ' " Text not containing " " ' Text not containing ' '

Why named constants ? Area := 6 * Length*Length... Tax := (Price * 6) DIV 100 CONST VAT = 6;... Area := 6 * Length*Length... Tax := (Price * VAT) DIV 100 Area := 21 * Length*Length... Tax := (Price * 21) DIV 100 CONST VAT = 21;... Area := 6 * Length*Length... Tax := (Price * VAT) DIV 100

Constants and Variables Constants –(Name) –Value The value of a constant belongs to a type. Variables –Name –Type Type = set of all values the variable can have The type determines the internal representation of data as well as the operations that can be performed on the data

Types in Modula 2 Simple Types: values can’t be decomposed –Ordinal Types: bijection with natural numbers –Reals: approx. representation for real values –Pointers: addresses in data memory Structured Types: Values have # components –Arrays: all components have same type –Records: components can have # types –Sets: small sets of ordinal values –Procedures: entire subprograms

Summary First example Terminal symbols Comments Data –Constants –Variables Actions –Assignment statement –Control statements

Assignments Assignment operator := –Fundamentally different from the = sign –The = sign denotes a timeless relation –The := operator describes a particular action: The right side expression is evaluated Its value is stored in the left side variable –Illustrative example : a := a + 1 area := pi * radius * radius;

Control Statements Influence the order of execution of instructions Selection –IF THEN ELSE –CASE Iteration –WHILE –REPEAT –FOR –LOOP Procedure Call WriteString("Hello, I comput WriteLn; WriteString("Give me the rad ReadReal(radius); area := pi * radius * radius WriteString("The area of thi WriteReal(area,0); WriteString(" square meters WriteLn;