1. Chapter 2.3 Modula 2 An Introduction

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

3. 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 = 3.1416; 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.

4. Terminal Symbols Upper- and Lower-case letters a b c d e... A B C D E... Digits 0 1 2 3 4 5 6 7 8 9 Special signs ;,. | + - * / = # > < ( ) [ ] { } Composed signs (* *) := >= <= Reserved Words MODULE BEGIN END IMPORT FROM

5. 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;

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

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

8. 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 3.1416 "The area of this circle is " pi = 3.1416 twopi = pi * 2.0

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

10. Whole Number Literal + - Decimal Literal Octal Literal Hexadecimal Literal

11. Decimal Literal DecimalDigit 01234 56789

12. Octal Literal OctalDigit 01234 567 B 10B 8B

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

14. Real Literal 3.1416 3.1416E0 0.31416E1 31416E-4 3,1416

15. 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 ' '

16. 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

17. 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

18. 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

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

20. 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;

21. 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;