1. Chapter 2.5 Modula 2 Control Instructions

2. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

3. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

4. IF B THEN S1 ELSE S2 END B S2S1 FALSETRUE B FALSE TRUE S2S1

5. IF B THEN S END B S1 FALSETRUE B FALSE TRUE S1

6. IF B1 THEN S1 ELSIF B2 THEN S2 ELSE S3 END S3S2 B2 FALSE TRUE B1 FALSE TRUE S3 S1 B1 FALSE TRUE S1 S2 FALSE TRUE B2

7. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

8. CASE e OF a:S1|b:S2|c:S3 END No e = a ? S1 Yes No e = b ? S2 Yes No e = c ? S3 Yes ???? e S1 S2 e = ae = be = c S3

9. CASE e OF a:S1|b:S2|c:S3 ELSE S4 END No e = a ? S1 Yes No e = b ? S2 Yes No e = c ? S3 Yes S4 e S1 S3 S2 e = ae = be = c S4 e # a e # b e # c

10. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

11. WHILE B DO S END B S TRUE FALSE S B

12. WHILE examples WHILE customers in shop DO serve customer END WHILE NOT end of tape DO play a song END

13. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

14. REPEAT S UNTIL B S B S B TRUE FALSE

15. REPEAT example REPEAT Remove bolt UNTIL No bolts left

16. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

17. S S LOOP S1 END

18. LOOP example LOOP Temperature := ReadSensor; IF Temperature > MaxTemp THEN StopReactor END

19. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

20. B S2 TRUE FALSE S2 B S1 LOOP S1; IF B THEN EXIT END; S2 END

21. LOOP with EXIT example Refinement of WHILE example NumberCustomers := CountCustomers; WHILE NumberCustomers > 0 DO ServeCustomer; NumberCustomers := CountCustomers END

22. LOOP with EXIT example LOOP NumberCustomers := CountCustomers; IF NumberCustomers = 0 THEN EXIT END; ServeCustomer END

23. Modula 2 Control Statements Selection statements –BOOLEAN Selector : IF statement –Ordinal Selector : CASE statement Iteration statements –Initial termination test : WHILE loop –Final termination test : REPEAT loop –Infinite loop : LOOP –Termination of LOOP : EXIT –Known number of iterations : FOR PROCEDURE call : chapter 7

24. FOR statement syntax (simple version)

25. FOR i := m TO n DO S END S m <= n S TRUE FALSE i := m inc(i) i = n TRUE FALSE exit loop when i = n i := m inc(i) TRUE

26. FOR example FOR NBolt := 1 TO 4 DO Remove bolt number NBolt END

27. FOR statement syntax (complete version)

28. FOR example FOR Count := 10 TO 1 BY -1 DO SpeakCardinal(Count,3) END; SpeakString("GO")

29. Computing the GCD (1) Specifications : –Given two cardinal numbers x and y –Compute G, the GCD of x and y Algorithm : GCD(x, y) = GCD(x, y-x)(x < y) GCD(x, y) = GCD(x-y, y)(x > y) WHILE x # y x > y x := x-yy := y-x TRUE

30. Computing the GCD (2) Top level design : Read value of x and y Compute g, the GCD of x and y Write the value of g MODULE GCD1;... VAR x,y,g : CARDINAL; BEGIN (* Read value of x and y *)... (* Compute g, the GCD of x and y *)... (* Write value of g *)... END GCD1.

31. Computing the GCD (3) MODULE GCD1; FROM InOut IMPORT WriteString, ReadCard; VAR x,y,g : CARDINAL; BEGIN (* Read value of x and y *) WriteString("Enter a cardinal number please "); ReadCard(x); WriteString("Enter a cardinal number please "); ReadCard(y); (* Compute g, the GCD of x and y *)... (* Write value of g *)... END GCD1.

32. Computing the GCD (4) MODULE GCD1; FROM InOut IMPORT WriteString, ReadCard; VAR x,y,g : CARDINAL; BEGIN (* Read value of x and y *)... (* Compute g, the GCD of x and y *) WHILE x # y DO IF x > y THEN x := x - y ELSE y := y - x END (* IF *) END; (* WHILE *) g := x; (* Write value of g *)... END GCD1.

33. Computing the GCD (5) MODULE GCD1; FROM InOut IMPORT WriteString, ReadCard, WriteCard, WriteLn; VAR x,y,g : CARDINAL; BEGIN (* Read value of x and y *)... (* Compute g, the GCD of x and y *)... (* Write value of g *) WriteString(" The GCD of these numbers is : "); WriteCard(g,10); WriteLn END GCD1.

34. Computing the GCD (6) MODULE GCD1; FROM InOut IMPORT WriteString, ReadCard, WriteCard, WriteLn, Read; VAR x,y,g : CARDINAL; ch : CHAR; BEGIN (* Read value of x and y *)... (* Compute g, the GCD of x and y *) WHILE x # y DO WriteString("x = "); WriteCard(x,4); WriteString("; y = "); WriteCard(y,4); WriteLn; Read(ch); IF x > y THEN x := x - y ELSE y := y - x END (* IF *) END; (* WHILE *) g := x; (* Write value of g *)... END GCD1.