1. CMP 131 Introduction to Computer Programming Violetta Cavalli-Sforza Week 11

2. THIS WEEK Today –Finish loops –Quiz results & going over.. Tomorrow (Tuesday): LAB 01 –Work on homework #5 Wednesday: Lecture –Start on File Processing Thursday: LAB 01 – More work on homework #5

3. NEXT WEEK Quiz #4 –Wednesday May 30th No make-up session but possibly start classes 15 minutes early and end 15 minutes late (since you have ½ hour breaks)

4. Nested Selection and Repetition Selection (IF-THEN-ELSE, CASE) and repetition (FOR-DO, WHILE-DO, REPEAT-UNTIL) are commonly occurring programming structures They are frequently nested within each other: –Selection inside repetition –Repetition inside selection

5. Selection Inside Repetition We could have done Fibonacci’s problem differently (not necessarily better) by incorporating the function in the loop:

6. Previous Program: Fibonacci PROGRAM Fibonacci; CONST Number = 20; VAR J, {counter variable} N1, N2, N3 {counter variable} : integer; BEGIN N1 := 0; writeln('Fibonacci 0:', N1:5); N2 := 1; writeln('Fibonacci 1:', N2:5); FOR J := 2 TO Number – 1 DO BEGIN N3 := N1 + N2; writeln('Fibonacci',J:3,':',N3:5); N1 := N2; N2 := N3; END; readln END.

7. New Program: Fibonacci PROGRAM Fibonacci; CONST Number = 20; VAR J, {counter variable} N1, N2, N3 {counter variable} : integer; BEGIN FOR J := 0 TO Number-1 DO IF J = 0 THEN BEGIN N1 := 0; writeln('Fibonacci 0: 0') END ELSE IF J = 1 THEN BEGIN N2 := 1; writeln('Fibonacci 1: 1') END ELSE BEGIN N3 := N1 + N2; writeln('Fibonacci',J:3,':',N3:5); N1 := N2; N2 := N3; END; readln END. Repeated at each iteration of the loop!

8. Selection Inside Repetition If unnecessary, avoid it. It makes for inefficient programs May be necessary and part of the design, e.g. Selecting a menu option inside a loop.

9. PROGRAM MenuSelect; VAR Option : char; BEGIN REPEAT {print menu of options} writeln; writeln('You have the following options:'); writeln(' 1 : to perform action 1'); writeln(' 2 : to perform action 2'); writeln('....'); writeln(' 9 : to perform action 9'); writeln(' Q : to QUIT'); {read user choice} write('Your choice: ') ; readln(Option); {handle user choice} CASE Option OF '1'..'9' : writeln('Performing action', Option:2); 'Q' : ; ELSE writeln('Unknown option', Option:2,'. Try again.') END; {CASE} UNTIL Option = 'Q' END.

10. PROGRAM MenuSelect; VAR Option : char; BEGIN REPEAT {print menu of options} writeln; writeln('You have the following options:'); writeln(' 1 : to perform action 1'); writeln(' 2 : to perform action 2'); writeln('....'); writeln(' 9 : to perform action 9'); writeln(' Q : to QUIT'); {read user choice} write('Your choice: ') ; readln(Option); {handle user choice} IF ((Option >= ‘1’) AND (Option <= ‘9’)) OR (Option = ‘Q’) THEN CASE Option OF '1'..'9' : writeln('Performing action', Option:2); 'Q' : ; END {CASE} ELSE writeln('Unknown option', Option:2,'. Try again.') UNTIL Option = 'Q' END.

11. Repetition Inside Selection Example: –Menu for printing out different types of pictures ExampleMenu for accessing other menus: (selection inside repetition + repetition inside selection)

12. Selection Inside Repetition + Repetition Inside Selection Example: Menu for accessing other menus Each menu is embedded inside a loop: selection inside repetition Each selection contains a menu within a loop: repetition inside selection

13. REPEAT {top-level menu} display options read user choice {handle user choice – CASE or IF-THEN-ELSE stmt} CASE Option OF choice1 : REPEAT display options read user choice handle user choice UNTIL exit from menu1... choiceN : REPEAT display options read user choice handle user choice UNTIL exit from menuN END; {CASE} UNTIL exit top level menu END.

14. Self-Check 1 What is displayed by the following program fragment, assuming N is 5? FOR I := 1 TO N DO BEGIN FOR J := 1 TO I DO write(‘*’); writeln END;

15. Self-Check 2 What is displayed by the following program fragment, assuming M is 3 and N is 5? FOR I := N DOWNTO 1 DO BEGIN FOR J := M DOWNTO 1 DO write(‘*’); writeLn END;

16. Self-Check 3 Show the output printed by the following nested loops: FOR I := 1 TO 2 DO BEGIN writeln(‘Outer’ : 5, I : 5); FOR J := 1 TO 3 DO writeln(‘Inner’ : 7, I : 3, J : 3) FOR K := 2 DOWNTO 1 DO writeln(‘Inner’ :7, I : 3, K : 3) END;

17. Self-Check 4 Write a program fragment that, given an input value N, displays N rows in the form 1 2... N, 2 3... N + 1, and so forth.

18. PROGRAM Check; VAR I,J,N : integer; BEGIN write('What is N? '); read(N); FOR I := 1 TO N DO BEGIN FOR J := I TO I + N - 1 DO write(J:2); writeln END; readln END.

19. Self-Check 5 Write a program that prints a nicely labeled multiplication table for the digits 1 through 9.

20. PROGRAM MultiplicationTable; VAR I,J : integer; BEGIN FOR I := 1 TO 9 DO BEGIN FOR J := 1 TO 9 DO write(I*J:3); writeln END; readln END.

21. Quiz Results Above Median: 60.5 49.5 46.5 43 40 40 37.5 Median: 35 Below Median: 26.5 22 21.5 21 16.5 11.5 11 Mean: 32.13333333 Total Points: 80 N = 15