Java Flow Control Techniques for Program Execution

1. Flow Control in Java

2. Controlling which instruction to execute next • Sequential • Similar to walking, one step after another • Branching • Similar to a fork in the road • Depending on the destination, you choose one way or the other, not both • Repetition • Similar to running on a track in the Olympics • Repeating the same track in a loop

3. Sequential x = 1; x = x + 1; • As expected • First instruction first • Second instruction second • What if we swap the two instructions? • That is • Instructions cannot be in arbitrary order

4. Branching (Conditional Statements) if( x < y ) // boolean condition x = x + 1; // execute if true else y = y * 10; // execute if false

5. Second/Else Branch is Optional if ( x < y ) // boolean condition x = x + 1; // execute if true

6. Multiple Instructions in One Branch if ( x < y ) { // note the matching braces x = x + 1; y = y – x; } else { y = y * 10; x = y / x; }

7. Nested Branching if ( x < y ) { x = x + 1; if ( y > 10) y = y – x; } else { y = y * 10; x = y / x; }

8. Cascaded Branching if (score >= 90) grade = ’A’; else if (score >= 80) grade = ’B’; else if (score >= 70) grade = ’C’; else if (score >= 60) grade = ’D’; else grade = ’F’;

9. Version 2: always correct answer? if (score >= 90) grade = ’A’; if (score >= 80) grade = ’B’; if (score >= 70) grade = ’C’; if (score >= 60) grade = ’D’; if (score < 60) grade = ’F’;

10. Version 3: always correct answer? if (score >= 90) grade = ’A’; if (score >= 80 && score < 90) grade = ’B’; if (score >= 70 && score < 80) grade = ’C’; if (score >= 60 && score < 70) grade = ’D’; if (score < 60) grade = ’F’;

11. Repetition (looping) • for • “counting” loops • frequently (sometimes inappropriately) used • while • general loops • most flexible • do-while (“Repeat until”) • at least once • least used

12. ICU • Initialize (start) • What is the initial/starting condition? • Continue (or stop) • When to continue/stop? • In what condition does it continue/stop? • Update • How to update the condition? • If ICU is not carefully designed (common mistake) • your program will be in ICU

13. Counting loop – 1, 2, 3, … 10 for (intnum = 1; num <= 10; num++) System.out.println(num); for (initialize; continue; update) body -- instruction(s) to be repeated Continue --boolean (continue if true, stop if false)

14. How about from 55 to 123? for (intnum = ?; ?; ?) System.out.println(num);

15. How about from 55 to 123? for (intnum = 55; num <= 123; num++) System.out.println(num);

16. How about 10 numbers from 55? for (intnum = ?; ?; ?) System.out.println(num);

17. How about 10 numbers from 55? // version 1? for (intnum = 55; num <= 64; num++) System.out.println(num); // version 2? for (intnum = 55; num<= 65; num++) System.out.println(num); // version 3? for (intnum = 55; num< 65; num++) System.out.println(num);

18. How about 10 even numbers from 2? for (intnum = ?; ?; ?) System.out.println(num);

19. How about 10 even numbers from 2? // version 1? for (intnum = 2; num <= 20; num=num+2) System.out.println(num); // version 2? for (intnum = 2; num <= 18; num=num+2) System.out.println(num); // version 3? for (intnum = 2; num< 20; num=num+2) System.out.println(num);

20. How about 10 even numbers down from 100? for (intnum= ? ; ?; ?) System.out.println(num);

21. How about 10 even numbers down from 100? // version 1? for (intnum=100; num >= 80; num=num-2) System.out.println(num); // version 2? for (intnum=100; num>= 82; num=num-2) System.out.println(num); // version 3? for (intnum=100; num > 82; num=num-2) System.out.println(num);

22. Anything that is strange? for (intnum=10;num < 10; num++) System.out.println(num);

23. Anything that is strange? for (intnum=10;num < 10; num++) System.out.println(num); continueis never true, body never executes

24. Anything that is strange? for (intnum=10;num >= 10; num++) System.out.println(num);

25. Anything that is strange? for (intnum=10;num >= 10; num++) System.out.println(num); Continue is always true, infinite loop (eventually stops since int has an upper limit and num overflows)

26. Finding Sum of 1 to 10 int sum = 0; for (intnum = 1; num <= 10; num++) sum = sum + num;

27. Finding Sum of 1 to 10 int sum = 0; for (intnum = 1; num <= 10; num++) sum = sum + num; // --- version 2 ? --- int sum = 0; for (intnum = 1; num < 10; num++) sum = sum + num;

28. Sum of first 10 even numbers int sum = 0; for (intnum = ?; ? ; ?) sum = sum + num;

29. Sum of first 10 even numbers int sum = 0; for (intnum = 2; num <= 18; num = num + 2) sum = sum + num;

30. Printing a Line of 5 Stars for (int star = 1; star <= 5; star++) { System.out.print(’*’); } System.out.println(); // new line // --- output: --- *****

31. Printing a Line of 5 Stars for (int star = 1; star <= 5; star++) { System.out.print(’*’); } System.out.println(); // new line // --- version 2 ? --- for (intstar = 0; star < 5; star++) { System.out.print(’*’); } System.out.println(); // new line

32. 4x5 Rectangle of Stars ?? for (int star = 1; star <= 5; star++) { System.out.print(’*’); } System.out.println(); // --- Output: --- ***** ***** ***** *****

33. 4x5 Rectangle of Stars – nested loop for (int line = 1; line <= 4; line++) { for (int star = 1; star <= 5; star++) { System.out.print(’*’); } System.out.println(); } // --- Output: --- ***** ***** ***** *****

34. Triangle of Stars * ** *** **** *****

35. “While” loop intnum = 1, sum = 0; while (num <= 10) { sum = sum + num; num++; } initialize while (continue) // repeat if continue is true { update }

36. A program with an exit command booleanexit = false; while (exit == false) { // do stuff if ( //exit command is entered ) exit = true; }

37. Is num a prime number?

38. Is num a prime number? intfactor = 2; booleanprime = true; while (prime == true && factor < num) { if (num % factor == 0) //remainder is 0 prime = false; else factor++; }

39. “Do-While” loop Execute the loop body at least once continue is checked after the loop body is executed initialize do { update } while (continue); // repeat if continue is true // note the semicolon at the end

40. Checking input intnum = 0; do { System.out.print(”Please enter a positive int: ”); num = keyboard.nextInt(); } while (num <= 0);

41. Checking Password String username = ””, password = ””; do { System.out.print(”Please enter username: ”); username= keyboard.next(); System.out.print(”Please enter password: ”); password = keyboard.next(); } while (!valid(username, password));

42. How to add at most 3 Trials? String username = ””, password = ””; do { System.out.print(”Please enter username: ”); username= keyboard.next(); System.out.print(”Please enter password: ”); password = keyboard.next(); } while (!valid(username, password));

43. At most 3 Trials String username = ””, password = ””; inttrials=0; do { System.out.print(”Please enter username: ”); username= keyboard.next(); System.out.print(”Please enter password: ”); password = keyboard.next(); trials++; } while (!valid(username, password) && trials < 3);