1 / 43

Flow Control in Java

Flow Control in Java. 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

jera
Download Presentation

Flow Control in Java

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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);

More Related