Mastering Loops in Computer Science: Fundamentals and Applications

1. Warm-Up: April 14 • What kind of loop would I use to complete the following: • Output all of the prime numbers that are less than 100,000 • Output the Fibonacci sequence until the numbers are greater than 1000 • Output the first 100 multiples of 13

2. Advanced Loops Pre-AP Computer Science, Cycle 6

3. Review • Loops are used for repeating tasks or sections of code • While loops are used when the number of times to loop is unknown, or may change • For loops are used when the number of times to loop is known, or can be represented by a variable

4. WHILE Loops • “While this condition is true, do this” • “Do this until this is true / do this until this isn’t true” int x=0; int y; while (condition) while (x < 100) { { //statements y = x*2; } x++; }

5. FOR Loops • “Repeat this ‘x’ times” • “Cycle through ‘x’ things” for (inti=0; i<100; i++) { int y=i*2; }

6. Counter Controlled Loops • Counter controlled loops are characterized by the use of a counter, or variable, that tracks the number of times a loop has executed • Some while loops are counter controlled, but ALL for loops are counter controlled

7. Counter Controlled Loops Example intx=0; int y; while (x < 100) { y = x*2; x++; } for (inti=0; i<100; i++) { int y=i/3; }

8. Non-Counter Controlled Loops • Non-counter controlled loops do NOT use a counter to keep track of loop iterations • Number of iterations doesn’t matter • Non-counter controlled loops are ALWAYS while or do-while loops • Examples: sentinel-controlled; flag-controlled

9. Sentinel-Controlled Loops • Sentinel: Special value that signal when the loop should stop running • Loop continues as long as the sentinel value is not encountered Structure while (variable != sentinel)

10. Sentinel-Controlled Example while (answer != “no”) { System.out.print(x); x++; System.out.print(“Keep looping?”); answer = console.next(); }

11. Flag-Controlled Loops • Uses a boolean variable to control the loop • The boolean variable is called a flag • Must eventually be set to true in loop body Structure boolean flag = false; while (!flag)

12. Flag-Controlled Example boolean stop = false; while (!stop) { System.out.println(“banana!”); System.out.println(“More bananas?”); String ans = console.next(); if (ans == “no” || ans == “No”) { stop = true; } }

13. Flag-Controlled Example B boolean loop = true; while (loop) { System.out.println(“banana!”); System.out.println(“More bananas?”); String ans = console.next(); if (ans.charAt(0) == ‘n’) { loop = false; } }

14. Warm-Up: April 15 • For each of the following kinds of loops, state whether a FOR loop, WHILE loop, or both are capable of performing that kind of loop. • Counter-Controlled • Sentinel-Controlled • Flag-Controlled

15. Warm-up: April 16(3rd period) • List at least 10 steps that the computer would have to complete for the following problem: A program continues to ask the user to input numbers until they input they number -1. Then, the computer outputs the minimum and maximum of the numbers typed in.

16. Steps • Set up variables for num, min, and max • Begin a loop that runs while num != -1 • Ask the user for a number • Save their response in “num” • IF num != -1 • Compare num to the minimum • Replace minimum with the smallest number • Compare num to the maximum • Replace max with the largest number • Output minimum • Output maximum

17. Do-While Loops Pre-AP Computer Science, Cycle 6

18. Review • Counter Controlled • Loop ends when the counter expires • for (int counter=0; counter<100; counter++) • Sentinel-Controlled Loop • Loop ends when the sentinel value is encountered • while (answer != -1) • Flag-Controlled Loop • Loop ends when the flag (boolean) is reversed • while (!stop)

19. Do-While Loops • Used for looping situations that MUST occur at least one time • Have the same expressive power as WHILE loops • Counter, flags, sentinels • Primary Difference: It checks the condition AFTER the loop statement

20. Do-While Loops - Structure • “Do this while this is true” do { //statements } while (condition);

21. Converting WHILE loops while (x != 0) { //statements; } do { //statements; } while (x !=0);

22. Do-While Loops - Example • Add up all user-inputted numbers until 0 is encountered do { System.out.print(“Enter a value: “); data = input.nextInt(); sum = sum + data; } while (data != 0); System.out.println(“Sum: “ + sum);

23. Do-While Loops – Example B • Print random numbers between 1-100 until the number 42 is encountered do { intnum = Math.random()*100 + 1; System.out.println(num); } while (num != 42);

24. Warm-up: April 17(3st period) • Convert the following while loop into a do-while loop while (number != 15) { System.out.println(number); number = Math.random()*20+1; }

25. Warm-up: April 17(1st period) • Which loop would be best for the following prompts? FOR loop, WHILE loop, or DO-WHILE loop • A program that displays all the numbers from 100 to 1000 that are divisible by 5 and 6 • A program that allows the user to type in test scores until they type “STOP”

26. Break and Continue Statements Pre-AP Computer Science, Cycle 6

27. Break Statement • Can be used to immediately terminate a loop early • Can be used in any kind of loop • Use by merely writing break;

28. Break Statement - Example int sum = 0; int number = 0; while (number < 20){ number++; sum = sum + number; if (sum >= 100) { break; } }

29. Break - Example 2 int number = (int)(Math.random()*100); while (true) { System.out.println(“Guess my number!); int guess = console.nextInt(); if (guess == number) { System.out.println(“Correct!”) break; } else if (guess > number) System.out.println(“Too high!”); else System.out.println(“Too low!”); }

30. Continue Keyword • Can be used to immediately break out of the current iteration of a loop, and skip to the next iteration • Can be used to skip certain situations without having to use complex logic for your condition (&& and ||)

31. Continue Statement - Example int number = 0; while (number < 5) { if (number == 3) { continue; } System.out.print(number + “ “); } Output: 0 1 2 4 5

32. Using Break and Continue • Typically, use of break and continue is discouraged as they can create loops with too many exit points that are difficult to debug • Break and continue should only be used if they simplify the loop, and make it easier to read