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 while do-while (“Repeat until”)
“counting” loops
frequently (sometimes inappropriately) used
while
general loops
most flexible
do-while (“Repeat until”)
at least once
least used

12. ICU Initialize (start) Continue (or stop) Update
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 (int num = 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 (int num = ?; ?; ?) System.out.println(num);

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

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

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

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

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

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

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

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

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

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

25. Anything that is strange?
for (int num=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 (int num = 1; num <= 10; num++) sum = sum + num;

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

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

29. Sum of first 10 even numbers
int sum = 0; for (int num = 0; 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 (int star = 0; star < 5; star++)

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

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

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

35. “While” loop
int num = 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
boolean exit = 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? Definition
Only divisible by 1 or itself
Check divisors between 2 and num – 1 to see if num is divisible by divisor
Don’t need to check divisors larger than num

39. Checking different divisors
Initialize
Start divisor with 2
Continue
Divisor is less than num and
num is not divisible by divisor
Update
Increment divisor

40. Checking different divisors
// I: start divisor with 2 int divisor = 2; // C: divisor less than num and // num not divisible by divisor while (divisor < num && num % divisor != 0) { divisor++; // U: increment factor }

41. Print the answer as well
// I: start divisor with 2 int divisor = 2; // C: divisor less than num and // num not divisible by divisor while (divisor < num && num % divisor != 0) { divisor++; // U: increment factor } System.out.println(?);

42. Print the answer as well
// I: start divisor with 2 int divisor = 2; // C: divisor less than num and // num not divisible by divisor while (divisor < num && num % divisor != 0) { divisor++; // U: increment factor } if (divisor == num)//not divisible by smaller divisors System.out.println(“prime”); else System.out.println(“not prime”);

43. Which do you prefer?
int divisor = 2; while (divisor < num && num % divisor != 0) { divisor++; } if (divisor == num) System.out.println(“prime”); else System.out.println(“not prime”);
int divisor = 2;
boolean prime = true;
while (divisor < num) {
if (num % divisor == 0)
prime = false;
divisor++; }
if (prime == true)
System.out.println(“prime”);
else
System.out.println(“not prime”);

44. “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

45. Checking input
int numTickets = 0; do { System.out.print(”Please enter # of tickets: ”); numtickets = keyboard.nextInt(); } while (numTickets <= 0);

46. 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));

47. 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));

48. At most 3 Trials
String username = ””, password = ””; int trials=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);