1 Conditionals Instructor: Mainak Chaudhuri

2 Announcements B1 batch will go for tutorial today –In CS102 (Computer Science department ground floor) –After you enter, take right –Ask people, if you are lost – (right after this class)

3 if statement if (condition) { statements } Nested if if (condition1) { statements1 if (condition2) { statements2 } statements3 }

4 Nested if Sometimes possible to simplify nested if if (condition1) { if (condition2) { statements } Same as if ((condition1) && (condition2)) { statements }

5 Example class exampleIf { public static void main(String arg[]) { int x=10, y, z; if ((x%2)==0) { System.out.println(x + “ is even.”); if ((x%3)==0) { System.out.println(x + “ is a multiple of 6.”); y = x/6; } z = x%6; }

6 if-else if (condition) { statements1 } else { statements2 } if within else if (condition1) { statements1 } else { if (condition2) { statements2 } statements3 }

7 if-else if-else if-…-else if (condition1) { statements1 } else if (condition2) { statements2 } else if (condition3) { statements3 } … else { statementsn }

8 Example class greetings { public static void main(String arg[]) { int hour = 3; if ((hour >= 0) && (hour < 12)) { System.out.println(“Good Morning!”); } else if ((hour >= 12) && (hour < 18)) { System.out.println(“Good Afternoon!”); } else if ((hour >=18) && (hour < 24)) { System.out.println(“Good Evening!”); } else { System.out.println(“Bad time!”); }

9 Conditional assignment if ((x%2)==0) { y = x/2; } else { y = (x+1)/2; } Same as y = ((x%2)==0) ? x/2 : (x+1)/2;

10 Integer part and absolute value class integerPart { public static void main(String arg[]) { double x = -3.7; int ipart; double aval; ipart = ((x >= 0) || ((int)x==x)) ? (int)x : (int)(x-1); aval = (x >= 0) ? x : -x; System.out.println(“Integer part of ” + x + “ is ” + ipart + “. Absolute value is ” + aval + “.”); }

11 Integer part and absolute value class integerPartAlternate { public static void main(String arg[]) { double x = -3.7; int ipart; double aval; ipart = ((x < 0) && ((int)x!=x)) ? (int)(x-1) : (int)x; aval = (x < 0) ? -x : x; System.out.println(“Integer part of ” + x + “ is ” + ipart + “. Absolute value is ” + aval + “.”); }

12 Sorting three numbers class sortThree { public static void main(String arg[]) { int x = 2, y = 5, z = 1; int max, mid, min; if ((x > y) && (x > z)) { max = x; if (y > z) { mid = y; min = z; } else { mid = z; min = y; } // next slide

13 Sorting three numbers else { if (y > z) { max = y; if (x > z) { mid = x; min = z; } else { mid = z; min = x; } else { // the remaining two permutations} } // end else }// end main }// end class

14 Loops Needed in problems that require solving the same subproblem over and over –Computing the sum of the first 100 natural numbers and putting the result in y –Algorithm: y = 1; y = y + 2; y = y + 3; … y = y + 100; –Cannot write 99 such additions: use a loop

15 while while (condition) { statements } Can put anything in “statements” –The entire construct is called a while loop –statements are executed until condition is true –Even before executing it the first time condition is evaluated A while loop may not execute even once

16 Example class justAnExample { public static void main(String arg[]) { int x = 5; int y = 0; while (x < 10) { y--; x++; } System.out.println(y); }

17 Example class justAnExample { public static void main(String arg[]) { int x = 15; int y = 0; while (x < 10) { y--; x++; } System.out.println(y); }

18 Sum of natural numbers class naturalSum { public static void main(String arg[]) { int n = 2; int y = 1; while (n <= 100) { y += n; n++; } System.out.println(“Sum of the first ” + (n- 1) + “ natural numbers is ” + y); }

19 Sum of natural numbers class naturalSumAnotherWay { public static void main(String arg[]) { int n = 99; int m = n+1; int y = 100; while (n > 0) { y += n; n--; } System.out.println(“Sum of the first ” + m + “ natural numbers is ” + y) }

20 Announcements There is a class this Saturday , in L7 Batches B9 and B10 will have labs on Saturday , in computer center Midterm-I is on 5 th September, 0930 to 1030, L1, L2, L3, L4, L5, L8, L9 –Odd row odd seats (OROS)

21 Announcements Seating arrangement (OROS) L1: 98292, Y2, Y3, Y4, Y5, up to Y6096 L2: Y6098 to Y6217 L3: Y6218 to Y6285 L4: Y6286 to Y6365 L5: Y6368 to Y6448 L8: Y6452 to Y6508 L9: Y6510 to Y6552 After arriving at your examination hall, if you fail to find an empty seat, please proceed to L9

22 Integer index class integerIndex { public static void main(String arg[]) { int n = 3; double x = 3.14, y = 1.0; int m = n; if (n < 0) { x = 1/x; m = -n; } while (m > 0) { y *= x; m--; } System.out.println(x + “ to the power ” + n + “ is ” + y); }

23 Positive Decimal to Binary class positiveDecimalToBinary { public static void main(String arg[]) { int n = 34, y=0, polyTerm = 1; if (n < 0) { System.out.println(“Sorry, cannot handle negative integers today!”); } else { while (n > 0) { y += (polyTerm*(n%2)); n /= 2; polyTerm *= 10; } System.out.println(“Required binary: ” + y); }

24 do-while do { statements } while (condition); “statements” execute at least once irrespective of condition

25 for loops for (expression1; condition; expression2) { statements } Same as expression1 while (condition) { statements expression2 }

26 Sum of natural numbers class naturalSum { public static void main(String arg[]) { int n; int y = 1; for (n=2; n <=100; n++) { y += n; } System.out.println(“Sum of the first ” + (n-1) + “ natural numbers is ” + y) }

27 Comma operator in for loop for (expression1a, expression2a, …; condition; expression1b, expression2b,…) { statements } Same as expression1a expression2a … while (condition) { statements expression1b expression2b … }

28 Sum of natural numbers class naturalSum { public static void main(String arg[]) { int n; int y; for (n=2, y=1; n <=100; y += n, n++) { } System.out.println(“Sum of the first ” + (n-1) + “ natural numbers is ” + y) }

29 Empty for loop body for (expression1; condition; expression2) { } Same as for (expression1; condition; expression2);

30 Infinite loops Loops that never terminate while (true) { statements } do { statements } while (true);

31 Infinite loops for (expression1; ;expression2) { statements } for (i=0; i > -10; i++) { statements } for (i=0; i<=100; i--) { statements }

32 Perfect squares class identifySquareButLessClever { public static void main (String arg[]) { int n = 48; int i; if (n < 0) { System.out.println (n + “ is not a perfect square.”); } else if ((n==0) || (n==1)) { System.out.println (n + “ is a perfect square.”); } else { for (i=2; i<=n/2; i++) { if ((i*i) == n) { System.out.println(n + “ is square of ” + i); }

33 break In the last example you may want to come out of the for loop as soon as you discover that n is a square –The computation done after this is useless –Use break –Breaks out of the loop (while, do-while, or for) currently you are in for (i=2; i<=n/2; i++) { if ((i*i)==n) { System.out.println(n + “ is square of ” + i); break; }

34 break Another way to achieve the same effect without using break for (i=2; (i<=n/2) && ((i*i) != n); i++) ; if ((i*i)==n) { System.out.println(n + “ is square of ” + i); }