1. Instructor: Mainak Chaudhuri mainakc@cse.iitk.ac.in
More Conditionals
Instructor: Mainak Chaudhuri

2. Primality testing class primalityTestSlow {
public static void main (String arg[]) {
int n = 42, d;
if (n <= 1) {
System.out.println(n + “ is not a prime.”); }
else {
for (d=2; d<=n/2; d++) {
if ((n%d)==0) {
break;
if (d > n/2) {
System.out.println(n + “ is a prime.”);
} } }

3. Primality testing class primalityTestLittleBetter {
public static void main (String arg[]) {
int n = 42, d;
if (n <= 1) {
System.out.println(n + “ is not a prime.”); }
else {
for (d=2; d*d<=n; d++) {
if ((n%d)==0) {
break;
if (d*d > n) {
System.out.println(n + “ is a prime.”);
} } }

4. continue
Allows you to skip parts of a for or while or do-while loop statements
Example (want to print two-digit numbers with both digits odd)
for (i=10; i<100; i++) {
if ((i%2)==0) {
continue; }
if (((i/10)%2)==1) {
System.out.println(i + “ has odd digits.”);

5. Perfect numbers class perfectNumber {
public static void main (String arg[]) {
int n = 24;
int d, sigma_n = 1+n;
for (d=2; d<=n/2; d++) {
if ((n%d) != 0) {
continue; }
sigma_n += d;
if (sigma_n == 2*n) {
System.out.println (n + “ is perfect.”);

6. switch-case An alternative of if-else if-…-else switch (expression) {
case constant1: // integer or character
statements1
case constant2:
statements2 …
case constantN:
statementsN
default:
statementsD }

7. switch-case Same as if (expression==constant1) { statements1…
statementsN
statementsD }
else if (expression==constant2) {
statements3
// continued on next slide

8. switch-case else if (expression==constant3) { statements3 statements4…
statementsN
statementsD }
else if (expression==constantN) {
else {

9. switch-case with break
switch (expression) {
case constant1:
statements1
break;
case constant2:
statements2 …
case constantN:
statementsN
default:
statementsD }

10. switch-case with break
Same as
if (expression==constant1) {
statements1 }
else if (expression==constant2) {
statements2 …
else if (expression==constantN) {
statementsN
else {
statementsD

11. switch-case: more flavors
switch (expression) {
case constant1:
case constant2:
statements1
break;
case constant3:
statements3 …
case constantN:
statementsN
default:
statementsD }

12. switch-case: more flavors
Same as
if ((expression==constant1) || (expression==constant2)) {
statements1 }
else if (expression==constant3) {
statements3 …
else if (expression==constantN) {
statementsN
else {
statementsD

13. Classification of numbers
class classifyNumbers {
public static void main (String arg[]) {
int n = 8;
switch (n) {
case 0 :
System.out.println(“Zero!”);
break;
case 1 :
System.out.println(“Smallest positive!”);
case 2 :
System.out.println(“Smallest prime!”);
// continued in next slide

14. Classification of numbers
case 3 :
System.out.println(“Smallest odd prime!”);
break;
case 4 :
System.out.println(“Smallest prime squared!”);
case 5 :
System.out.println(“Number of fingers!”);
case 6 :
System.out.println(“Smallest perfect!”);
// continued in next slide

15. Classification of numbers
case 7 :
System.out.println(“North seven stars!”);
break;
case 8 :
System.out.println(“Smallest prime cubed!”);
case 9 :
System.out.println(“Smallest odd prime squared!”);
default :
System.out.println(“Not a digit!”);
} // end of switch
} // end of main
} // end of class

16. More classification class differentClassification {
public static void main (String arg[]) {
int n = 8;
switch (n) {
case 2:
case 3:
case 5:
case 7:
System.out.println(“Prime!”);
break;
case 1:
case 4:
case 9:
System.out.println(“Square!”);
// continued in next slide

17. More classification case 6: System.out.println(“Perfect!”); break;
System.out.println(“Cube!”);
case 0:
System.out.println(“Zero the Great!”);
default:
System.out.println(“Not a digit!”);
} // end switch
} // end main
} // end class

18. More example of switch class rainbow {
public static void main (String arg[]) {
char c = ‘V’;
switch (c) {
case ‘V’ :
case ‘v’ :
System.out.println (“Violet”);
break;
case ‘I’ :
case ‘i’ :
System.out.println (“Indigo”);
case ‘B’ :
case ‘b’ :
System.out.println (“Blue”);

19. More example of switch case ‘G’ : case ‘g’ :
System.out.println (“Green”);
break;
case ‘Y’:
case ‘y’ :
System.out.println (“Yellow”);
case ‘O’ :
case ‘o’ :
System.out.println (“Orange”);
case ‘R’ :
case ‘r’ :
System.out.println (“Red”);
// continued in next slide

20. More example of switch default :
System.out.println (“You are not in rainbow!”);
break; }

21. Nested loops Loop within loop for (i=0; i<=100; i++) {
for (j=0; j<=100; j++) {
System.out.println (i+j); }
Number of loops is called the depth of the nest
The innermost loop executes most frequently
The outermost loop executes least
You can nest while loops within for loops and vice-versa
Loop variables should be different for different loops e.g., i and j in this case (what happens if both are i ?)

22. Nested loops for (i1=p1; i1<q1; i1++) { // outermost
statements1 // can be empty
for (i2=p2; i2<q2; i2++) {
statements2 // can be empty
for (i3=p3; i3<q3; i3++) {
statements3 // can be empty …
for (iN=pN; iN<qN; iN++) { // innermost
statementsN }
} // How many times does statementsK execute?

23. All perfect numbers class allPerfectNumbersUptoOneLakh {
public static void main (String arg[]) {
int n, d, sigma_n;
for (n=2; n<=100000; n++) {
sigma_n = 1+n;
for (d=2; d<=n/2; d++) {
if ((n%d)==0) {
sigma_n += d; }
if (sigma_n == 2*n) {
System.out.println (n + “is perfect.”);

24. Euclid’s division algorithm
A fast way to compute the greatest common divisor of two numbers a and b
Algorithm (this is not a program)
Divide b by a, assign a to b and the remainder to a; loop until a is zero. The value of b at this point is the gcd.
Assumes a is less than b
Main observation: gcd (a, b) = gcd (a, b-a)

25. GCD class gcd { public static void main (String arg[]) {
int a = 40, b = 24, r, gcd;
if ((a==0) || (b==0)) {
gcd = (a > b) ? a : b; }
else if (a < b) {
while (a!=0) {
r = b%a;
b = a;
a = r;
gcd = b;
// next slide

26. GCD else { while (b!=0) { r = a%b; a = b; b = r; } gcd = a;
System.out.println(“GCD: ” + gcd);

27. Announcements Mid-term exam I on 31st August 0930 to 1030
Seating arrangement:
Y4, Y5, Y7001 to Y7085: L1 OROS
Y7092 to Y7233: L2 ERES
Y7234 to Y7388: L16 OROS
Y7391 to Y7518: L17 ERES
Please take seats 15 minutes before the scheduled time
Exam syllabus: up to and including conditionals