Repetition Statements Repetition statements allow us to execute a statement multiple times Often they are referred to as loops Like conditional statements, they are controlled by boolean expressions Java has three kinds of repetition statements: while loop do loop for loop The programmer should choose the right kind of loop for the situation 4/29/2019 CS102 - Algorithms & Programming II

Java Repetition Statements while (condition) statement for ( init; condition; update) statement do statement while (condition); where statement is any Java statement condition is a boolean expression 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II while Statement The most general one of these loop statements is while. while ( boolean-expression ) statement where statement can be any statement including a compound statement, an if-statement, another loop statement, ... Statement is repeated as long as the condition (boolean-expression) is true. When the condition gets false, the statement is not executed anymore. If the condition never gets false  infinite loop If the condition gets immediately false  the loop will be repeated zero times. 4/29/2019 CS102 - Algorithms & Programming II

while Statement – Flow Diagram while ( condition ) statement true condition statement false 4/29/2019 CS102 - Algorithms & Programming II

while Statement – Counter Controlled Loop(1) Print 5 asterisk characters This loop will be repeated for 5 times (count: 0,1,...,4) count = 0; while ( count < 5 ) { System.out.println( “*”); count = count + 1; } System.out.println( “done”); * * * * * done 4/29/2019 CS102 - Algorithms & Programming II

while Statement – Counter Controlled Loop(2) Read & sum 5 values sum = 0; count = 0; while ( count < 5 ) { value = scan.nextInt(); sum = sum + value; count = count + 1; } System.out.println( “sum is ” + sum); 5 3 7 4 1 sum is 20 4/29/2019 CS102 - Algorithms & Programming II

Counter Controlled Loops Template of counter-controlled loops initialize counter while ( condition depending on counter) { other statements to be repeated update counter } 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II Infinite Loops The body of a while loop eventually must make the condition false If not, it is called an infinite loop, which will execute until the user interrupts the program in console applications use Ctrl-C to exit This is a common logical error You should always double check the logic of a program to ensure that your loops will terminate normally int count = 1; while (count <= 25){ System.out.println (count); count = count - 1; } 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II Infinite Loops (2) i = 1; while ( i != 50 ) { System.out.println( i); i = i + 2; } System.out.println( “done”); i = scan.nextInt(); while ( i != 1 ) { if ( i % 2 == 0) i = i / 2; else i = 3 * i + 1; Sometimes, it is difficult to check loop terminating condition. 4/29/2019 CS102 - Algorithms & Programming II

Sentinel-Controlled Loops What happens if we don’t know how many times a loop will run. Ex: a loop which reads the scores of the students in an exam and find the average of the scores; and we we don’t know the number of students. How are we going to stop the loops?  use a sentinel-value We choose a sentinel-value which can not be a score (e.g. –1) We read the scores until this sentinel-value has been entered. When this sentinel-value has been read, we stop the loop. 4/29/2019 CS102 - Algorithms & Programming II

Sentinel-Controlled Loops – Example(1) int sum = 0; int numOfStudents = 0; int ascore; double avg; System.out.print(“Enter a score (-1 to stop) >”); ascore = scan.nextInt(); while (ascore != -1) { sum = sum + ascore; numOfStudents = numOfStudents + 1; } avg = (double) sum / numOfStudents; System.out.println(“The number of students : “ + numOfStudents); System.out.println(“Average Score : “ + avg); 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II Example Program 1 // This program finds the factorial value of the given positive integer import java.util.Scanner; public class Factorial { public static void main(String[] args){ int num, factVal, counter; // Create a Scanner object Scanner scan = new Scanner(System.in); // Read the positive integer System.out.print("A Positive Integer: "); num = scan.nextInt(); // Find its factorial value counter = 2; factVal = 1; while (counter <= num) { factVal = factVal * counter; counter = counter + 1; } // Write the result System.out.println("Given Positive Integer: " + num); System.out.println("Its Factorial value : " + factVal); 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II for Statement Another loop statement in Java is for-statement. for-statement is more suitable for counter-controlled loops. for ( initialization ; condition ; increment ) statement which is equivalent to initialization ; while (condition ){ statement ; increment ; } 4/29/2019 CS102 - Algorithms & Programming II

for Statement – Flow Diagram initialization false condition true statement increment 4/29/2019 CS102 - Algorithms & Programming II

for statement -- Example int i; int i; int sum = 0; int sum = 0; for (i=1; i<=20; i++) i = 1; sum = sum + i; while (i<=20) { sum = sum + i; i++; } int i; int i; int sum = 0; int sum = 0; for (i=100; i>=1; i--) i = 100; sum = sum + i; while (i>=1) { 4/29/2019 CS102 - Algorithms & Programming II

for statement -- Example int i, j; int count=0; for (i=1, j=10; i<j; i++, j--)  count is 5 count++; i=1; j=10; for (; i<j; ) { i++; j--; } 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II do-while statement The third loop statement in Java is do-while statement. do statement while ( condition ) ; which is equivalent to while (condition ) 4/29/2019 CS102 - Algorithms & Programming II

do-while statement – Flow Diagram true condition false 4/29/2019 CS102 - Algorithms & Programming II

do-while statement -- Example int i=1; do { System.out.println(i); i++; } while (i<10); String s; System.out.print(“Enter a word > “); System.out.flush(); s = scan.nextLine(); System.out.println(s); } while (! s.equals(“quit”)); 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II Nested Loops If the body of a loop contains another loop, this is called as a nested loop. int sum=0; int i,j; for (i=1; i<=5; i++) for (j=1; j<=6; j++) sum=sum+1;  sum is 5*6=30 for (j=1; j<=i; j++) sum=sum+1;  sum is 1+2+3+4+5=15 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II Nested Loops – Example1 a right angled triangle (n lines, ith line contains i stars) * for (i=1; i<=n; i++) { // for each line ** for (j=1; j<=i; j++) *** System.out.print(“*”); . System.out.println(“”); . } *** .. * 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II Nested Loops – Example2 a triangle shape (1st line contains 1 star, 2nd line contains 3 star,..., nth line contains 2n-1 stars) * for (i=1; i<=n; i++) { // for each line *** for (j=1; j<=(n-i); j++) ***** System.out.print(“ ”); . for (j=1; j<=(2*i-1); j++) . System.out.print(“*”); ***.....*** System.out.println(“”); } 4/29/2019 CS102 - Algorithms & Programming II

CS102 - Algorithms & Programming II Nested Loops Nesting can be more than one level int sum=0; for (i=1; i<=5; i++) for (j=1; j<=5; j++) for (k=1; k<=5; k++) sum=sum+1;  sum is 125 4/29/2019 CS102 - Algorithms & Programming II