1. 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
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CS102 - Algorithms & Programming II

2. 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
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CS102 - Algorithms & Programming II

3. 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

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

5. 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

6. 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);
sum is 20
4/29/2019
CS102 - Algorithms & Programming II

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

8. 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

9. 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

10. 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

11. 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);
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CS102 - Algorithms & Programming II

12. 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

13. 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 ; }
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CS102 - Algorithms & Programming II

14. for Statement – Flow Diagram
initialization
false
condition
true
statement
increment
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CS102 - Algorithms & Programming II

15. 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) {
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CS102 - Algorithms & Programming II

16. 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--; }
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CS102 - Algorithms & Programming II

17. 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 )
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CS102 - Algorithms & Programming II

18. do-while statement – Flow Diagram
true
condition
false
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CS102 - Algorithms & Programming II

19. 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”));
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CS102 - Algorithms & Programming II

20. 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 =15
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CS102 - Algorithms & Programming II

21. 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(“”); }
*** .. *
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CS102 - Algorithms & Programming II

22. 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(“”); }
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CS102 - Algorithms & Programming II

23. 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