3. Loops
• A loop can be used to execute a block of statements repeatedly. If the
repeating time is:
• predictable: usually use for loop
• unpredictable: usually use while or do-while loop
for ( initialization; condition; expression ){ ... }
• Example:
for( int i=1 ; i<10 ; i++ ){
...
}
4. Alternatives for loops
• Q1: How about only printing the odd numbers smaller than 10?
for ( initialization; condition; expression ){ ... }
for( int i=1; i<10; i++ ){
if( I % 2 != 0 ){
System.out.println( i );
}
}
• Q2: How about printing the numbers in a descending order?
for( int i=9; i>=1; i--){ ... }
5. for-each loops
for ( Type var : array ) {
...
}
• Example:
int[] numArray = {7,2,6,0}
for ( int i : numArray ) {
System.out.println("number is: " + i);
}
Note: the use of a colon
6. while and do while loops
while(Boolean expression) {
...
}
do {
...
} while ( <Boolean expression> )
• When the condition becomes false, program control passes to the line
immediately following the loop.
• while loop might not ever run.
• do-while loop runs at least once.
7. Examples
Calculate the factorial of n (=1*2*...*n) using the three types of loops.
Option 1: for loop:
int n = 5;
int factorial = 1;
for (int number = 1; number <= n; ++number) {
factorial *= number;
}
8. Examples
Calculate the factorial of n (=1*2*...*n) using the three types of loops.
Option 2: while loop:
int n = 5;
int factorial = 1;
int number = 1;
while (number<=n) {
factorial *= number;
number++; //update
}
9. Examples
Calculate the factorial of n (=1*2*...*n) using the three types of loops.
Option 3: do-while loop:
int n = 5;
int factorial = 1;
int number = 1;
do{
factorial *= number;
number++; //update
} while (number<=n);
Should we print or return
the factorial?
10. break and continue
• The break statement is used to jump out of the loop or switch
statement.
• The continue statement is used to skip to the next iteration of the
loop.
11. break
• Note: Only jump out of the innermost enclosing loop of the statement.
int n = 5;
for ( int i = 1 ; i < n ; i++ ){
for ( int j = 1 ; j < n ; j++ ){
if ( j > i) {
break;
} else {
System.out.print( j + " ");
}
} // break comes here if it runs
System.out.println();
}
What is the output?
12. continue
• The continue statement skips the current iteration of a loop. Only skip the
innermost enclosing loop of the statement.
for (int i=0;i<=10;i++){
if (i==5) {
continue;
}
System.out.println(i);
}
System.out.println("Done!");
1. What is the output?
2. What is the output if replacing continue; with break;?
3. What is the output if replacing continue; with return;?
13. Summary
• Loop control statements
• for loop
• for ( int i=0; i<n; i++) { … }
• while loop and do-while loop
• Both for and while loops might not ever run, do-while loop runs at least once
• break and continue
• break: jump out of the loop or switch statement.
• continue: skips the current iteration of a loop.
• Both only have effect on the innermost enclosing loop.
14. Learning resources
The workshop homepage
https://mcollison.github.io/JPMC-java-intro-2021/
The course materials
https://mcollison.github.io/java-programming-foundations/
• Session worksheets – updated each week
15. Additional resources
• Think Java: How to think like a computer scientist
• Allen B Downey (O’Reilly Press)
• Available under Creative Commons license
• https://greenteapress.com/wp/think-java-2e/
• Oracle central Java Documentation –
https://docs.oracle.com/javase/8/docs/api/
• Other sources:
• W3Schools Java - https://www.w3schools.com/java/
• stack overflow - https://stackoverflow.com/
• Coding bat - https://codingbat.com/java
