1. CSS161: Fundamentals of Computing
Loops
This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials.
CSS161: Fundamentals of Computing

2. CSS161: Fundamentals of Computing
Loops
Loop:
Loop body, (i.e., Statements) to be repeated
A Boolean expression to terminate the repetition
Iteration:
Each repetition of the loop body
Three types of loop statements
while loop: a Boolean expression evaluated at top of each iteration
do-while loop: a Boolean expression evaluated at bottom of each iteration
for loop: is similar to while loop but additionally includes initialization statements and post-iteration updating statements
CSS161: Fundamentals of Computing

3. CSS161: Fundamentals of Computing
while Statement
Syntax
while (Boolean_Expression)
Statement Or {
Statement_1
Statement_2
Statement_Last }
Example
int countDown = 3;
while ( countDown > 0 )
System.out.println( “Hello” );
countDown = countDown - 1;
Boolean
Expression?
false
true
Statement(s)
CSS161: Fundamentals of Computing

4. CSS161: Fundamentals of Computing
do-while Statement
Syntax do
Statement
while (Boolean_Expression); Or {
Statement_1
Statement_2
Statement_Last
} while (Boolean_Expression);
Example
countDown = 3;
System.out.println( “Hello” );
countDown = countDown – 1;
} while ( countDown > 0 );
Statement(s)
true
Boolean
Expression?
false
CSS161: Fundamentals of Computing

5. Algorithms and Pseudocode
Algorithm: a set of precise instructions that leads a solution.
Example: Sorting Algorithms (Selection, Bubble, Insertion, Quick, Merge)
Sorting three integers:
Declare three variables: a, b, and c;
Read keyboard inputs into a, b, and c;
Swap a and b if a < b
Swap a and c if a < c
Swap b and c if b < c
Print out a, b, and c
Pseudocode: a mixture of a programming language and human language to write an algorithm
int a, b, c, tmp;
// read keyboard inputs into a, b, and c;
if ( a < b ) {
tmp = a;
a = b;
b = tmp; }
if ( a < c ) {
a = c;
c = tmp;
if ( b < c ) {
tmp = b;
b = c;
// print out a, b, and c
CSS161: Fundamentals of Computing

6. CSS161: Fundamentals of Computing
Pseudocode to Program
import java.util.Scannaer;
public class Averager {
public static void main( String[] args )
Scanner keyboard = new Scanner( System.in );
System.out.println( “Enter nonnegative scores.” );
System.out.println( “End with a negative number.” );
System.out.println( “I will compute their average” );
double next, sum = 0;
int count = 0;
next = keyboard.nextDouble( );
while( next >= 0 )
sum += next;
count++; }
if ( count == 0 )
System.out.println( “No scores entered.” );
else
double average = sum/count;
System.out.println( count + “ scores read” );
System.out.println( “The average is ” + average );
Give the user instructions.
count = 0;
sum = 0;
Read a number and store it in a variable named text.
While ( next >= 0 ) {
sum = sum + next;
count++;
Read a number and store it in next. }
The average is sum/count provided count is not zero.
Output the results
Sentinel Value
CSS161: Fundamentals of Computing

7. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook p131’s exercises 22 ~ 27.
CSS161: Fundamentals of Computing

8. CSS161: Fundamentals of Computing
for Statement
Syntax
for (Initialization; Boolean_Expression; Updates)
Body
Examples
int next, sum = 0;
for ( next = 0; next <= 10; next++ ) {
sum = sum + next;
System.out.println( “sum up to ” + next + “ is ” + sum ); }
Scanner keyboard = new Scanner( System.in );
int value = keyboard.nextInt( );
int limit = keyboard.nextInt( );
for ( int mult = value; mult <= limit; mult += value )
System.out.println( mult );
Initialization
false
Boolean
Expression?
true
Statement(s)
Updates
CSS161: Fundamentals of Computing

9. for-Equivalent while Loop Syntax
CSS161: Fundamentals of Computing

10. Comma in for Statements
Multiple initializing actions, each separated by a comma
Multiple updating actions, each separated by a comma
Syntax:
for ( Init1, Init2, ..; Boolean_Expression; update1, update2, … )
body
Notes:
If Initialization also declares variables (valid only within the for-loop), all declared variables must be the same type.
Updates can replace body that can be even eliminated. (the last semicolon is necessary.)
Example:
for ( term = 1; sum = 0; term <= 10; sum += term, term++ )
; // empty body
CSS161: Fundamentals of Computing

11. CSS161: Fundamentals of Computing
Examples
Syntactically illegal
for ( int term = 1, double sum = 0; term <= 10; term++ )
sum = sum + term;
double sum;
for ( int term = 1, sum = 0; term <=10; term++ )
Syntactically legal
for ( int term = 1; sum = 0; term <= 10; term++ )
Should be int
CSS161: Fundamentals of Computing

12. CSS161: Fundamentals of Computing
Repeat N Times Loops
Print N times (1-indexed counting)
public static final int N = 30;
for ( int count = 1; count <= N; count++ )
System.out.println( “Hop, Step, Jump” );
Alternative way to count (0-indexed counting)
for ( int count = 0; count < N; count++ )
System.out.println( “Hop, Stem, Jump” );
int[] array = new int[N];
for ( int index = 0; index < N; index++ )
array[index] = 100;
CSS161: Fundamentals of Computing

13. Empty Statement in a for Loop
Pitfall
for ( int count = 0; count <= 10; count++ );
System.out.println( “count = ” + count );
Correct
for ( int count = 0; count <= 10; count++ )
Intentional
for ( int count = 0; count <= 10; System.out.println( “count = “ + (count++ ) );
An empty statement: meaninglessly repeated 10 times;
CSS161: Fundamentals of Computing

14. CSS161: Fundamentals of Computing
Infinite Loops
The Boolean expression in a do-while/while/for loop must eventually end up with “false”.
Otherwise the loop can’t get finished.
Examples
number = 1;
while ( number != 12 ) {
System.out.println( number );
number = number + 2; }
for ( int count = 0, limit = 5; count < limit; limit++ )
System.out.println( count );
for( ; ; ) {
while ( true ) {
number <= 12
count++
if ( Boolean_expression )
break;
CSS161: Fundamentals of Computing

15. CSS161: Fundamentals of Computing
Nested Loops
When nested, the inner loop iterates from beginning to end for each single iteration of the outer loop
Examples
What results will be printed out?
int rowNum, columnNum;
for (rowNum = 1; rowNum <=3; rowNum++) {
for (columnNum = 1; columnNum <=2; columnNum++)
System.out.print(" row " + rowNum + " column " + columnNum);
System.out.println(); }
for ( int row = 1; row <= 10; row++ )
for ( int star = 1; start <= row; star++ )
System.out.print( “*” );
System.out.println( );
CSS161: Fundamentals of Computing

16. CSS161: Fundamentals of Computing
break Statement
Breaking out of the current loop
for ( int x = 0; x < MAX; x++ )
if ( ( next = keyboard.nextInt( ) ) <= 0 )
break;
for ( int y = 0; y < MAX; y++ )
for ( int z = 0; z < MAX; z++ )
Breaking out of the current and outer loops all under a give label
outerLoop:
middleLoop:
innerLoop:
for ( int z = 0; z < MAX; z++ ) {
if ( ( next = keyboard.nextInt( ) ) < 0 )
break innerLoop;
if ( next == 0 )
break middleLoop;
if ( next < 100 )
break outerLoop; }
CSS161: Fundamentals of Computing

17. CSS161: Fundamentals of Computing
continue Statement
Ends the current iteration of the nearest enclosing loop, and then
Starts the next iteration.
for ( int num = 0; num < MAX; num++ ) {
if ( ( div = keyboard.nextInt( ) ) == 0 )
continue;
System.out.println( num + “/” + div + “=“ + num/div ) }
for ( int x = 0; x < MAX; x++ )
for ( int y = 0; y < MAX; y++ )
for ( int z = 0; z < MAX; z++ ) {
if ( ( div = keyboard.nextInt( ) ) <= 0 )
System.out.println( ( x + y + z ) / div )
CSS161: Fundamentals of Computing

18. CSS161: Fundamentals of Computing
exit Statement
break ends a loop, but does not end the program.
System.exit(0) immediately ends the program.
0 used to indicate a normal ending of the program.
System.out.println( “Enter a negative number:” );
int negNumber = Kyeboard.nextInt( );
if ( negNumber >= 0 ) {
System.out.println( negNumber + “>= Program aborting.” );
System.exit( 0 ); }
CSS161: Fundamentals of Computing

19. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook P140’s exercises 28 ~ 39
CSS161: Fundamentals of Computing