Looping

Yong ChoiSchool of BusinessCSU, Bakersfield

Objectives

• Learn about the loop structure• Use a while loop• Use shortcut arithmetic operators• Use a for loop• Learn how and when to use a do…while loop• Learn about nested loops

Learning about the Loop Structure

• Loop: A structure that allows repeated execution of a block of statements

• Loop body: A block of statements; as long as the expression is true, the loop body executes

• Iteration- One execution of any loop

Using the while Loop

• while Loop: execute a body of statements continually as long as the Boolean expression continues to be true– Consists of the keyword while followed by a Boolean

expression within parentheses followed by the body of the loop

– Use when you need to perform a task a predetermined number of times

Using a while Loop

• Incrementing – Altering a loop by adding one to loop control variable

• Decrementing – Altering a loop by subtracting one from a loop control variable

While Loop Examplepublic class loopExample {

public static void main (String[] args ) { int count = 1; // start count out at one while ( count <= 3 ) // loop while count is <= 3

{ System.out.println( "count is:" + count ); count = count + 1; // add one to count, same as (count++)

} System.out.println( "Done with the loop" ); } }

Syntax of the while statement

while ( condition ) loop body // a statement or block

statement• When the condition is true, the loop body is exectued. • When the condition is false, the loop body is skipped,

and the statment after the loop is executed. • Once execution has passed to the statement after the

loop, the while statement is finished, at least for now. • If the condition is false the very first time it is evaluated,

the loop body will not be executed even once.

Using Shortcut Arithmetic Operators

To increase a variable’s value by exactly one:• prefix ++

– Used before the variable name• ++someValue;

• postfix ++ (recommend)– Used after the variable name

• anotherValue++;

Counting Upwards by Two's

int count = 0; // count is initialized while ( count <= 6 ) // count is tested {System.out.println( "count is:" + count ); count = count + 2; // count is changed by 2 } System.out.println( "Done counting by two's." );

Decrementing the Loop Control Variable

• The loop control variable in a counting loop can be changed by a negative value.

• Here is a program fragment that decrements the loop control variable at the bottom of each iteration:

int count = 2; // count is initialized while ( count >= 0 ) // count is tested { System.out.println( "count is:" + count ); count = count - 1; // count is changed by -1 } System.out.println( "Done counting down." );

Infinite Loop

• What’s the result of below loop?

int count = 13;

int decrement = -1; while ( count >= 0 ) { System.out.println( "count is:" + count ); count = count - decrement; } System.out.println( "Count was " + count + " when it failed the test");

Using a for Loop

• For loop: A special loop that is used when a definite number of loop iterations is required

• Keyword for• Use a set of parentheses• Three sections within parentheses

– Initializing the loop control variable– Testing the loop control variable– Updating the loop control variable

Example of For Statement

public class loopExample { public static void main (String[] args ) {

int count, sum; sum = 0; for ( count = 0; count <= 5; count++ )

{ sum = sum + count ; System.out.print( count + " " );

} System.out.println( "sum is: " + sum );

Syntax of for Statement

• Java (and several other languages) has a for statement which combines the three aspects of a loop into one statement. In general, it looks like this:

for ( initialize ; test ; change ) loopBody ;

• The initialize, test , and change are statements or expressions that (usually) perform the named action. The loopBody can be a single statement or a block statement.

• Here is an example of a for statement: for ( count = 0; count < 10; count++ )

System.out.print( count + " " );

Side By Side

for loop

int count, sum; sum = 0; for ( count = 0; count <= 5; count+

+ ) { sum = sum + count ; System.out.print( count + " " ); } System.out.println( "sum is: " + sum );

While loop

int count, sum; sum = 0; count = 0; while ( count <= 5 ) { sum = sum + count ; System.out.print( count + " " ); count++ ; } System.out.println( "sum is: " + sum );

Using a do…while Loop

• The while loop can be used to implement any loop.

• However, the for loop is very convenient. • The do…while loop is occasionally

convenient. – Of the three looping statements, it is used the least.

• Some programmers prefer not to use it at all.

Learning How and When to Use a do…while loop

• Checks at the bottom of the loop after one repetition has occurred– Bottom-driven loop

• Loop body executes at least one time• The loop starts with the keyword do• The body of the loop is contained within curly

braces

Example of do…while Statement

int count = 0; // initialize count to 0

do { System.out.println( count ); // loop body: includes code to count++ ; // change the count } while ( count < 10 ); // test if the loop body should be

// executed again.

while Loop with Alternatives

import java.io.* ; public class SqrtCalc { public static void main( String[] args ) throws IOException { String chars ;

double x;BufferedReader stdin = new BufferedReader( new

InputStreamReader(System.in) ); chars = "yes" ; while ( chars.equals( "yes" ) || chars.equals( "YES" ) ||

chars.equals( "y" ) || chars.equals( "Y" ) ) { System.out.print("Enter a number-->"); chars = stdin.readLine(); x = (Double.valueOf(chars)).doubleValue(); System.out.println("Square root of " + x + " is " + Math.sqrt( x ) ); System.out.print("Do you wish to continue? (yes or no) -->"); chars = stdin.readLine(); } } }

Nested Loops

• Loops can be nested much like if statements• You can place a while loop within a while loop,

a for loop within a for loop, a do…while loop within a do…while loop, or use any combination of these loops

• Try the program on page 196 – 198