Looping

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# Looping - PowerPoint PPT Presentation

Looping. Yong Choi School of Business CSU, 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.

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### Looping

Yong Choi

CSU, 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
• 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 Example

public 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 + " " );

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 );

Side By Side
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;

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) -->");