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 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 + " " );

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