Iteration Structures

Iteration Structures • Iteration is the third control structure we will explore. • Iteration simply means to do something repeatedly. • All iteration control structures test a condition each time through the loop.

Types of Loops • There are three types of loops that Java employs. • Pretest Loop- test a condition each time before the loop is executed. • Posttest Loop- test a condition after each loop execution. • Fixed repetition- cause a loop to be executed a predetermined number of times.

Three Iteration Control Structures • The three iteration control structures are the while, do/while, and for. • The difference between them is the means by which they control the exiting of the loop. • The while is a pretest loop. • The do/while is a posttest loop. • The for is a fixed repetition loop.

The WHILE loop • If the test expression is true the loop statements are executed. • If the test expression is false the loop statements are bypassed. • To get out of the loop something must change the expression to false, otherwise the result will be an infinite loop.

Syntax for the While Loop while (test expression) { statement1; statement2; statementn; }

The DO/WHILE loop • The do/while loop is tested at the end of the loop compared to the while which is tested at the beginning. • The loop statements will always be executed at least once. • To break the loop the test expression must become false, otherwise, just like the while loop, the result will be an infinite loop.

Syntax for DO/WHILE do { statement1; statement2; statementn; } while (test expression);

Using Counters and Accumulators • Numeric variables used within a repetition structure to calculate subtotals, totals, averages and run the loop. • Counter • Used for counting at constant rate. • Ex. counter += 1 or counter++ • Accumulator • Used for accumulating (adding together) changing values. • Ex. accumulator += value

Sentinel Values • Values used to end loops • Should be easily distinguishable from the valid data used by the program • Also called trip values or trailer values • Should either be declared as a constant variable or regular variable and ask the user to enter a value.

The FOR loop • The for loop runs a fixed number of times • The first thing that is done is the initialization of the counter. • If the results of the test expression is true, then the statements are executed. • Each time the loop is executed, the loop counter must be incremented or decremented.

Syntax for FOR loop for (initialize counter; test counter; update counter) { statement1; statement2; statementn; }

Nested Loops • Many times it is desirable to have looping operations within loops. • This is called nested looping. • Each inner loop will run as many times as desired X the number of times the outer loop runs.

Break and Continue Option • If you want a loop to terminate when a desired value is found, it is possible to use the break statement to force the loop to quit. • If you want one iteration of the loop to be terminated, it is possible to use a continue statement to force the loop to quit one iteration and continue on with another.

Syntax for BREAK while (test expression) { statement1; statement2; if (test expression) break; statementn; }

Syntax for CONTINUE for (initialize counter; test counter; update counter) { statement1; statement2; if (test expression) continue; statementn; }

Number Formatting • Import • import java.text.DecimalFormat; • Instantiation • DecimalFormat twoDecimal = new DecimalFormat("0.00"); • The numbers inside of parenthesis are the way in which you want your numbers to show up when you output. • Ouput • twoDecimal.format(variable);

Example Formatting Program • import TerminalIO.KeyboardReader; • import java.text.DecimalFormat; • public class FormatExample • { • public static void main(String [] args) • { • KeyboardReader reader = new KeyboardReader(); • DecimalFormat twoDecimal = new DecimalFormat("0.00"); • DecimalFormat oneDecimal = new DecimalFormat("0.0"); • double number; • number = reader.readDouble("Please enter a number: "); • System.out.println("Two decimal format: " + twoDecimal.format(number)); • System.out.println("One decimal format: " + oneDecimal.format(number)); • } • }

Iteration Structures