Chapter 4: Loops and Files

1. Chapter 4: Loops and Files Lecture 11: Wednesday October 11

2. Running Totals • Loops allow the program to keep running totals while evaluating data. • Imagine needing to keep a running total of user input.

3. Example: TotalSales.java int days; // The number of days double sales; // A day's sales figure double totalSales; // Accumulator String input; // To hold the user's input // Create a DecimalFormat object to format output. DecimalFormat dollar = new DecimalFormat("#,##0.00"); // Get the number of days. input = JOptionPane.showInputDialog("For how many days " + "do you have sales figures?"); days = Integer.parseInt(input);

4. Example: TotalSales.java totalSales = 0.0; for (int count = 1; count <= days; count++) { input = JOptionPane.showInputDialog("Enter the ”+“salesfor day " + count + ": "); sales = Double.parseDouble(input); totalSales += sales; // Add sales to totalSales. } JOptionPane.showMessageDialog(null, "The total sales are $" +dollar.format(totalSales));

5. Sentinel Values • Sometimes (usually) the end point of input data is not known. • A sentinel value can be used to notify the program to stop acquiring input. • If it is a user input, the user could be prompted to input data that is not normally in the input data range (i.e. –1 where normal input would be positive.)

6. Example: SoccerPoints.java int points; // Game points int totalPoints = 0; // Accumulator initialized to 0 // Display general instructions. System.out.println("Enter the number of points your team"); System.out.println( "has earned for each game this season."); System.out.println("Enter -1 when finished."); System.out.println(); // Get the first number of points. points = Integer.parseInt( JOptionPane.showInputDialog( "Enter game points or -1 to end: "));

7. SoccerPoints.java // Accumulate the points until -1 is entered. while (points != -1) { totalPoints += points; points = Integer.parseInt( JOptionPane.showInputDialog( "Enter game points or -1 to end: ")); } // Display the total number of points. System.out.println("The total points are " + totalPoints);

8. Nested Loops • Like if statements, loops can be nested. • If a loop is nested, the inner loop will execute all of its iterations for each time the outer loop executes once. for(int i = 0; i < 10; i++) for(int j = 0; j < 10; j++) loop statements; • The loop statements in this example will execute 100 times.

9. Example: Clock.java // Create a DecimalFormat object to format output. DecimalFormat fmt = new DecimalFormat("00"); // Simulate the clock. for (int hours = 1; hours <= 12; hours++) { for (int minutes = 0; minutes <= 59; minutes++) { for (int seconds = 0; seconds <= 59; seconds++) { System.out.print(fmt.format(hours) + ":"); System.out.print(fmt.format(minutes) + ":"); System.out.println(fmt.format(seconds)); } } }

10. The break And continue Statements • The break statement can be used to abnormally terminate a loop. • The use of the break statement in loops bypasses the normal mechanisms and makes the code hard to read and maintain. • It is considered bad form to use the break statement in this manner.

11. The continue Statement • The continue statement will cause the currently executing iteration of a loop to terminate and the next iteration will begin. • The continue statement will cause the evaluation of the condition in while and for loops. • Like the break statement, the continue statement should be avoided because it makes the code hard to read and debug.

12. Deciding Which Loops to Use • The while loop: • Performs the test before entering the loop • Use it where you do not want the statements to execute if the condition is false in the beginning. • The do-while loop: • Performs the test after entering the loop • Use it where you want the statements to execute at least one time. • The for loop: • Performs the test before entering the loop • Use it where there is some type of counting variable that can be evaluated.

13. Math.random() • Returns a randomvalue between 0 and 1. • Can be used to generate a number between x and y: • double r = Math.random()*(y-x) +x;

14. Math.round() • Rounds doubles to the nearest integer. • Example: • Math.round(Math.random()*(y-x))+x; • Returns a long value between x and y.