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
for (int count = 1; count <= days; count++) {
input = JOptionPane.showInputDialog("Enter the ” +“sales for 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:
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.