What Part of a Loop Continues Until a Special Value is Input
The preceding example executes the loop ten times. If you want the user to decide whether to take another question, you can use a confirmation dialog to control the loop. A confirmation dialog can be created using the following statement:
When a button is clicked, the method returns an option value. The value is JOptionPane.YES_OPTION ( 0 ) for the Yes button, JOptionPane.NO_OPTION ( 1 ) for the No button, and JOptionPane.CANCEL_OPTION ( 2 ) for the Cancel button. For example, the following loop continues to execute until the user clicks the No or Cancel button.
int option = 0;
while (option == JOptionPane.YES_OPTION) {
System.out.println("continue loop");
option = JOptionPane.showConfirmDialog(null, "Continue?");
}
Java: Controlling a Loop with a Sentinel Value
Another common technique for controlling a loop is to designate a special value when reading and processing a set of values. This special input value, known as a sentinel value, signifies the end of the loop.
writes a program that reads and calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input. Do you need to declare a new variable for each input value? No. Just use one variable named data (line 9) to store the input value and use a variable named sum (line 12) to store the total. Whenever a value is read, assign it to data and added to sum (line 14) if it is not zero.
Example: SentinelValue.java
1 import javax.swing.JOptionPane;
2
3 public class SentinelValue {
4 /** Main method */
5 public static void main(String[] args) {
6 // Read an initial data
7 String dataString = JOptionPane.showInputDialog(
8"Enter an int value:\n(the program exits if the input is 0)");
9 int data = Integer.parseInt(dataString);
10
11 // Keep reading data until the input is 0
12 int sum = 0;
13 while (data != 0 ) {
14 sum += data;
15
16 // Read the next data
17 dataString = JOptionPane.showInputDialog(
18 "Enter an int value:\n(the program exits if the input is 0)");
19 data = Integer.parseInt(dataString);
20 }
21
22 JOptionPane.showMessageDialog(null, "The sum is " + sum);
23 }
24 }
A sample run of the program is shown in. If data is not 0 , it is added to the sum (line 14) and the next items of input data are read (lines 12–19). If data is 0 , the loop body is no longer executed and the while loop terminates. The input value 0 is the sentinel value for this loop. Note that if the first input read is 0 , the loop body never executes, and the resulting sum is 0 .
The program uses a while loop to add an unspecified number of integers.
Caution
| | Don't use floating-point values for equality checking in a loop control. Since floating-point values are approximations for some values, using them could result in imprecise counter values and inaccurate results. This example uses int value for data . If a floating-point type value is used for data , (data != 0) may be true even though data is exactly 0 . Here is a good example provided by a reviewer of this book: // data should be zero double data = Math.pow(Math.sqrt(2), 2) – 2; if (data == 0) System.out.println("data is zero"); else System.out.println("data is not zero"); Like pow , sqrt is a method in the Math class for computing the square root of a number. The variable data in the above code should be zero, but it is not, because of rounding-off errors. |
The do-while Loop
The do-while loop is a variation of the while loop. Its syntax is given below:
do {
// Loop body;
Statement(s);
} while (loop-continuation-condition); .
The loop body is executed first. Then the loop-continuation-condition is evaluated. If the evaluation is true , the loop body is executed again; if it is false , the do-while loop terminates. The major difference between a while loop and a do-while loop is the order in which the loop-continuation-condition is evaluated and the loop body executed.
The while loop and the do-while loop have equal expressive power. Sometimes one is a more convenient choice than the other. For example,
TestDo.java
1 import javax.swing.JOptionPane;
2
3 public class TestDoWhile {
4 /** Main method */
5 public static void main(String[] args) {
6 int data;
7 int sum = 0;
8
9 // Keep reading data until the input is 0
10 do {
11 // Read the next data
12 String dataString = JOptionPane.showInputDialog(null,
13 "Enter an int value:\n(the program exits if the input is 0)",
14 "TestDo", JOptionPane.QUESTION_MESSAGE);
15
16 data = Integer.parseInt(dataString);
17
18 sum += data;
19 } while (data != 0);
20
21 JOptionPane.showMessageDialog(null, "The sum is " + sum,
22 "TestDo", JOptionPane.INFORMATION_MESSAGE);
23 }
24 }
Source: https://lessonsonjava.blogspot.com/2014/07/java-controlling-loop-with-confirmation.html
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