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Chapter 4
Loops
4.1 Introduction
- Loops are structures that control repeated executions of a block of statements.
- Java provides a powerful control structure called a loop , which controls how many times an operation or a sequence of operation is performed in succession.
- Java provides three types of loop statements while loops, do-while loops, and for loops.
4.2 The while Loop
- The syntax for the while loop is as follows:
while (loop-continuation-condition) { // loop-body Statement(s); }
- The braces enclosing a while loop or any other loop can be omitted only if the loop body contains one or no statement. The while loop flowchart is in Figure (a).
- The loop-continuation-condition, a Boolean expression, must appear inside the parentheses. It is always evaluated before the loop body is executed.
- If its evaluation is true , the loop body is executed; if its evaluation is false, the entire loop terminates, and the program control turns to the statement that follows the while loop.
- For example, the following while loop prints Welcome to Java! 100 times.
int count = 0; while (count < 100) { System.out.println("Welcome to Java!"); count++; }
FIGURE 4.1 The while loop repeatedly executes the statements in the loop body when the loop-continuation-condition evaluates to true.
Caution
- Make sure that the loop-continuation-condition eventually becomes false so that the program will terminate.
- A common programming error involves infinite loops.
Loop Continuation Condition? true Statement(s) (loop body)
false (count < 100)?
true System.out.println("Welcome to Java!"); count++;
false
(A) (B)
count = 0;
4.2.3 Example: An Advanced Math Learning Tool (Page 121)
- The Math subtraction learning tool program generates just one question for each run. You can use a loop to generate questions repeatedly. This example gives a program that generates five questions and reports the number of the correct answers after a student answers all five questions.
- LISTING 4.3 SubtractionQuizLoop.java
import java.util.Scanner;
public class SubtractionQuizLoop { public static void main(String[] args) { final int NUMBER_OF_QUESTIONS = 5; // Number of questions int correctCount = 0; // Count the number of correct answers int count = 0; // Count the number of questions long startTime = System.currentTimeMillis(); String output = ""; // output string is initially empty Scanner input = new Scanner(System.in);
while (count < NUMBER_OF_QUESTIONS) { // 1. Generate two random single-digit integers int number1 = (int)(Math.random() * 10); int number2 = (int)(Math.random() * 10);
// 2. If number1 < number2, swap number1 with number if (number1 < number2) { int temp = number1; number1 = number2; number2 = temp; }
// 3. Prompt the student to answer “What is number1 – number2?” System.out.print( "What is " + number1 + " - " + number2 + "? "); int answer = input.nextInt();
// 4. Grade the answer and display the result if (number1 - number2 == answer) { System.out.println("You are correct!"); correctCount++; } else System.out.println("Your answer is wrong.\n" + number
- " - " + number2 + " should be " + (number1 - number2));
// Increase the count count++;
output += "\n" + number1 + "-" + number2 + "=" + answer + ((number1 - number2 == answer)? " correct" : " wrong"); }
long endTime = System.currentTimeMillis(); long testTime = endTime - startTime;
System.out.println("Correct count is " + correctCount + "\nTest time is " + testTime / 1000 + " seconds\n" + output); } }
What is 9 - 2? 7 Your answer is correct!
What is 3 - 0? 3 Your answer is correct!
What is 3 - 2? 1 Your answer is correct!
What is 7 - 4? 4 Your answer is wrong. 7 - 4 should be 3
What is 7 - 5? 4 Your answer is wrong. 7 - 5 should be 2
Correct count is 3 Test time is 1021 seconds Ï 9-2=7 correct 3-0=3 correct 3-2=1 correct 7-4=4 wrong 7-5=4 wrong
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. Consider the following code for computing 1 + 0.9 + 0.8 + ... + 0.1:
double item = 1; double sum = 0; while (item != 0) { // No guarantee item will be 0 sum += item; item -= 0.1; } System.out.println(sum);
- Variable item starts with 1 and is reduced by 0.1 every time the loop body is executed. The loop should terminate when item becomes 0. However, there is no guarantee that item will be exactly 0 , because the floating-point arithmetic is approximated. This loop seems OK on the surface, but it is actually an infinite loop.
4.2.5 Input and Output Redirections
- If you have a large number of data to enter, it would be cumbersome to type from the key board. You may store the data separated by whitespaces in a text file, say input.txt, and run the program using the following command:
java SentinelValue < input.txt
- This command is called input redirection. The program takes the input from the file input.txt rather thatn having the user to type the data from the keyboard at runtime.
- There is output redirection which sends the output to a file rather than displaying it on the console. The command for output redirection is:
java ClassName > output.txt
- Input and output redirection can be used in the same command. For example, the following command gets input from input.txt and sends output to output.txt:
java SentinelValue < input.txt > output.txt
- You can rewrite the TestWhile program shown previously as follows:
- LIST 4.5 TestDoWhile.java ( Page 125)
import java.util.Scanner;
public class TestDoWhile { /** Main method */ public static void main(String[] args) { int data; int sum = 0;
// Create a Scanner Scanner input = new Scanner(System.in);
// Keep reading data until the input is 0 do { // Read the next data System.out.print( "Enter an int value (the program exits if the input is 0): "); data = input.nextInt();
sum += data; } while (data != 0);
System.out.println("The sum is " + sum); } }
Enter an int value (the program exits if the input is 0): 2 Enter an int value (the program exits if the input is 0): 3 Enter an int value (the program exits if the input is 0): 4 Enter an int value (the program exits if the input is 0): 0 The sum is 9
4.4 The for Loop
- The syntax of a for loop is as shown below.
for (initial-action; loop-continuation-condition; action-after-each-iteration) { //loop body; Statement(s); }
- The for loop statement starts with the keyword for , followed by a pair of parentheses enclosing initial- action, loop-continuation-condition , and action-after-each-iteration, and the loop body, enclosed inside braces.
- initial- action, loop-continuation-condition , and action-after-each-iteration are separated by semicolons ;
- A for loop generally uses a variable to control how many times the loop body is executed and when the loop terminates.
- This variable is referred to as a control variable. The initial-action often initializes a control variable, the action-after-each-iteration usually increments or decrements the control variable, and the loop-continuation-condition tests whether the control variable has reached a termination value.
- Example: The following for loop prints Welcome to Java! 100 times.
int i; for (i = 0; i < 100; i++) { System.out.println("Welcome to Java! ”); }
FIGURE 4.3 A for loop performs an initial action one, then repeatedly executes the statements in the loop body, and performs an action after an iteration when the loop- continuation-condition evaluates as true
Loop Continuation Condition? true Statement(s) (loop body)
false
(A)
Action-After-Each-Iteration
Initial-Action
(i < 100)?
true System.out.println( "Welcome to Java");
false
(B)
i++
i = 0
4.5 Which Loop to Use?
- The three forms of loop statements, while , do , and for , are expressively equivalent; that is, you can write a loop in any of these three forms.
- For example, a while loop in (a) in the following figure can always be converted into the following for loop in (b):
- A for loop in (a) in the following figure can generally be converted into the following while loop in (b) except in certain special cases.
Recommendations
- The author recommends that you use the one that is most intuitive and comfortable for you.
- In general, a for loop may be used if the number of repetitions is known , as, for example, when you need to print a message 100 times.
- A while loop may be used if the number of repetitions is not known , as in the case of reading the numbers until the input is 0.
- A do-while loop can be used to replace a while loop if the loop body has to be executed before testing the continuation condition.
while (loop-continuation-condition) { // Loop body } (a)
Equivalent
(b)
for ( ; loop-continuation-condition; ) // Loop body }
for (initial-action; loop-continuation-condition; action-after-each-iteration) { // Loop body; } (a)
Equivalent
(b)
initial-action; while (loop-continuation-condition) { // Loop body; action-after-each-iteration; }
Caution
- Adding a semicolon at the end of the for clause before the loop body is a common mistake, as shown below:
for (int i = 0; i < 10; i++); // Logic Error (‘;’) { System.out.println("i is " + i); }
- Similarly, the following loop is also wrong:
int i=0; while (i<10); // Logic Error (‘;’) { System.out.println("i is " + i); i++; }
- In the case of the do loop, the following semicolon is needed to end the loop.
int i=0; do { System.out.println("i is " + i); i++; } while (i<10); // Correct, The semicolon is needed
4.7 Minimizing Numerical Errors
- Numeric errors involving floating-point numbers are inevitable.
- Write a program that sums a series that starts with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows 0.01 + 0.02 + 0.03 and so on.
- LISTING 4.7 TestSum.java ( Page 130)
public class TestSum { public static void main(String[] args) { // Initialize sum float sum = 0;
// Add 0.01, 0.02, ..., 0.99, 1 to sum for (float i = 0.01f; i <= 1.0f; i = i + 0.01f) sum += i;
// Display result System.out.println("The sum is " + sum); } }
o The for loop repeatedly adds the control variable i to the sum. This variable, which begins with 0.01, is incremented by 0.01 after each iteration. The loop terminates when i exceeds 1.0. o The exact sum should be 50.50 , but the answer is 50.499985. The result is not precise because computers use a fixed number of bits to represent floating-point numbers, and thus cannot represent some floating-point number exactly.
- If you change float in the program to double as follows, you should see a slight improvement in precision because a double variable takes 64 bits, whereas a float variable takes 32 bits.
public class TestSum { public static void main(String[] args) { // Initialize sum double sum = 0;
// Add 0.01, 0.02, ..., 0.99, 1 to sum for (double i = 0.01; i <= 1.0; i = i + 0.01) sum += i;
// Display result System.out.println("The sum is " + sum); } }
The sum is 50.
The sum is 49.
- To fix the problem: Using an integer count to ensure that all the numbers are processed.
public class TestSum { public static void main(String[] args) { // Initialize sum double sum = 0; double currentValue = 0.01;
// Add 0.01, 0.02, ..., 0.99, 1 to sum for (int count = 0; count < 100; count++) { sum += currentValue; currentValue += 0.01; }
// Display result System.out.println("The sum is " + sum); } }
The sum is 50.
4.8.2 Problem: Predicating the Future Tuition (Page 133)
- Problem: Suppose that the tuition for a university is $10,000 this year and tuition increases 7% every year. In how many years will the tuition be doubled?
double tuition = 10000; int year = 1 // Year 1 tuition = tuition * 1.07; year++; // Year 2 tuition = tuition * 1.07; year++; // Year 3 tuition = tuition * 1.07; year++; // Year 4 ...
- LISTING 4.9 FutureTuition.java ( Page 133 )
public class FutureTuition { public static void main(String[] args) { double tuition = 10000; // Year 1 int year = 1; while (tuition < 20000) { tuition = tuition * 1.07; year++; }
System.out.println("Tuition will be doubled in "
Tuition will be doubled in 12 years
4.9 Keywords break and continue
- The break control immediately ends the innermost loop that contains it. It is generally used with an if statement.
- The continue control only ends the current iteration. Program control goes to the end of the loop body. This keyword is generally used with an if statement.
- The break statement forces its containing loop to exit.
- The continue statement forces the current iteration of the loop to end.
false
true
Statement(s)
Next Statement
Continue condition?
Statement(s)
continue
false
true
Statement(s)
Next Statement
Continuation condition?
Statement(s)
break
