1136 CS Lab 6
While, Do-While and For Loops
While Loops If statements allow the program to skip the execution of a statement or choose between one of two statements to be executed based on the value of a Boolean expression. In contrast, the while statement allows a program to continue executing a statement as long as the value of a Boolean expression is true. When the Boolean expression becomes false, the execution of the program continues with the statement immediately following the while statement. What would happen in this program if value were initialized to 1 instead of 0?
Scanner keyboard = new Scanner(System.in()); int value = 0; while (value >= 0) { System.out.println( "Please enter a number."); value = keyboard.nextInt(); } System.out.println("You entered a negative number");
For obvious reasons, the while statement is called a loop. The statements that are being executed within the loop are called the body of the loop. There are three basic steps that must be done for a loop to execute properly. 1. The variables in the expression (value in the above example) must be initialized before the while statement is executed the first time. 2. The variables in the expression must test the status of the expression correctly so that the body of the loop executes when it is supposed to and terminates at the proper time. 3. The variables in the expression must be updated within the loop. If the variables are not updated, the loop never stops executing. This is called an infinite loop. For example, what is the output of the following segment of code?
int counter = 0; int sum = 0; while (counter <= 10) { sum = sum + 1; System.out.println("Loop " + counter); } System.out.println("Sum is " + sum);
The answer is Loop ... forever. Since counter is never incremented inside the body of the loop, the loop runs forever and the statement following the loop is never
executed. You can tell when you're inside an infinite loop because the program never stops running and in this statement because the cout statement inside the loop never prints anything but "Loop 0". If you find yourself in this situation, press <Ctrl> and <Break> (which is at the top right of your keyboard) at the same time. This will "break you out" of your program. Usually, a light blue line will appear somewhere inside your infinite loop after returning to the editor. Other times, the loop will actually crash your computer, so ALWAYS remember to save your program before you run it. Do-While Loops Do-while loops work the same as while loops do-while EXCEPT loops check the Boolean expression after the statements inside the loop are executed for the first time. That means the body of the loop will always be executed at least once. Why is it not necessary to set value equal to zero before the following loop?
Scanner keyboard = new Scanner(System.in()); int value; do { System.out.print("Please enter a number: "); value = keyboard.nextInt(); // do you want to print // something for >= 0 // inputs? } while (value >= 0); // this loop continues until a // negative number is entered. System.out.println("You entered a negative number");
For Loops The for loop is designed specifically to allow the loop to run a set number of times. So, if we want to print "Hello World" exactly 10 times, we would use a for loop. For example,
for (counter = 1; counter <= 10; counter++) System.out.println("Hello World");
Here, counter, the loop control variable, is initialized to 1. While counter is less than or equal to 10, the block of code is executed, and counter is incremented by 1 (using the post-increment operator). The loop is executed with counter equal to the initial value, the final value and all values in between. Many programmers begin their loop counters at zero (for reasons you will see later). For now, remember that the following loop is equivalent to the previous loop. If you're not sure this is true, count the number of times each loop is executed on paper.
for (counter = 0; counter < 10; counter++) System.out.println("Hello World");
Exercise 1:
Write a program with the .java file called Lab6_Ex1 that requests the user to enter a positive integer between 1 and 100. The program then should not accept any number outside of the range of 1 to 100 but should issue an error message and reread the number. Display the number accepted by your program on the monitor. Exercise 2: Write a program with the .java file called Lab6_Ex2 that requests the user to enter two integers. The program then should calculate and report the sum of all the integers between and including the two integers. Assume that the smaller integer is entered first. For instance, if the user enters 2 and 9, the program would report that the sum of all the integers from 2 through 9 is 44. Exercise #3 Write a program with the .java called Lab6_Ex3 that asks the user to type in numbers. After each entry, the program reports the cumulative sum of the entries to date. The program terminates when the user enters a zero. Please upload and submit your .java and output files to WEBCT for all exercises.