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While loops

1public class WhileLoops {
2  public static void main(String[] args) {
3    // A different type of loop
4    int i = 1;
5    while(i < 11) {
6      System.out.println(i);
7      i++;
8    }
9    System.out.println("End: " + i);
10
11    // The condition can be anything
12    i = 1;
13    while(i % 5 != 0) {
14      System.out.println(i);
15      i++;
16    }
17    System.out.println("End: " + i);
18
19    i = 10;
20    while(i > 0) {
21      System.out.println(i);
22      i--;
23    }
24    System.out.println("End: " + i);
25  }
26}

Explanation

  1. In this exercise, we introduce while loops, which are another kind of loop. They are like for loops, but they only have one condition that has to be met, and no initialisation or increment. That is why variables used in the condition are defined beforehand.

  2. Like for loops, the while loops continue until the condition specified in parentheses is no longer met.

  3. The first while loop repeats until i equals 11, whereas the second while loop repeats until i is divisible by 5.

  4. In the third while loop, i is decremented. It continues to run until i becomes 0.

  5. While loops are always in the form:

    while(CONDITION) {
 BODY;
}

  6. Note: You must remember to change i when you code a while loop. Imagine line 7 were omitted - the value of i would never change and the while loop's condition would always be true. This would create an infinite while loop, and for now must be avoided.

Exercises

Note: These are the same exercises as in Part VI, but using while loops.

  1. Write a program, Ex7B, that uses a while loop to produce the following sequence: 1 -2 4 -8 16 -32, stopping once it gets to 2000.

  2. Write a program, Ex7C that uses a while loop to produce the following sequence: 1 3 6 10 15 21 28 36. Hint: Use a counter to store the last number you've printed out, and think about what number to start at.

  3. Write a program, Ex7D, that prints out all the multiples of 9 between 0 and 108.

  4. Write a program, Ex7E, that prints out the sum of all the multiples of 9 between 0 and 108.