Building Java Programs

Lab: While Loops

Except where otherwise noted, the contents of this document are Copyright 2019 Stuart Reges and Marty Stepp.

Lab goals

Goals for this problem set:

• use while loops for indefinite repetition
• practice the fencepost and sentinel loop patterns

while Loops

A while loop repeats indefinitely until a given condition is met.

while (test) {
    statement(s);
}

Example:

int num = 1;
while (num < 5) {
    System.out.print(n + " ");     // output: 1 2 3 4
    n++;
}

Exercise : while loop basics

Consider the following loop.

int x = 1;
System.out.print(x);
while (x < 100) {
    x = x + x;
    System.out.print(", " + x);
}

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery(int x) {
    int y = 1;
    int z = 0;
    while (2 * y <= x) {
        y = y * 2;
        z++;
    }
    System.out.println(y + " " + z);
}

1 0

4 2

16 4

32 5

64 6

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery2(int x, int y) {
    int z = 0;
    while (x % y != 0) {
        x = x / y;
        z++;
        System.out.print(x + ", ");
    }

    System.out.println(z);
}

12, 1

0

1034, 103, 10, 3

31, 15, 7, 3, 1, 0, 6

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery3(int x) {
    int y = 0;
    while (x % 2 == 0) {
        y++;
        x = x / 2;
    }
    System.out.println(x + " " + y);
}

19 0

21 1

3 4

5 3

1 6

Exercise : while loop mystery

Fill in the boxes at right with the output produced by each method call.

public static void mystery4(int n) {
    int x = 1;
    int y = 2;
    while (y < n) {
        if (n % y == 0) {
            n = n / y;
            x++;
        } else {
            y++;
        }
    }
    System.out.println(x + " " + n);
}

1 2

1 5

4 3

3 7

Exercise : digitSum

• Write a method named digitSum that accepts an integer as a parameter and returns the sum of the digits of that number. For example, the call digitSum(29107) returns 2+9+1+0+7 or 19. For negative numbers, return the same value that would result if the number were positive. For example, digitSum(-456) returns 4+5+6 or 15. The call digitSum(0) returns 0.
• (Hint: This is a cumulative algorithm. To extract a digit from a number, use / 10 and % 10 operations.)

Exercise : ProcessName2

Modify your previous ProcessName program so that it re-prompts until the user types a name that is at least 5 letters total in length and has at least one space in it. Example:

Type your name: Joe
Error, must be at least 5 chars with a space.
Type your name: O K!
Error, must be at least 5 chars with a space.
Type your name: what
Error, must be at least 5 chars with a space.
Type your name: Tyler Durden
Your name is: Durden, T.

Fencepost Loops

A fencepost loop is a common algorithmic pattern where you want to perform N tasks with N-1 things between them. It's like a fence with N posts with N-1 wires between the posts.

To achieve this, place one "post" outside your loop, then alternate between "wires" and "posts" inside the loop.

Example:

System.out.print(1);                 // |==|==|==|==| fence
for (int i = 2; i <= 5; i++) {
    System.out.print(".." + i);      // 1..2..3..4..5
}

Exercise : printLetters

• Write a method named printLetters that takes a String as its parameter and that prints the letters of the String, separated by dashes. For example, the call of printLetters("Rabbit") should print:

  R-a-b-b-i-t
  

• (Hint: This is a fencepost problem. Remember that each String object has a length method that tells you how many characters are in the String and a charAt method that gets you individual characters of the String.)

Exercise : printFactors

• Write a method named printFactors that accepts an integer as its parameter and uses a fencepost loop to print the factors of that number, separated by the word "and". For example, the call printFactors(24) should print the following output:

  1 and 2 and 3 and 4 and 6 and 8 and 12 and 24
  

• You may assume that the parameter value passed is greater than 0.
• (Hint: This is an example of a fencepost problem.)