Building Java Programs

Lab: While Loops

 

 

Lab goals

Goals for this problem set:

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);
}
How many times does the code in the while loop execute? 7
What output is produced by the overall code? 1, 2, 4, 8, 16, 32, 64, 128

Exercise 2: 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);
}
mystery(1);
1 0
mystery(6);
4 2
mystery(19);
16 4
mystery(39);
32 5
mystery(74);
64 6

Exercise 3: 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);
}
mystery2(25, 2);
12, 1
mystery2(32, 4);
0
mystery2(10345, 10);
1034, 103, 10, 3
mystery2(63, 2);
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);
}
mystery3(19);
19 0
mystery3(42);
21 1
mystery3(48);
3 4
mystery3(40);
5 3
mystery3(64);
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);
}
mystery4(2);
1 2
mystery4(5);
1 5
mystery4(24);
4 3
mystery4(28);
3 7

Exercise : digitSum

Exercise 7: 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

Exercise : printFactors