Unit 4: Practice

Note on all problems
Practice
1. Grades
2. Linear Search
3. Max
4. Prime Number
5. Sum Row
Challenges

Note on all problems

If you can write JavaDoc comments, do so for all of the methods you write!

Practice

Grades

methods

return

if statement

Create a class called Grades which contains your solution code.

Create a method called printGrade which takes the test score of a student and prints the letter grade. It does not return anything. Call printGrade in the main method with 3 different scores.

Create another method called getGrade which takes the test score of a student and returns the grade, a char. Print the result of calling printGrade in the main method with 3 different scores.

Template Code

public class Grades {
    public static void main(String[] args) {
        // write your code here
    }

    // write methods here
}

Solution

Linear Search

methods

arrays

for loops

return

Create a class called LinearSearch which contains your solution code.

Create a method called search (method signature is given in template code) that returns the index of the item in the list if the key matches it and -1 otherwise. Call the method to test it in the main method. (Print the value that is returned.)

Example Input

Array: { 5, 1, 2, 4, 3 } Key: 3

Example Output

Template Code

public class LinearSearch {
    public static void main(String[] args) {
        // write your code here
    }

    public static int search(int[] list, int key) {
        // write your code here
    }
}

Solution

Max

methods

if statements

return

Create a class called Max which contains your solution code.

Create a method called max which has 2 parameters, which are numbers (both double). Return the maximum of those 2 numbers. Call the method in the main method and print the result of calling it on 3 sets of 2 numbers.

Template Code

public class Max {
    public static void main(String[] args) {
        // write your code here
    }

    // write your method here
}

Solution

Prime Number

methods

if statements

loops

return

Create a class called PrimeNumber which contains your solution code.

Create a method called isPrime that returns true if the number (int) given as an argument is prime and false otherwise. Call it in the main method and print the result of that call.

Template Code

public class PrimeNumber {
    public static void main(String[] args) {
        // write your code here
    }

    // write your method here
}

Solution

Sum Row

methods

for loops

return

Create a class called SumRow which contains your solution code.

Given a 2D array of integers, print the sum of each row. Note: The subarrays are not necessarily of equal length. To do this, create a method called sumRow (see template code for the method signature). Test sumRow by calling it in the main method. You should use a loop to iterate through each array in a 2D array and call sumRow on the current inner array.

Example Input Array:

int[][] numbers = {
    { 1, 2, 3, 4, 5 },
    { 1, 2, 3, 4 },
    { 1, 2, 3 },
    { 1, 2 },
    { 1 },
};

Example Output

Sum of row 0: 15
Sum of row 1: 10
Sum of row 2: 6
Sum of row 3: 3
Sum of row 4: 1

Template Code

public class SumRow {
    public static void main(String[] args) {
        // write your code here
    }

    public static int sumRow(int[][] list, int rowIndex) {
        // write your code here
    }
}

Solution

Challenges

Digit Sum

Scanner

while loops

methods

arithmetic operators

return

Create a class called DigitSum which contains your solution code.

Write a method called sumDigits that computes the sum of the digits in a positive integer. The method header is in the template code below.

The main method will repeatedly ask for a number, call sumDigits with that number, and print the result, until the user enters -1.

Example Output

Enter a positive integer, or -1 to quit: 12345 [Enter ↩]
Total of the sum of the digits in the number: 15
Enter a positive integer, or -1 to quit: 1000 [Enter ↩]
Total of the sum of the digits in the number: 1
Enter a positive integer, or -1 to quit: 0 [Enter ↩]
Total of the sum of the digits in the number: 0
Enter a positive integer, or -1 to quit: 34 [Enter ↩]
Total of the sum of the digits in the number: 7
Enter a positive integer, or -1 to quit: -1 [Enter ↩]

Template Code

public class DigitSum {
    public static void main(String[] args) {
        // write your code here
    }

    public static int sumDigits(long n) {
        // write your code here
    }
}

Solution

Find Closest Pair

2D arrays

methods

for loops

nested loops

Math class

Create a class called FindClosestPair which contains your solution code.

Given a 2D double array of (x, y) coordinates, find the closest pair of points. Print the coordinates of that pair and the distance between them.

Hint: Create multiple methods to break the problem into parts.

distance - returns the distance between 2 points
findClosestPair - returns a 2D double array containing the 2 closest points
printPair - prints the 2 closest points and the distance between them in a readable way

See the template code below for method signatures.

Example Input

double[][] coordinates = {
    { -1, 3 },
    { -1, -1 },
    { 1, 1 },
    { 2, 0.5 },
    { 2, -1 },
    { 3, 3 },
    { 4, 2 },
    { 4, -0.5 },
};

Example Output

The closest pair is:
(1.000000, 1.000000) and (2.000000, 0.500000)
The distance between those points is 1.118033988749895

Template Code

public class FindClosestPair {
    public static void main(String[] args) {
        // write your code here
    }

    /** Returns the distance between 2 points */
    public static double distance(double x1, double y1, double x2, double y2) {
        // write your code here
    }

    /** Returns a 2D double array containing the 2 closest points */
    public static double[][] findClosestPair(double[][] coordinates) {
        // write your code here
    }

    /**
     * Prints the 2 closest points and
     * the distance between them in a readable way
     */
    public static void printPair(double[][] pair) {
        // write your code here
    }
}

Solution

Matrix Multiplication

2D arrays

methods

arithmetic operators

Scanner

Create a class called MatrixMult which asks the user to enter values for 2 matrices A, B and then multiplies them (if possible).

If AB exists, the program displays the product matrix C. If AB doesn’t exist, an error message is displayed.

You can read more about matrix multiplication here.

Hint: Break the problem into multiple methods.

multiplyMatrix - returns a 2D array which is the result of multiplying 2 matrices
getMatrix - returns a 2D array which represents a matrix from user input

See the template code below for method signatures.

Example Output

Matrix A
Enter number of rows: 3 [Enter ↩]
Enter number of columns: 2 [Enter ↩]
Enter row 1: 1 2 [Enter ↩]
Enter row 2: 3 4 [Enter ↩]
Enter row 3: 5 6 [Enter ↩]

Matrix B
Enter number of rows: 2 [Enter ↩]
Enter number of columns: 2 [Enter ↩]
Enter row 1: 7 8 [Enter ↩]
Enter row 2: 9 10 [Enter ↩]

Matrix C is
25.0 28.0
57.0 64.0
89.0 100.0

Template Code

public class MatrixMult {
    public static void main(String[] args) {
        // write your code here
    }

    /**
     * Returns a 2D array which is the
     * result of multiplying 2 matrices
     */
    public static double[][] multiplyMatrix(double[][] a, double[][] b) {
        // write your code here
    }

    /**
     * Returns a 2D array which represents
     * a matrix from user input
     */
    public static double[][] getMatrix() {
        // write your code here
    }
}

Solution

Merge

methods

arrays

loops

if statements

Scanner

Create a class called Merge which asks the user to enter 2 lists of integers ordered from least to greatest, then prints each list and a merged list (with duplicates).

Create 3 methods:

merge - Merges 2 arrays of integers that are sorted in ascending order
getArray - Prompts user to enter elements of an array of integers
printArray - Prints the array like so: [ 1 2 3 4 5 … n ]

See the template code below for method signatures.

You might find this animation to be helpful when figuring out how to merge the arrays.

Example Output

Enter size of list: 4 [Enter ↩]
Enter items in list: 1 3 6 10 [Enter ↩]
Enter size of list: 3 [Enter ↩]
Enter items in list: 2 4 7 [Enter ↩]
list1 is [ 1 3 6 10 ]
list2 is [ 2 4 7 ]
The merged list is [ 1 2 3 4 6 7 10 ]

Template Code

public class Merge {
    public static void main(String[] args) {
        // write your code here
    }

    /**
     * Merges 2 arrays of integers
     * that are sorted in ascending order
     */
    public static int[] merge(int[] list1, int[] list2) {
        // write your code here
    }

    /**
     * Prompts user to enter elements
     * of an array of integers
     */
    public static int[] getArray() {
        // write your code here
    }

    /**
     * Prints the array like so:
     * [ 1 2 3 4 5 ... n ]
     */
    public static void printArray(int[] arr) {
        // write your code here
    }
}

Solution

Selection Sort

arrays

methods

for loop

nested loop

if statement

Create a class called SelectionSort which contains your solution code.

Create a method called sort which sorts an array of integers using the Selection Sort algorithm.

In the main method, call the sort method on an unsorted array of integers. Print the unsorted array and then the sorted array.

Example Output

5 23 -25 0 -14 100 34
-25 -14 0 5 23 34 100

Template Code

public class SelectionSort {
    public static void main(String[] args) {
        // write your code here
    }

    /** Sorts an array using Selection Sort */
    public static void sort(int[] list) {
        // write your code here
    }
}

Solution

Unit 4: Practice

Table of Contents

Note on all problems

Practice

Grades

Linear Search

Max

Prime Number

Sum Row

Challenges

Digit Sum

Find Closest Pair

Matrix Multiplication

Merge

Selection Sort