We Keep Coding

How to create a Java matrix that consists of numbers that are the product of 2?

I'm trying to create a 2D int array of numbers that are the product of 2 via user input defining the length of the rows and columns, as the one below:
1 2 4 8
2 4 8 16
4 8 16 32
8 16 32 64
I've only got up to here, and cannot figure out how to make the matrix to start from 1 and to look like the one above. I'd appreciate your help on this one!
Scanner scan = new Scanner(System.in);
int n = Integer.parseInt(scan.nextLine());
int[][] matrix = new int[n][n];
matrix[0][0] = 1;
int temp = 1;
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
matrix[i][j] = temp * 2;
temp *= 2;
}
}
System.out.println(Arrays.deepToString(matrix));

I'd say that each element of the matrix would be:
matrix[i][j] = (int) Math.pow(2, i+j) ;
So, your loop would look like this:
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
matrix[i][j] = (int) Math.pow(2, i+j);
}
}

public static void main (String[] args) throws java.lang.Exception
{
Scanner scan = new Scanner(System.in);
int n = Integer.parseInt(scan.nextLine());
int[][] matrix = new int[n][n];
matrix[0][0] = 1;
int temp = 1;
for (int i = 0; i < n; i++) {
temp = (int)Math.pow(2, i);
for (int j = 0; j < n; j++) {
matrix[i][j] = temp;
temp *= 2;
}
}
for (int i = 0; i < n; i++) {
for (int j = 0; j < n; j++) {
System.out.print(matrix[i][j]+" ");
}
System.out.println();
}
}

Sum the elements of 2D array

I made 2D arrray which prints some random elements.
Now i need a method which calculates the sum of that elements but just elements below the main diagonal.
Here is my code...
class Init {
public static void main(String[] args) {
int n = 0;
int m = 0;
int aray[][];
Random random = new Random();
Scanner tastatura = new Scanner(System.in);
int[][] array = new int[n][m];
n = tastatura.nextInt();
m = tastatura.nextInt();
array = new int[n][m];
for (int i = 0; i < array.length; i++) {
for (int j = 0; j < array[i].length; j++) {
array[i][j] = random.nextInt(20);
}
}
for (int[] a : array) {
System.out.println(Arrays.toString(a));
}
}
}
I did it like this... Now i can sum, but when i try to multyply same numbers i am geting 0 Why is that?
Scanner scanner = new Scanner(System.in);
System.out.print("Unesite duzinu kolona i redova : ");
int rows = scanner.nextInt();
int columns = rows;
int[][] matrix = new int[rows][rows];
Random random = new Random();
System.out.println("Nasumicni/random brojevi su :");
for (int i = 0; i < matrix.length; i++) {
for (int j = 0; j < matrix[i].length; j++) {
matrix[i][j] = random.nextInt(20);
}
}
for (int[] a : matrix) {
System.out.println(Arrays.toString(a));
}
//here is the logic which sum those elements
int sum = 0;
for (int i = 1; i < rows; i++) {
for (int j = i - 1; j >= 0; j--) {
sum = sum + matrix[i][j];
}
}
System.out.println("\nMatrix is : ");
for (int i = 0; i < rows; i++) {
for (int j = 0; j < columns; j++) {
System.out.print(matrix[i][j] + " ");
}
System.out.println();
}
System.out.println("Proizvod elemenata ispod glavne dijagonale je: " + sum);

What about this?
int s = 0;
for(int i = 1; i < m; ++i)
for(int j = 0; j < i; ++j)
s += a[i][j];
This selectively loops through the elements below the main diagonal and sums them up, without looping through the entire matrix and making it lengthier.

The main diagonal of a matrix consists of those elements that lie on the diagonal that runs from top left to bottom right. But since you want those elements "below" the main diagonal, here is an algorithm I came up with for that.
int sum = 0;
for (int i = 0; i < n; i++)
for (int j = 0; j < m; j++)
if (i == j && (i + 1 < n))
{
int temp = i + 1;
while (temp < n)
{
sum += arr[temp][j];
temp++;
}
}
Also, you declare int[][] array multiple times. You need to declare it only once, after you get the values for n and m.

for(i=0;i
        for(j=0;j
        {
                if(j>i)
                    d1+=a[i][j];. // Above the diagon
                else
                    if(i>j)
                        d2+=a[i][j];. // Below the diagonal
        }
    

Two dimensional array to one dimensional array in Java

I have a two dimensional array and I fill it with scanner. I want to copy the elements that start with letter 'a' to a new one dimensional array without using ArrayList. Please advise on what I can do to get this code functioning properly. the question is how can I know the new array size while I don't know how many words start with letter a
Here is what I have so far:
import java.util.Scanner;
class Untitled {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
String[][] name = new String[2][2];
for (int i = 0; i < name.length; i++) {
for (int j = 0; j < name[i].length; j++) {
name[i][j] = input.next();
}
}
student(name);
}
public static void student(String[][] arr) {
int count = 0;
int c2 = -1;
String[] name2 = new String[count];
String temp = "";
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j].charAt(0) == 'a') {
c2++;
temp = arr[i][j];
name2[c2] = temp;
count++;
temp = "";
}
}//inner
}//outer
for (int i = 0; i < name2.length; i++) {
System.out.println(name2[i]);
}
}
}

A two dimensional arrray of size [n][n] is equal to one dimensional array of size n. If you want to copy them on proper place then you can use this formula, it is useful if you later want to copy these elements back to twodimensional array at proper places:
int v = i * n + j; // i and j your loops and n is length of rows or colums.
array[v] = array[i][j];
for in your codes it's like:
int v = 0;
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j].charAt(0) == 'a') {
v = i * arra.length +j;
name2[v] = arr[i][j];
count++;
Ok here is a working code:
public static void main(String [] args) {
Scanner input = new Scanner(System.in);
String[][] name = new String[2][2];
System.out.println("Enter the name: ");
for (int i = 0; i < name.length; i++) {
for (int j = 0; j < name[i].length; j++) {
name[i][j] = input.next();
}
}
student(name);
}
public static void student(String[][] arr) {
int count = 0;
int v = 0;
String[] name2 = new String[arr.length*arr[0].length];
String temp = "";
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j].charAt(0) == 'a') {
v = i *+arr[0].length + j;
name2[v] = arr[i][j];
count++;
}
}//inner
}//outer
for (int i = 0; i < name2.length; i++) {
System.out.println(name2[i]);
}
System.out.println("printing without nulls");
//if you don't want null to be printed then do this:
for (int i = 0; i < name2.length; i++) {
if(name2[i] != null)
System.out.println(name2[i]);
}
}

I did it with two nested for loop one for indicating the array size and the other for filling the elements into the array, it does the work but is there any way to do this better
public static void student(String[][] arr) {
int size = 0;
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j].charAt(0) == 'a') {
size++;
}
}//inner
}//outer
String[] name2 = new String[size];
int count = 0;
for (int i = 0; i < arr.length; i++) {
for (int j = 0; j < arr[i].length; j++) {
if (arr[i][j].charAt(0) == 'a') {
name2[count] = arr[i][j];
count++;
}
}//inner
}//outer
for (int i = 0; i < name2.length; i++) {
System.out.println(name2[i]);
}

Filling a vertical matrix Java

I'm trying to fill a matrix vertically, but 1 row is missing. Can you help me ? There is the code. Maybe there is an easier way to fill a matrix verically, but i cant find it.
public static void main(String[]args){
Scanner input = new Scanner(System.in);
System.out.print("Enter the value of matrix: ");
int n = input.nextInt();
int [][] matrix = new int [n][n];
for (int i = 1; i < matrix.length; i++) {
matrix[0][i] = matrix[0][i -1] + n;
}
for(int i = 1; i < matrix.length; i++){
for (int j = 0; j < matrix.length; j++){
matrix[i][j] = matrix[i -1][j] + 1;
System.out.print(matrix[i][j] + " ");
}
System.out.println();
}
input.close();
}
Output:
Enter the value of matrix: 4
1 5 9 13
2 6 10 14
3 7 11 15

Your row is missing because you never printed it in your first loop (the one that is initializing your first line) - you should have a row of 0 4 10 12 at the beginning. But you could do it much easier with only one nested loop.

Try
public static void main(String[]args){
Scanner input = new Scanner(System.in);
System.out.print("Enter the value of matrix: ");
int n = input.nextInt();
int [][] matrix = new int [n][n];
matrix[0][0]=0; //you have forgotten the first value
for (int i = 1; i < matrix.length; i++) {
matrix[0][i] = matrix[0][i -1] + n;
//initializing the first line
}
for(int i = 1; i < matrix.length; i++){
for (int j = 0; j < matrix.length; j++){
matrix[i][j] = matrix[i -1][j] + 1;
}
// re-loop to display but this time start with i=0
for(int i = 0; i < matrix.length; i++){
for (int j = 0; j < matrix.length; j++){
System.out.print(matrix[i][j] + " ");
}
System.out.println();
}
input.close();
}

To fill a matrix vertically, you must loop through columns in the outer loop and through rows in the inner(nested) loop. For example:
for(int j = 0; j < matrix.length; j++) {
for(int i = 0; i < matrix.length; i++) {
matrix[i][j] = /* The value you want to fill */;
.../* Other stuff you wanna do */
}
}

There is an easier way of doing this:
keep a variable like count and iterate the matrix on columns first then rows:
int count = 1; // or 0 if you start with 0
int[][] a = new int[n][n];
for (int i = 0; i < n; i++)
for (int j = 0; j < n; j++) {
a[j][i] = count; // notice j first then i
count++;
}
After that you can easly print out the values:
for (int i = 0; i < n; i++)
for (int j = 0; j < n; j++)
System.out.println(a[i][j]);

Multiplying Muti-Dimensional Arrays

Been having a bit of trouble finishing this program involving two 2D arrays and multiplying them together. Now I was able to construct these arrays with set lengths and then using a number generator to create each array. As for a third array I was able to establish the length of the array, but when placing the three arrays into a method I am still having out of bounds issues.
public class arrayTest1{
public static void main ( String [] args){
int matrix1[][] = new int [5][2];
for (int i = 0; i < matrix1.length; i++)
for (int j = 0; j < matrix1[i].length; j++)
matrix1[i][j] = (int)(Math.random() * 1000);
System.out.println("The array 1 is: ");
for (int i = 0; i < matrix1.length; i++){
for (int j = 0; j < matrix1[i].length; j++){
System.out.print(matrix1[i][j]+" ");
}
System.out.println();
}
int matrix2[][] = new int [2][5];
for (int i = 0; i < matrix2.length; i++)
for (int j = 0; j < matrix2[i].length; j++)
matrix2[i][j] = (int)(Math.random() * 1000);
System.out.println("The array 2 is: ");
for (int i = 0; i < matrix2.length; i++){
for (int j = 0; j < matrix2[i].length; j++){
System.out.print(matrix2[i][j]+" ");
}
System.out.println();
}
int matrixSum[][] = new int [matrix1.length][matrix2[0].length];
matrixMulti(matrix1,matrix2,matrixSum);
System.out.println("The array mutliplied is: ");
for (int i = 0; i < matrixSum.length; i++){
for (int j = 0; j < matrixSum[i].length; j++){
System.out.print(matrixSum[i][j]+" ");
}
System.out.println();
}
}
public static void matrixMutli(int [][] m1, int [][] m2,int [][] totalMatrix){
for(int i = 0; i < m1.length; i++)
for(int j = 0; j < m2[0].length; j++)
for(int k = 0; k < totalMatrix.length; k++)
totalMatrix [i][j] += m1[i][k] * m2[k][j];
}
}

The ArrayIndexOutOfBounds is caused by the fact that your k variable goes from 0 to 5. It needs to go from 0 to 2. The problem is because of the line :
for(int k = 0; k < totalMatrix.length; k++)
The matrix totalMatrix is initialized with int matrixSum[][] = new int [matrix1.length][matrix2[0].length]; and matrix1.length is 5. So totalMatrix.length is 5.
To correct this you need to make sure that k is bounded by either matrix1[i].length or by matrix2.length. These two values need to be the same and you can choose either to be the bound for k.
So this is the code:
for(int i = 0; i < m1.length; i++)
for(int j = 0; j < m2[0].length; j++)
for(int k = 0; k < m2.length; k++)
totalMatrix [i][j] += m1[i][k] * m2[k][j];
Also consider adding code that checks whether m1[i].length == m2.length and throws an IllegalArgumentException if not. If you are still in trouble have a look at matrix multiplication.