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Trouble with java.lang.ArrayIndexOutOfBoundsException: [duplicate]

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What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?
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Closed 7 years ago.
I'm trying to compute the row and column sums of a 2D array but I'm always getting this error java.lang.ArrayIndexOutOfBoundsException: 2
It says there is a error in method computeRowSums but I cannot figure out why it would be array index out of bounds.
// RowColSums.java
// To compute the row and column sums of a 2D array.
import java.util.*;
public class RowColSums {
public static void main(String[] args) {
int row_size, col_size, row, col;
int[][] array2D;
Scanner sc = new Scanner(System.in);
System.out.println("Enter number of rows and columns: ");
row_size = sc.nextInt();
col_size = sc.nextInt();
array2D = new int[row_size][col_size];
System.out.println("Enter values for 2D array: ");
for(row = 0; row < row_size; row++)
for(col = 0; col < col_size; col++)
array2D[row][col] = sc.nextInt();
int[] rowSums = computeRowSums(array2D);
System.out.print("Row sums: ");
System.out.println(Arrays.toString(rowSums));
int[] colSums = computeColSums(array2D);
System.out.print("Column sums: ");
System.out.println(Arrays.toString(colSums));
}
public static int[] computeRowSums(int[][] arr)
{
int i, j;
int[] row_sum = new int[arr[0].length];
for(i = 0; i < arr[0].length; i++)
for(j = 0; j < arr.length; j++)
row_sum[i] += arr[i][j];
return row_sum;
}
public static int[] computeColSums(int[][] arr)
{
int z, r;
int[] col_sum = new int[arr.length];
for(z = 0; z < arr.length; z++)
for(r = 0; r < arr[0].length; r++)
col_sum[z] += arr[z][r];
return col_sum;
}
}

In computeRowSums you got the indices in the wrong order. It should be :
for(i = 0; i < arr[0].length; i++)
for(j = 0; j < arr.length; j++)
row_sum[i] += arr[j][i];
I'm assuming your 2D array has a different number of rows and columns, which explains the exception you got.

Sorting Two-Dimensional Array by Row

The requirement is to sort the rows of a two-dimensional array. I feel like my code is very close to being done, but I can't figure out why it isn't displaying the sorted array. I forgot to mention that we are not allowed to use the premade sorting methods. The problem is most likely in the sortRows method. Anyways, here's my code:
public class RowSorting
{
public static void main(String[] args)
{
double[][] numbers = new double[3][3];
double[][] number = new double[3][3];
int run = 0;
String answer = "";
while (run == 0)
{
Scanner input = new Scanner(System.in);
System.out.print("Enter a 3-by-3 matrix row by row: ");
for(int row = 0; row < numbers.length; row++)
{
for(int column = 0; column < numbers[row].length; column++)
{
numbers[row][column] = input.nextDouble();
}
}
for(int row = 0; row < numbers.length; row++)
{
for(int column = 0; column < numbers[row].length; column++)
{
System.out.print(numbers[row][column] + " ");
}
System.out.print("\n");
}
System.out.println("The sorted array is: \n");
number = sortRows(numbers);
for(int row = 0; row < number.length; row++)
{
for(int column = 0; column < number[row].length; column++)
{
System.out.print(number[row][column] + " ");
}
System.out.print("\n");
}
System.out.print("\nWould you like to continue the program (y for yes or anything else exits): ");
answer = input.next();
if(answer.equals("y"))
{
continue;
}
else
break;
}
}
public static double[][] sortRows(double[][] m)
{
for(int j = 0; j < m[j].length - 1; j++)
{
for(int i = 0; i < m.length; i++)
{
double currentMin = m[j][i];
int currentMinIndex = i;
for(int k = i + 1; k < m[j].length; k++)
{
if(currentMin > m[j][i])
{
currentMin = m[j][i];
currentMinIndex = k;
}
}
if(currentMinIndex != i)
{
m[currentMinIndex][j] = m[j][i];
m[j][i] = currentMin;
}
}
}
return m;
}
}

It looks like this block:
if(currentMin > m[j][i])
{
currentMin = m[j][i];
currentMinIndex = k;
}
Will never happen. Because you just assigned currentMin to m[j][i] two lines before it. I believe you want to use k in that if check. Something like
if (currentMin > m[j][k]){
currentMin = m[j][k];
currentMinIndex = k;
}

As cited per ergonaut, you have a problem with the codeblock
if(currentMin > m[j][i]) ...
and also
m[currentMinIndex][j] = m[j][i];
However, you also have a problem with your for-loops.
for(int j = 0; j < m[j].length - 1; j++) ...
for(int i = 0; i < m.length; i++) ...
Both of these are oddly structured. You probably want to swap these for-loops so that you don't throw index exceptions. This will also cause you address the indices in your code. And modify your j-index for-loop to include the entire range.

java performing array calculation [closed]

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Edit the question to include desired behavior, a specific problem or error, and the shortest code necessary to reproduce the problem. This will help others answer the question.
Closed 8 years ago.
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The code below will print two square matrices and I need them to perform multiplication between the two matrices but i cant seem to get that part working. I put a comment right before that block of code where the problem is. But for now all it prints is zeros. Iv been looking online at a lot of sites but cant seem to get mine to work.
import java.util.*;
import java.math.*;
public class Main {
public static void main(String[] args) {
//create the grid
final int rowWidth = 9;
final int colHeight = 9;
Random rand = new Random();
int [][] board = new int [rowWidth][colHeight];
//fill the grid
for (int row = 0; row < board.length; row++) {
for (int col = 0; col < board[row].length; col++) {
board[row][col] = rand.nextInt(10);
}
}
//display output
for(int i = 0; i < board.length; i++) {
for(int j = 0; j < board[i].length; j++) {
System.out.print(board[i][j] + " ");
//System.out.println();
}
System.out.println();
}
System.out.println();
int [][] board2 = new int [rowWidth][colHeight];
//fill the grid
for (int row2 = 0; row2 < board2.length; row2++) {
for (int col2 = 0; col2 < board[row2].length; col2++) {
board[row2][col2] = rand.nextInt(10);
}
}
//display output
for(int m = 0; m < board2.length; m++) {
for(int n = 0; n < board[m].length; n++) {
System.out.print(board[m][n] + " ");
}
System.out.println();
}
//error is somewhere here
int[][] calculationMultiplication = new int[rowWidth][colHeight];
for (int l = 0; l < rowWidth; l++) {
for (int t = 0; t < colHeight; t++) {
for (int z = 0; z < rowWidth; z++) {
calculationMultiplication[l][t] = calculationMultiplication[l][t] + board[l][z] * board2[z][t];
}
}
}
//display output
System.out.println("\nProduct of the 2 matrices is ");
for (int i = 0; i < calculationMultiplication.length; i++) {
for (int j = 0; j < calculationMultiplication[0].length; j++) {
System.out.print(calculationMultiplication[i][j] + " ");
}
System.out.println();
}
} //end of main
} //end of class Main

The problem is that you've not filled the board2 array, so all its elements will be 0. In the for loop, you should also assign random values to board2. You are doing it twice for the board array.
//fill the grid
for (int row2 = 0; row2 < board2.length; row2++) {
for (int col2 = 0; col2 < board2[row2].length; col2++) { // notice 'board2[row].length'
board2[row2][col2] = rand.nextInt(10);
}
}
You should do something similar in the for loops where you display the array:
//display output
for (int m = 0; m < board2.length; m++) {
for (int n = 0; n < board2[m].length; n++) { // notice 'board2[row].length'
System.out.print(board2[m][n] + " ");
}
System.out.println();
}

board2 is always zero.
You fill the wrong array here:
//fill the grid
for (int row2 = 0; row2 < board2.length; row2++) {
for (int col2 = 0; col2 < board[row2].length; col2++) {
board[row2][col2] = rand.nextInt(10);
}
}

Basically you just did simple copy paste mistakes that can be avoidable if you used methods. I'll tell you how but first here's how you can discover your mistake:
System.out.println(String.format("%d * %d = %d",board[l][z], board2[z][t], board[l][z] * board2[z][t]));
Add this just before doing the calculation for calculationMultiplication[l][t]. Try if you want...
Anyway getting back to your mistakes, you have populated your first matrix only, the second one contain zeros (verify your code in the for loop for inserting random numbers into the second matrix), therefore any number multiplied by zero is equal to zero.
I haven't looked at all your code, because it contain a lot of copy paste which confused you and could confuse any one as well, so here's a better way to avoid mistakes in printing the matrix and inserting random numbers:
Printing:
void printMatrix(int[][] matrix){
for(int row =0; row < numOfRows; row++){
for(int col =0; col < numOfCols; col++){
System.out.print(matrix[row][col]+" ");
}
System.out.println(); // new line
}
}
Inserting:
void insertMatrix(int[][] matrix){
for(int row =0; row < numOfRows; row++){
for(int col =0; col < numOfCols; col++){
matrix[row][col] = rand.nextInt(10); // rand must be declared outside any method
}
}
}
Putting all together:
import java.util.*;
public class Main {
// create the grid
final static int rowWidth = 9;
final static int colHeight = 9;
static Random rand;
public static void main(String[] args) {
rand = new Random();
int[][] board = new int[rowWidth][colHeight];
int[][] board2 = new int[rowWidth][colHeight];
int[][] calculationMultiplication = new int[rowWidth][colHeight];
// fill
insertMatrxi(board);
// display output
printMatrix(board);
System.out.println();
// fill
insertMatrxi(board2);
// display output
printMatrix(board2);
for (int l = 0; l < rowWidth; l++) {
for (int t = 0; t < colHeight; t++) {
for (int z = 0; z < rowWidth; z++) {
calculationMultiplication[l][t] += board[l][z] * board2[z][t];
}
}
}
// display output
System.out.println("\nProduct of the 2 matrices is ");
printMatrix(calculationMultiplication);
} // end of main
public static void printMatrix(int[][] matrix) {
for (int row = 0; row < rowWidth; row++) {
for (int col = 0; col < colHeight; col++) {
System.out.print(matrix[row][col] + " ");
}
System.out.println(); // new line
}
}
public static void insertMatrxi(int[][] matrix) {
for (int row = 0; row < rowWidth; row++) {
for (int col = 0; col < colHeight; col++) {
matrix[row][col] = rand.nextInt(10);
}
}
}
} // end of class Main

Reverse the rows of a 2d array

Yesterday I asked a very similar question and I kind of messed up with asking it.
I need to pass an array to a method and inside of that method I need to swap the rows around so if it's
1 2 3
3 2 1
2 1 3
it needs to be
3 2 1
1 2 3
3 1 2
With the code I have right now it swaps the last column to the first column spot correctly then it puts the column that's supposed to be last.
3 1 2
1 3 2
3 2 1
Also, it needs to stay a void because I need to be modifying the original array so I can't set it as a temp array but I can use a temp integer to store.
Here is the code I have right now that's sort of working
public static void reverseRows(int[][] inTwoDArray)
{
for (int row = 0; row < inTwoDArray.length; row++)
{
for (int col = 0; col < inTwoDArray[row].length; col++)
{
int tempHolder = inTwoDArray[row][col];
inTwoDArray[row][col] = inTwoDArray[row][inTwoDArray[0].length - 1];
inTwoDArray[row][inTwoDArray[0].length - 1] = tempHolder;
}
}
}
any help would be great, I'm running out of hair to pull out! Thanks!

First, how to reverse a single 1-D array:
for(int i = 0; i < array.length / 2; i++) {
int temp = array[i];
array[i] = array[array.length - i - 1];
array[array.length - i - 1] = temp;
}
Note that you must stop in half of your array or you would swap it twice (it would be the same one you started with).
Then put it in another for loop:
for(int j = 0; j < array.length; j++){
for(int i = 0; i < array[j].length / 2; i++) {
int temp = array[j][i];
array[j][i] = array[j][array[j].length - i - 1];
array[j][array[j].length - i - 1] = temp;
}
}
Another approach would be to use some library method such as from ArrayUtils#reverse():
ArrayUtils.reverse(array);
And then again put into a cycle:
for(int i = 0; i < array.length; i++){
ArrayUtils.reverse(array[i]);
}

I guess this the easiest approach, tried and tested
For instance, you have
1 2
3 4
and you want
2 1
4 3
You can reverse the loop, without any extra space or inbuilt function.
Solution:
for(int i =0;i<arr.length;i++) //arr.length=no of rows
{
for(int j = arr[i].length-1;j>=0;j--)//arr[i].length=no of col in a ith row
{
System.out.print(arr[i][j]+" ");
}
System.out.println();
}

Not sure if I didn't confuse what array stores the rows and which one the columns.... but this should work (long time since I've done Java last, so be nice to me when spotting any errors please ^^):
public static void reverseRows(int[][] array)
{
for (int i = 0 ; i < array.length ; i++) { // for each row...
int[] reversed = new int[array[i].length]; // ... create a temporary array that will hold the reversed inner one ...
for(int j = 0 ; j < array[i].length ; j++) { // ... and for each column ...
reversed[reversed.length - 1 - j] = array[i][j]; // ... insert the current element at the mirrored position of our temporary array
}
array[i] = reversed; // finally use the reversed array as new row.
}
}

Java Code :-
import java.util.Scanner;
public class Rev_Two_D {
static int col;
static int row;
static int[][] trans_arr = new int[col][row];
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner sc = new Scanner(System.in);
int m = sc.nextInt();
row = m;
int n = sc.nextInt();
col = n;
int[][] arr = new int[row][col];
for (int i = 0; i < row; i++) {
for (int j = 0; j < col; j++) {
arr[i][j] = sc.nextInt();
System.out.print(arr[i][j] + " ");
}
System.out.println();
}
for (int j = 0; j < arr.length; j++) {
for (int i = 0; i < arr[j].length / 2; i++) {
int temp = arr[j][i];
arr[j][i] = arr[j][arr[j].length - i - 1];
arr[j][arr[j].length - i - 1] = temp;
}
}
for (int i = 0; i < row; i++) {
for (int j = 0; j < col; j++) {
System.out.print(arr[i][j] + " ");
}
System.out.println();
}
}
}

Reverse of two D array - Print your two D array in reverse order
public void reverse(){
int row = 3;
int col = 3;
int[][] arr = new int[row][col];
int k=0;
for (int i = 0; i < row; i++) {
for (int j = 0; j < col; j++,k++) {
arr[i][j] = k;
System.out.print(arr[i][j] + " ");
}
System.out.println();
}
System.out.println();
for (int i = arr.length -1; i >=0 ; i--) {
for (int j = arr.length -1; j >=0 ; j--) {
System.out.print(arr[i][j] + " ");
}
System.out.println();
}
}

public static void main(String[] args) {
int [][] a={{1,2,3,4},{5,6,7,8},{9,10,11,12},{13,14,15,16}};
for(int i=0 ; i<a.length;i++)
{
for(int j=0 ; j<a.length;j++)
{
System.out.print(a[i][j]+",");
}
System.out.println();
}
System.out.println("***************************");
for(int i=0 ; i<a.length;i++)
{
for(int j=a.length-1 ; j>=0;j--)
{
System.out.print(a[i][j]+",");
}
System.out.println();
}
}

Reverse 2 D Array
public static void main(String[] args) {
int a[][] = {{1,2,3},
{4,5,6},
{8,9,10,12,15}
};
for(int i=0 ; i<a.length;i++)
{
for(int j=0 ; j<a[i].length;j++)
{
System.out.print(a[i][j]+",");
}
System.out.println();
}
for(int i=0 ; i<a.length;i++)
{
for(int j=a[i].length-1 ; j>=0;j--)
{
System.out.print(a[i][j]+",");
}
System.out.println();
}

Add Matrices in Java

I need to write a short program on how to add two matrices.. The first matrix should look like this:
1 2 3 4 5 6 7 8 9 10
11 12 13.......19 20
21................30
31................40
41................50
etc..
91...............100
But I don't really come to a solution how to increment the first array.. :S
Here is what I got so far:
package uebung05;
public class MatrixAddition
{
public static void main(String[] argv)
{
int firstArray[][] = new int[10][10];
int secondArray[][] = new int[10][10];
int ergArray[][] = new int[10][10];
System.out.println("Matrix 1\n----------------------------");
// Inkrementieren der ersten Matrix
for(int row = 0; row < firstArray.length; row++)
{
for(int column = 0; column < firstArray[row].length; column++)
{
// Increment Array here???
System.out.print(firstArray[row][column] + " ");
}
System.out.println();
}
System.out.println("\nMatrix 2\n----------------------------");
// Dekrementieren der zweiten Matrix
for(int row = 0; row < secondArray.length; row++)
{
for(int column = 0; column < secondArray[row].length; column++)
{
// Array mit Werten befüllen
secondArray[row][column] = column + 1;
System.out.print(secondArray[row][column] + " ");
}
System.out.println();
}
System.out.println("\nAddition beider Matrizen\n----------------------------");
// Addition firstArray & secondArray
for(int row = 0; row < ergArray.length; row++)
{
for(int column = 0; column < ergArray[row].length; column++)
{
// Addition
ergArray[row][column] = firstArray[row][column] +
secondArray[row][column];
System.out.print(ergArray[row][column] + " ");
}
System.out.println();
}
}
}

Method to add the first and second matrices together:
public static int[][] matrixAdd(int[][] A, int[][] B)
{
// Check if matrices have contents
if ((A.length < 0) || (A[0].length < 0)) return B;
if ((B.length < 0) || (B[0].length < 0)) return A;
// create new matrix to store added values in
int[][] C = new int[A.length][A[0].length];
for (int i = 0; i < A.length; i++)
{
for (int j = 0; j < A[i].length; j++)
{
C[i][j] = A[i][j] + B[i][j];
}
}
return C;
}

But I don't really come to a solution how to increment the first array.
// Inkrementieren der ersten Matrix
for(int row = 0; row < firstArray.length; row++)
{
for(int column = 0; column < firstArray[row].length; column++)
{
firstArray[row][column] = 1+ row*10 + column;
System.out.print(firstArray[row][column] + " ");
}
System.out.println();
}

Sum two matrices in the new one and return:
public int[][] addMatrixes(int[][] src1, int[][] src2){
int[][] dst = new int[src1.length][src1[0].length];
for(int i=0;i<src1.length;i++){
for(int j=0;j<src1[0].length;j++){
dst[i][j] = src1[i][j] + src2[i][j];
}
}
return dst;
}

Not very generic, but you can define your first matrix with only one easy loop :
int dim = 10;
int size = dim*dim;
int firstArray[][] = new int[dim][dim];
int row, column;
for (int index = 0; index < size; index++ ){
row = index/dim;
column = index%dim;
firstArray[row][column]=row*10+column+1;
System.out.print(String.valueOf(firstArray[row][column])+"\t");
if (column == 9){ System.out.println("");}
}