I try to convert 1D to 2D array, but I keep getting java.lang.ArrayIndexOutOfBoundsException, and I have tried whatever I could find on the stackoverflow or internet, but I do not understand why I have this issue?
public class Arrayto2DArray {
/**
* #param args the command line arguments
*/
public static void main(String[] args) {
// TODO code application logic here
int[] a = {0,1, 6, 83, 4, 5, 12, 7};
int[][] b = new int[4][4];
for (int i = 0; i < b.length; i++) {
for (int j = 0; j < b[i].length; j++) {
b[i][j]=0;
System.out.print(b[i][j]);
}
System.out.println();
}
System.out.println("--------------------------");
for (int i = 0; i < 4; i++) {
for (int j = 0; j < b[i].length; j++) {
try{
b[i][j] = a[i+j*4];
}catch(Exception e){
e.printStackTrace();
System.out.println(e);
}
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
System.out.print(b[i][j]);
}
System.out.println();
}
}
}
I kind of know why I get this error and it because of this line
b[i][j] = a[i+j*4];
but I cannot come up any formula better than this.
Consider the second for-loop
Lets say when i = 3 and j= 3
a[i+j*4] evaluates to a[15] which is out of the array
When you declare you 2-d array, you specified int[][] b = new int[4][];, meaning that the first inner for loop for (int j = 0; j < b[i].length; j++) should result in a NullPointerException since b[i].length has no predefined length. Before intering the inner for loop, you should define the size of each b[i] like b[i] = new int[somenumber]
In regards to convert the 1d loop to a 2d, you need to define the rule around spliting it into the 2-d array. Then accordingly the second for loop need modification
EDIT:You modified your code to have an int[4][4] array, which means you have 16 placeholders. Your 1-d array contain only 8 placeholders. It depends on how you want to sort the array, like it can be
b 0 1 2 3
0 0 1 6 83
1 4 5 12 7
2 0 0 0 0
3 0 0 0 0
or any other pattern
Assuming the length of 1-d array is 8 and the total index of 2-d array is 16, the following is more of a general solution:
public static void main(String[] args) {
// TODO code application logic here
int[] a = { 0, 1, 6, 83, 4, 5, 12, 7 };
int[][] b = new int[4][4];
for (int i = 0; i < b.length; i++) {
for (int j = 0; j < b[i].length; j++) {
b[i][j] = 0;
System.out.print(b[i][j] + "\t");
}
System.out.println();
}
System.out.println("--------------------------");
for (int i = 0; i < 4; i++) {
for (int j = 0; j < b[i].length; j++) {
try {
if ((j + i * 4) < a.length)
b[i][j] = a[j + i * 4];
} catch (Exception e) {
e.printStackTrace();
System.out.println(e);
}
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
System.out.print(b[i][j] + "\t");
}
System.out.println();
}
}
What is wrong is your 2D array is a 4*4, so that means for each index, there will be 4 elements i.e. say values. So there would be a total of 16 values, so you would need a 1D array with 16 elements for the above code to work. Right now you have only 8 values in your 1D array.
I think your code should change like this
public class Arrayto2DArray {
public static void main(String[] args) {
// TODO code application logic here
int[] a = {0,1, 6, 83, 4, 5, 12, 7};
int[][] b = new int[4][2];
for (int i = 0; i < b.length; i++) {
for (int j = 0; j < b[i].length; j++) {
b[i][j]=0;
System.out.print(b[i][j]);
}
System.out.println();
}
System.out.println("--------------------------");
for (int i = 0; i < 4; i++) {
for (int j = 0; j < b[i].length; j++) {
try{
b[i][j] = a[i+j*4];
}catch(Exception e){
e.printStackTrace();
System.out.println(e);
}
}
}
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 2; j++) {
System.out.print(b[i][j]+" ");
}
System.out.println();
}
}
}
Your correct formula should be like.
int[] a = {0,1, 6, 83, 4, 5, 12, 7};
int[][] b = new int[4][2];
int k=0;
for (int i = 0; i < b.length; i++) { // Column of 2D array
for (int j = 0; j < b[0].length; j++) { // Row of 2D array
b[i][j]= a[k++];
System.out.println(b[i][j]);
}
}
Edit:
For general case, If your row is fixed at 4 but column is not fixed than your 2D conversion formula should be like below.
int col = a.length / 4;
int remainder = a.length % 4;
if (remainder > 0) {
col = col + 1; // Get the correct column size.
}
int[][] b = new int[4][col];
int k = 0;
for (int i = 0; i < 4; i++) {
for (int j = 0; j < col; j++) {
if (k < a.length) {
b[i][j] = a[k];
System.out.println(b[i][j]);
}
k++;
}
}
Related
So I'm currently playing around with multidimensional arrays (2D) and I'm trying to reverse the order of each array in a 2-d array.
So I have a 2D-array set as:
int firstArray[][] = {{5,6,7,8,9,10}, {11,12,13,14,15,16}}
I have manually looked through the issue to see where I may have went wrong, to see which part of my code would end up going out of bounds in regards to my for-loops. The -1 part also caught me off guard.
I have began doing reverses on a regular 1-d array, and tried to apply the same concept to multidimensional arrays.
class Test2 {
public static void main (String[] args) {
int firstArray[][] = {{5,6,7,8,9,10}, {10,11, 12, 13, 14, 15}};
System.out.println("FIRST ARRAY");
display(firstArray);
}
public void display(int [][]num) {
for (int i = 0; i < num.length; i++) {
for (int j = 0; j < num[i].length/2; j++) {
int temp = num[i][j];
num[i][j] = num[i][num.length-1-j];
num[i][num.length-1-j] = temp;
}
}
for (int a = 0; a < num.length; a++) {
for (int b = 0; b < num[a].length; b++) {
System.out.print(num[a][b] + "\t");
}
System.out.println();
}
}
}
I want the output using my display method to basically be a reverse of the arrays in my 2-d array:
10 9 8 7 6 5
15 14 13 12 11 10
The issue that I'm getting is an
Exception in thread "main"java.lang.ArrayIndexOutOfBoundsException: -1
ArrayIndexOutOfBoundsException: -1
at Test2.display(Test2.java:30)
at Test2.main(Test2.java:20)
You are using the length of the wrong dimension.
With num.length you are using the number of rows and not the number of columns of the current row.
You need to change that to num[i].length.
public static void display(int [][]num) {
for (int i = 0; i < num.length; i++) {
for (int j = 0; j < num[i].length/2; j++) {
int temp = num[i][j];
num[i][j] = num[i][num[i].length-1-j];
num[i][num[i].length-1-j] = temp;
}
}
for (int a = 0; a < num.length; a++) {
for (int b = 0; b < num[a].length; b++) {
System.out.print(num[a][b] + "\t");
}
System.out.println();
}
}
Notice you wrote num[i][num.length-1-j];
num.length-1-j is basically 2 - 1 -j.
public static void display(int [][]num) {
for (int i = 0; i < num.length; i++) {
for (int j = 0; j < num[i].length/2 ; j++) {
int temp = num[i][j];
num[i][j] = num[i][num[i].length-1-j];
num[i][num[i].length-1-j] = temp;
}
}
for (int a = 0; a < num.length; a++) {
for (int b = 0; b < num[a].length; b++) {
System.out.print(num[a][b] + "\t");
}
System.out.println();
}
}
This question already has answers here:
What causes a java.lang.ArrayIndexOutOfBoundsException and how do I prevent it?
(26 answers)
Closed 4 years ago.
I'm trying to write a code that multiplies two given matrices together and displays the results. Here is the code I have down so far:
public static void main(String[] args) {
double firstMatrix[][] = {{1, 2}, {3, 4}, {5, 6}};
double secondMatrix[][] = {{1, 2, 3}, {4, 5, 6}};
double[][] thirdMatrix = multiplyMatrix((firstMatrix), (secondMatrix));
System.out.println("The product of the matrices is ");
for (int i = 0; i < thirdMatrix.length; i++) {
for (int j = 0; j < thirdMatrix[0].length; j++) {
System.out.print(firstMatrix[i][j] + " ");
if (i == 1 && j == 2) {
System.out.print(" * ");
} else {
System.out.print(" ");
}
}
for (int j = 0; j < thirdMatrix[0].length; j++) {
System.out.print(secondMatrix[i][j] + " ");
if (i == 1 && j == 2) {
System.out.print(" = ");
} else {
System.out.print(" ");
}
}
for (int j = 0; j < thirdMatrix[0].length; j++) {
System.out.printf("%.1f", thirdMatrix[i][j]);
}
System.out.println();
}
}
public static double[][] multiplyMatrix(double[][] a, double[][] b) {
double c[][] = new double[3][3];
for (int i = 0; i < c.length; i++) {
for (int j = 0; j < b.length; j++) {
for (int k = 0; k < a.length; k++) {
c[i][j] += ((a[i][k]) * (b[k][j]));
}
}
}
return c;
}
But when I try to execute the program I receive an Array Index out of Bounds Error which shouldn't be the case since my for loops are making sure not to step out of bounds from the array. I don't understand where to change my code because it looks fine until I try to execute. Any help is really appreciated.
If 1st matrix's dimensions is mxn and the 2nd's nxp then the product's dimensions mxp. Right?
So this 2nd level of the triple loop should iterate through the 2nd dimension of the product array:
for (int j = 0; j < b.length; j++)
should be:
for (int j = 0; j < b[0].length; j++)
b[0].length is the number of columns of the matrix b.
Also the 3d level of the triple loop should be:
for (int k = 0; k < b.length; k++)
b.length is the number of rows of the matrix b which is equal to the number of columns of the matrix a.
Edit
In a generalized version you could change this:
double c[][] = new double[3][3];
to
double c[][] = new double[a.length][b[0].length];
public static void main(String[] args) {
double firstMatrix[][] = {{1, 2}, {3, 4}, {5, 6}};
double secondMatrix[][] = {{1, 2, 3}, {4, 5, 6}};
double[][] thirdMatrix = multiplyMatrix((firstMatrix), (secondMatrix));
System.out.println("The product of the matrices is ");
for (int i = 0; i < thirdMatrix.length; i++) {
for (int j = 0; j < firstMatrix[0].length; j++) {// check this one
System.out.print(firstMatrix[i][j] + " ");
if (i == 1 && j == 2) {
System.out.print(" * ");
} else {
System.out.print(" ");
}
}
for (int j = 0; j < thirdMatrix.length; j++) {
if (i <= 1) {// you need are condition to stop
System.out.print(secondMatrix[i][j] + " ");
if (i == 1 && j == 2) {
System.out.print(" = ");
} else {
System.out.print(" ");
}
}
}
for (int j = 0; j < thirdMatrix[0].length; j++) {
System.out.print(thirdMatrix[i][j]);
}
System.out.println();
}
}
public static double[][] multiplyMatrix(double[][] a, double[][] b) {
double c[][] = new double[3][3];
for (int i = 0; i < c.length; i++) {
// Change b.length to b[0].length;
for (int j = 0; j < b[0].length; j++) {
// Change a.length to a[0].length;
for (int k = 0; k < a[0].length; k++) {
c[i][j] += ((a[i][k]) * (b[k][j]));
}
}
}
return c;
}
The program should multiply 2 matrices 4x1 and 1x4 and output the result to the console (matrix 4X4). But nothing displays. What's the problem?
public class Matrix {
public static void main(String[] args) {
int[][] matrixA = new int[4][1];
int[][] matrixB = new int[1][4];
int[][] matrixC = new int[4][4];
matrixA[0][0] = 1;
matrixA[1][0] = 2;
matrixA[2][0] = 3;
matrixA[3][0] = 4;
matrixB[0][0] = 4;
matrixB[0][1] = 3;
matrixB[0][2] = 2;
matrixB[0][3] = 1;
for (int i = 0; i < 4; i++) { // A rows
for (int j = 0; j < 4; j++) { // B columns
for (int k = 0; k < 1; k++) { // A columns
matrixC[i][j] += matrixA[i][k] * matrixB[k][j];
System.out.print(matrixC[i][j] + " ");
}
}
}
int j = 0;
for (int i = 0; i < 4; i++) {
for (int k = 0; k < 1; k++)
System.out.print(matrixC[i][j] + " ");
System.out.println();
}
} //end main
} //end class
You introduced a variable j before your second set of for loops. Also, even if they're optional, I would highly recommend always including braces. And k < 4. Like,
for (int i = 0; i < 4; i++) {
for (int k = 0; k < 4; k++) {
System.out.print(matrixC[i][k] + " "); // not [i][j]
}
System.out.println();
}
or just use Arrays.deepToString(Object[]) like
System.out.println(Arrays.deepToString(matrixC));
Hope this helps:
public class Matrix {
public static void main(String[] args) {
int[][] matrixA = {{1}, {2}, {3}, {4}};
int[][] matrixB = {{4, 3, 2, 1}};
int[][] matrixC = new int[4][4];
for (int i = 0; i < 4; i++) { // A rows
for (int j = 0; j < 4; j++) { // B columns
for (int k = 0; k < 1; k++) { // A columns
matrixC[i][j] += matrixA[i][k] * matrixB[k][j];
System.out.print(matrixC[i][j] + " ");
}
}
System.out.println();
}
}
}
Output:
4 3 2 1
8 6 4 2
12 9 6 3
16 12 8 4
I've got the following code, I'm trying to get it to multiply array a by array b and produce array c. I've browsed several questions on here and for some reason I can't get the array to produce the correct results, I believe because it's not multiplying the right indices by one another. Any help would be greatly appreciated, as I am still trying to grasp how array multiplication works. Here is the code:
class Main
{
public static void main (String[] args)
{
int[][]a =
{
{1,2,3,4},
{5,6,7,8},
{9,10,11,12},
//print array a
};
for(int i = 0; i < a[0].length-1; i++)
{
for(int j = 0; j < a[0].length; j++)
{
System.out.print(a[i][j] + " ");
}
System.out.println();
}
int[][]b =
{
{-1,-2,-3},
{-4,-5,-6},
{-7,-8,-9},
{-10,-11,-12},
};
System.out.println();
//print array b
for(int i = 0; i < b.length; i++)
{
for(int j = 0; j < b.length-1; j++)
{
System.out.print(b[i][j] + " ");
}
System.out.println();
}
System.out.println();
int[][]c = new int[a.length][b[0].length];
if(b.length == a[0].length)
{
for(int i = 0; i < a.length; i++)
{
for (int j = 0; j < b[0].length; j++)
{
for (int k = 0; k < a[0].length; k++)
{
c[i][j] = a[i][k]*b[k][j];
}
}
}
}
else
{
System.out.println("Dimension requirements not satisfied for accurate calculation");
}
for(int i = 0; i < c.length; i++)
{
for(int j = 0; j < c.length; j++)
{
System.out.print(c[i][j] + " ");
}
System.out.println();
}
}
}
Change c[i][j] = a[i][k]*b[k][j]; to c[i][j] += a[i][k]*b[k][j];
The answer by #Jeffrey Chen is correct because in the matrix multiplication each row of the first matrix multiply by each column of second matrix
In your case row 1 of matrix a is 1, 2, 3, 4
Column 1 of matrix b is -1, -4, -7, -10
So the answer is (1*-1)+(2*-4)+(3*-7)+(4*-10)
= -1-8-21-40 = -70
Then the first row of matrix a multiply by second column of matrix b
(1*-2)+(2*-5)+(3*-8)+(4*-11)
= -2-10-24-44 = -80
And so on
So the following is correct
c[i][j] += a[i][k] * b[k][j];
Sorting a 2-dimensional can be done in two ways :
Converting 2-dimensional to 1-dimensional and then sorting it.
Directly sorting the 2-dimensional array.
I would like to know the second method. Sorting it directly.
Try this. You can sort two dimensional array Directly
int array[][] = { { 12, 43, 21, 87, 32 }, { 43, 75, 21, 45, 65 } };
int t = 0;
for (int x = 0; x < 2; x++) {
for (int y = 0; y < 5; y++) {
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 5; j++) {
if (array[i][j] > array[x][y]) {
t = array[x][y];
array[x][y] = array[i][j];
array[i][j] = t;
}
}
}
}
}
System.out.println("The Sorted Array:");
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 5; j++) {
System.out.print(array[i][j] + "\t");
}
System.out.println();
}