I'm trying to make a simple matrix multiplication method using multidimensional arrays ([2][2]). I'm kinda new at this, and I just can't find what it is I'm doing wrong. I'd really appreciate any help in telling me what it is. I'd rather not use libraries or anything like that, I'm mostly doing this to learn how it works. Thank you so much in advance.
I'm declaring my arays in the main method as follows:
Double[][] A={{4.00,3.00},{2.00,1.00}};
Double[][] B={{-0.500,1.500},{1.000,-2.0000}};A*B should return the identity matrix. It doesn't.
public static Double[][] multiplicar(Double[][] A, Double[][] B){
//the method runs and returns a matrix of the correct dimensions
//(I actually changed the .length function to a specific value to eliminate
//it as a possible issue), but not the correct values Double[][] C= new Double[2][2]; int i,j; ////I fill the matrix with zeroes, if I don't do this it gives me an error for(i=0;i<2;i++) { for(j=0;j<2;j++){ C[i][j]=0.00000; } } ///this is where I'm supposed to perform the adding of every element in //a row of A multiplied by the corresponding element in the //corresponding column of B, for all columns in B and all rows in A for(i=0;i<2;i++){ for(j=0;j<2;j++) C[i][j]+=(A[i][j]*B[j][i]); } return C;
}9 Answers
You can try this code:
public class MyMatrix { Double[][] A = { { 4.00, 3.00 }, { 2.00, 1.00 } }; Double[][] B = { { -0.500, 1.500 }, { 1.000, -2.0000 } }; public static Double[][] multiplicar(Double[][] A, Double[][] B) { int aRows = A.length; int aColumns = A[0].length; int bRows = B.length; int bColumns = B[0].length; if (aColumns != bRows) { throw new IllegalArgumentException("A:Rows: " + aColumns + " did not match B:Columns " + bRows + "."); } Double[][] C = new Double[aRows][bColumns]; for (int i = 0; i < aRows; i++) { for (int j = 0; j < bColumns; j++) { C[i][j] = 0.00000; } } for (int i = 0; i < aRows; i++) { // aRow for (int j = 0; j < bColumns; j++) { // bColumn for (int k = 0; k < aColumns; k++) { // aColumn C[i][j] += A[i][k] * B[k][j]; } } } return C; } public static void main(String[] args) { MyMatrix matrix = new MyMatrix(); Double[][] result = multiplicar(matrix.A, matrix.B); for (int i = 0; i < 2; i++) { for (int j = 0; j < 2; j++) System.out.print(result[i][j] + " "); System.out.println(); } }
}Java. Matrix multiplication.
Tested with matrices of different size.
public class Matrix {
/** * Matrix multiplication method. * @param m1 Multiplicand * @param m2 Multiplier * @return Product */ public static double[][] multiplyByMatrix(double[][] m1, double[][] m2) { int m1ColLength = m1[0].length; // m1 columns length int m2RowLength = m2.length; // m2 rows length if(m1ColLength != m2RowLength) return null; // matrix multiplication is not possible int mRRowLength = m1.length; // m result rows length int mRColLength = m2[0].length; // m result columns length double[][] mResult = new double[mRRowLength][mRColLength]; for(int i = 0; i < mRRowLength; i++) { // rows from m1 for(int j = 0; j < mRColLength; j++) { // columns from m2 for(int k = 0; k < m1ColLength; k++) { // columns from m1 mResult[i][j] += m1[i][k] * m2[k][j]; } } } return mResult; } public static String toString(double[][] m) { String result = ""; for(int i = 0; i < m.length; i++) { for(int j = 0; j < m[i].length; j++) { result += String.format("%11.2f", m[i][j]); } result += "\n"; } return result; } public static void main(String[] args) { // #1 double[][] multiplicand = new double[][] { {3, -1, 2}, {2, 0, 1}, {1, 2, 1} }; double[][] multiplier = new double[][] { {2, -1, 1}, {0, -2, 3}, {3, 0, 1} }; System.out.println("#1\n" + toString(multiplyByMatrix(multiplicand, multiplier))); // #2 multiplicand = new double[][] { {1, 2, 0}, {-1, 3, 1}, {2, -2, 1} }; multiplier = new double[][] { {2}, {-1}, {1} }; System.out.println("#2\n" + toString(multiplyByMatrix(multiplicand, multiplier))); // #3 multiplicand = new double[][] { {1, 2, -1}, {0, 1, 0} }; multiplier = new double[][] { {1, 1, 0, 0}, {0, 2, 1, 1}, {1, 1, 2, 2} }; System.out.println("#3\n" + toString(multiplyByMatrix(multiplicand, multiplier))); }
}Output:
#1 12.00 -1.00 2.00 7.00 -2.00 3.00 5.00 -5.00 8.00
#2 0.00 -4.00 7.00
#3 0.00 4.00 0.00 0.00 0.00 2.00 1.00 1.00static int b[][]={{21,21},{22,22}};
static int a[][] ={{1,1},{2,2}};
public static void mul(){ int c[][] = new int[2][2]; for(int i=0;i<b.length;i++){ for(int j=0;j<b.length;j++){ c[i][j] =0; } } for(int i=0;i<a.length;i++){ for(int j=0;j<b.length;j++){ for(int k=0;k<b.length;k++){ c[i][j]= c[i][j] +(a[i][k] * b[k][j]); } } } 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("\n"); }
}Try this,
public static Double[][] multiplicar(Double A[][],Double B[][]){ Double[][] C= new Double[2][2]; int i,j,k; for (i = 0; i < 2; i++) { for (j = 0; j < 2; j++) { C[i][j] = 0.00000; } } for(i=0;i<2;i++){ for(j=0;j<2;j++){ for (k=0;k<2;k++){ C[i][j]+=(A[i][k]*B[k][j]); } } } return C;
}try this,it may help you
import java.util.Scanner;
public class MulTwoArray {
public static void main(String[] args) { int i, j, k; int[][] a = new int[3][3]; int[][] b = new int[3][3]; int[][] c = new int[3][3]; Scanner sc = new Scanner(System.in); System.out.println("Enter size of array a"); int rowa = sc.nextInt(); int cola = sc.nextInt(); System.out.println("Enter size of array b"); int rowb = sc.nextInt(); int colb = sc.nextInt(); //read and b System.out.println("Enter elements of array a"); for (i = 0; i < rowa; ++i) { for (j = 0; j < cola; ++j) { a[i][j] = sc.nextInt(); } System.out.println(); } System.out.println("Enter elements of array b"); for (i = 0; i < rowb; ++i) { for (j = 0; j < colb; ++j) { b[i][j] = sc.nextInt(); } System.out.println("\n"); } //print a and b System.out.println("the elements of array a"); for (i = 0; i < rowa; ++i) { for (j = 0; j < cola; ++j) { System.out.print(a[i][j]); System.out.print("\t"); } System.out.println("\n"); } System.out.println("the elements of array b"); for (i = 0; i < rowb; ++i) { for (j = 0; j < colb; ++j) { System.out.print(b[i][j]); System.out.print("\t"); } System.out.println("\n"); } //multiply a and b for (i = 0; i < rowa; ++i) { for (j = 0; j < colb; ++j) { c[i][j] = 0; for (k = 0; k < cola; ++k) { c[i][j] += a[i][k] * b[k][j]; } } } //print multi result System.out.println("result of multiplication of array a and b is "); for (i = 0; i < rowa; ++i) { for (j = 0; j < colb; ++j) { System.out.print(c[i][j]); System.out.print("\t"); } System.out.println("\n"); }
}
}The method mults is a procedure(Pascal) or subroutine(Fortran)
The method multMatrix is a function(Pascal,Fortran)
import java.util.*;
public class MatmultE
{
private static Scanner sc = new Scanner(System.in); public static void main(String [] args) { double[][] A={{4.00,3.00},{2.00,1.00}}; double[][] B={{-0.500,1.500},{1.000,-2.0000}}; double[][] C=multMatrix(A,B); printMatrix(A); printMatrix(B); printMatrix(C); double a[][] = {{1, 2, -2, 0}, {-3, 4, 7, 2}, {6, 0, 3, 1}}; double b[][] = {{-1, 3}, {0, 9}, {1, -11}, {4, -5}}; double[][] c=multMatrix(a,b); printMatrix(a); printMatrix(b); printMatrix(c); double[][] a1 = readMatrix(); double[][] b1 = readMatrix(); double[][] c1 = new double[a1.length][b1[0].length]; mults(a1,b1,c1,a1.length,a1[0].length,b1.length,b1[0].length); printMatrix(c1); printMatrixE(c1); } public static double[][] readMatrix() { int rows = sc.nextInt(); int cols = sc.nextInt(); double[][] result = new double[rows][cols]; for (int i = 0; i < rows; i++) { for (int j = 0; j < cols; j++) { result[i][j] = sc.nextDouble(); } } return result; } public static void printMatrix(double[][] mat) { System.out.println("Matrix["+mat.length+"]["+mat[0].length+"]"); int rows = mat.length; int columns = mat[0].length; for (int i = 0; i < rows; i++) { for (int j = 0; j < columns; j++) { System.out.printf("%8.3f " , mat[i][j]); } System.out.println(); } System.out.println(); } public static void printMatrixE(double[][] mat) { System.out.println("Matrix["+mat.length+"]["+mat[0].length+"]"); int rows = mat.length; int columns = mat[0].length; for (int i = 0; i < rows; i++) { for (int j = 0; j < columns; j++) { System.out.printf("%9.2e " , mat[i][j]); } System.out.println(); } System.out.println(); } public static double[][] multMatrix(double a[][], double b[][]){//a[m][n], b[n][p] if(a.length == 0) return new double[0][0]; if(a[0].length != b.length) return null; //invalid dims int n = a[0].length; int m = a.length; int p = b[0].length; double ans[][] = new double[m][p]; for(int i = 0;i < m;i++){ for(int j = 0;j < p;j++){ ans[i][j]=0; for(int k = 0;k < n;k++){ ans[i][j] += a[i][k] * b[k][j]; } } } return ans; } public static void mults(double a[][], double b[][], double c[][], int r1, int c1, int r2, int c2){ for(int i = 0;i < r1;i++){ for(int j = 0;j < c2;j++){ c[i][j]=0; for(int k = 0;k < c1;k++){ c[i][j] += a[i][k] * b[k][j]; } } } }
}where as input matrix you can enter
inE.txt
4 4
1 1 1 1
2 4 8 16
3 9 27 81
4 16 64 256
4 3
4.0 -3.0 4.0
-13.0 19.0 -7.0
3.0 -2.0 7.0
-1.0 1.0 -1.0in unix like cmmd line execute the command:
$ java MatmultE < inE.txt > outE.txt
and you get the output
outC.txt
Matrix[2][2] 4.000 3.000 2.000 1.000
Matrix[2][2] -0.500 1.500 1.000 -2.000
Matrix[2][2] 1.000 0.000 0.000 1.000
Matrix[3][4] 1.000 2.000 -2.000 0.000 -3.000 4.000 7.000 2.000 6.000 0.000 3.000 1.000
Matrix[4][2] -1.000 3.000 0.000 9.000 1.000 -11.000 4.000 -5.000
Matrix[3][2] -3.000 43.000 18.000 -60.000 1.000 -20.000
Matrix[4][3] -7.000 15.000 3.000 -36.000 70.000 20.000
-105.000 189.000 57.000
-256.000 420.000 96.000
Matrix[4][3]
-7.00e+00 1.50e+01 3.00e+00
-3.60e+01 7.00e+01 2.00e+01
-1.05e+02 1.89e+02 5.70e+01
-2.56e+02 4.20e+02 9.60e+01 My code is super easy and works for any order of matrix
public static void main(String[] args) { Scanner sc = new Scanner(System.in); System.out.println(" Enter No. of rows in matrix 1 : "); int arows = sc.nextInt(); System.out.println(" Enter No. of columns in matrix 1 : "); int acols = sc.nextInt(); System.out.println(" Enter No. of rows in matrix 2 : "); int brows = sc.nextInt(); System.out.println(" Enter No. of columns in matrix 2 : "); int bcols = sc.nextInt(); if (acols == brows) { System.out.println(" Enter elements of matrix 1 "); int a[][] = new int[arows][acols]; int b[][] = new int[brows][bcols]; for (int i = 0; i < arows; i++) { for (int j = 0; j < acols; j++) { a[i][j] = sc.nextInt(); } } System.out.println(" Enter elements of matrix 2 "); for (int i = 0; i < brows; i++) { for (int j = 0; j < bcols; j++) { b[i][j] = sc.nextInt(); } } System.out.println(" The Multiplied matrix is : "); int sum = 0; int c[][] = new int[arows][bcols]; for (int i = 0; i < arows; i++) { for (int j = 0; j < bcols; j++) { for (int k = 0; k < brows; k++) { sum = sum + a[i][k] * b[k][j]; c[i][j] = sum; } System.out.print(c[i][j] + " "); sum = 0; } System.out.println(); } } else { System.out.println("Order of matrix in invalid"); }
}multiply 4x4 Matrixes
float[] mul(float[] l, float[] r) { float[] res = new float[16]; for (int i = 0; i < 16; i++) { int y = i / 4; int x = i % 4; res[i] = l[x] * r[y] + l[x + 4] * r[y + 4] + l[x + 8] * r[y + 8] + l[x + 12] * r[y + 12]; } return res;
}import java.util.*;
public class Mult { public static int[][] C; public static void main(String[] args) { Scanner s = new Scanner(System.in); System.out.println("Enter Row of Matrix A"); int Rowa = s.nextInt(); System.out.println("Enter Column of Matrix A"); int Cola = s.nextInt(); System.out.println("Enter Row of Matrix B"); int Rowb = s.nextInt(); System.out.println("Enter Column of Matrix B"); int Colb = s.nextInt(); int[][] A = new int[Rowa][Cola]; int[][] B = new int[Rowb][Colb]; C = new int[Rowa][Colb]; //int[][] C = new int; System.out.println("Enter Values of Matrix A"); for (int i = 0; i < A.length; i++) { for (int j = 0; j < A.length; j++) { A[i][j] = s.nextInt(); } } System.out.println("Enter Values of Matrix B"); for (int i = 0; i < B.length; i++) { for (int j = 0; j < B.length; j++) { B[i][j] = s.nextInt(); } } if (Cola == Rowb) { for (int i = 0; i < A.length; i++) { for (int j = 0; j < A.length; j++) { C[i][j] = 0; for (int k = 0; k < B.length; k++) { C[i][j] += A[i][k] * B[k][j]; } } } } else { System.out.println("Cannot multiply"); } // Printing matrix A /* for (int i = 0; i < A.length; i++) { for (int j = 0; j < A.length; j++) { System.out.print(A[i][j] + "\t"); } System.out.println(); } */ for (int i = 0; i < A.length; i++) { for (int j = 0; j < A.length; j++) { System.out.print(C[i][j] + "\t"); } System.out.println(); } }
}
