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matrix multiply
im doing a project for my class, and this is my code that i am using..
i am currently having errors in this code which are:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3
at arrayMatrix.multMatrix(arrayMatrix.java:67)
at arrayMatrix.main(arrayMatrix.java:32)
i am using Eclipse to type this code, so if anyone can help me fix these problems i would greatly appreciate it
Code:
/* This program multiplies 2 matrices times each other.
* If the matrices are not compatible, it warns the user. */
public class arrayMatrix
{
//static List list;
public static void main(String[] args)
{
int j, k, l;
int a[][], b[][], ab[][];
System.out.println("Hello TV land!");
j = 1;
k = 2;
l = 3;
a = makeIdent(2, 3);
a[0][0] = 1;a[0][1] = 2;a[0][2] = 4;
a[1][0] = 2;a[1][1] = 6;a[1][2] = 0;
showMatrix(2, 3, a, j);
b = makeIdent(3, 4);
b[0][0] = 4;b[0][1] = 1;b[0][2] = 4;b[0][3] = 3;
b[1][0] = 0;b[1][1] = -1;b[1][2] = 3;b[1][3] = 1;
b[2][0] = 2;b[2][1] = 7;b[2][2] = 5;b[2][3] = 2;
showMatrix(3, 4, b, k);
ab = multMatrix(2, 3, a, 3, 4, b);
showMatrix(2, 4, ab, l);
}
static int[][] makeIdent(int nRows, int nCols){
int a[][] = new int[nRows][nCols];
return a;
}
static void showMatrix(int nRows, int nCols, int m[][], int x){
System.out.println("Matrix " + x + ":");
for(int i = 0; i < nRows; i++) {
for(int j = 0; j < nCols; j++) {
System.out.println("Row: " + i + " Column: " + j
+ " = "+ m[i][j]);
}
}
}
static int[][] multMatrix(int aRows, int aCols, int a[][],
int bRows, int bCols, int b[][]){
int mult[][] = new int[aRows][bCols];
if(aCols != bRows){
System.out.println("These matrices are not compatible");
}else{
for(int i = 0; i < aRows; i++) {
for(int j = 0; j < bCols-1; j++) {
for(int k = 0; k < bCols; k++){
mult[i][j] += a[i][k]*b[k][j];
}
}
}
}
return mult;
}
}
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a[i].length and b.length are only 3 each, and k has a value of 3, which means it's trying to read the fourth element.
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how do i fix that? maybe j < bCols instead of bCols -1?
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No--instead of guessing, think about this logically.
Your k value is going too high--it should go 2 instead of 3. Changing the j values will not fix this. Instead, you should be focused on this line:
Code:
for(int k = 0; k < bCols; k++){
You're going to want to change something in that line.
Edit: Fixed incorrect numerical values above.
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from what im understanding, i should change Code:
for(int j = 0; j < bCols - 1; j++) {
for(int k = 0; k < bCols; k++){
to
f Code:
or(int j = 0; j < bCols; j++) {
for(int k = 0; k < bCols-1; k++){
am i misunderstanding what im doing wrong?
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When in doubt, give it a try!
But yes, you are correct, assuming this is the desired output:
Code:
Hello TV land!
Matrix 1:
Row: 0 Column: 0 = 1
Row: 0 Column: 1 = 2
Row: 0 Column: 2 = 4
Row: 1 Column: 0 = 2
Row: 1 Column: 1 = 6
Row: 1 Column: 2 = 0
Matrix 2:
Row: 0 Column: 0 = 4
Row: 0 Column: 1 = 1
Row: 0 Column: 2 = 4
Row: 0 Column: 3 = 3
Row: 1 Column: 0 = 0
Row: 1 Column: 1 = -1
Row: 1 Column: 2 = 3
Row: 1 Column: 3 = 1
Row: 2 Column: 0 = 2
Row: 2 Column: 1 = 7
Row: 2 Column: 2 = 5
Row: 2 Column: 3 = 2
Matrix 3:
Row: 0 Column: 0 = 12
Row: 0 Column: 1 = 27
Row: 0 Column: 2 = 30
Row: 0 Column: 3 = 0
Row: 1 Column: 0 = 8
Row: 1 Column: 1 = -4
Row: 1 Column: 2 = 26
Row: 1 Column: 3 = 0
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when i basically swap the J and K (bCols) and (bCols - 1) i get actual values for [0][3] and [1][3] instead of the 0 that shows up on yours
I do greatly appreciate you helping me put my brain back on the tracks on what i need to fix
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Ah, yes, I didn't notice the change in the j statement (just the k statement). My apologies. Is the program now working as you expected?
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yes it is, Thanks for your help
