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Index › Programming Questions & Help › Syntax Questions › Matrix multiplication
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Matrix multiplication (Read 684 times)
firefox950
Matrix multiplication
Mar 30th, 2008, 7:45am  
i need help with simple matrix multiplication code in Processing
am using this code.

int[][] matrixA = {{3,2,-1},{0,4,6}};
int[][] matrixB= {{1,0},{5,3},{6,4}};
int[][] matrixC;


void setup()
{

 
  for(int i=0; i<2; i++){
           for(int j=0; j<2; j++){
               for(int k=0; k<3; k++){
               
               
                 matrixC[i][j]= matrixC[i][j]+ matrixA[i][k] * matrixB[k][j];
             
               }
           }
       }
}

 
it gives me the error

java.lang.NullPointerException at temporary_8356_2317.setup(Temporary_8356_2317.java:32)
at processing.core.PApplet.handleDisplay(PApplet.java:1390)
at processing.core.PGraphics.requestDisplay(PGraphics.java:690)
at processing.core.PApplet.run(PApplet.java:1562)
at java.lang.Thread.run(Unknown Source)
pleas help
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JohnG
Re: Matrix multiplication
Reply #1 - Mar 30th, 2008, 11:43am  
You need to give MatrixC a size before you can assign values to it.
So chance int[][] matrixC;
to
int[][]matrixC=new int[3][3];
//change 3 3 to the right values, I'm not sure what dimensions a 2x3 * 3x2 gives.
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firefox950
Re: Matrix multiplication
Reply #2 - Mar 31st, 2008, 1:58am  
thanks...i figured that out after a couple of tries..
but thanks for the answer.
it was really lame of me..
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