Matrix multiplication using 1-d arrays

This is a discussion on Matrix multiplication using 1-d arrays within the C++ Programming forums, part of the General Programming Boards category; I know how to properly multiply matrices, so that isn't the problem. However, I'm working on a program that multiplies ...

  1. #1
    Befuddled
    Guest

    Matrix multiplication using 1-d arrays

    I know how to properly multiply matrices, so that isn't the problem.

    However, I'm working on a program that multiplies 2 dynamically created matrices, but the data is stored in one dimensional arrays. I thought it would be easier than creating an array of pointers that would point to arrays, but I can't figure out how to properly implement matrix multiplication on the 2 arrays.

    In the followoing code, Matrix is a struct containing (int Row, int Col, and int *Data). Of course, this code doesn't multiply properly:

    Code:
    void matrixMultiply(int MatrixOne, int MatrixTwo)
    { 
        cout << "Multiplying..." << endl;
        int colCount = 0,
            rowInc = 0,
            colInc = 0,
            Mat1Size = mList[MatrixOne].Row * mList[MatrixOne].Col,
            Mat2Size = mList[MatrixTwo].Row * mList[MatrixTwo].Col;
        
        Matrix Answer; //creates answer matrix
        Answer.Row = mList[MatrixOne].Row;
        Answer.Col = mList[MatrixTwo].Col;
        Answer.Data = new int[Answer.Row*Answer.Col]; //allocates matrix space
        for (int ii = 0; ii < Answer.Row*Answer.Col; ii++)
        {
            Answer.Data[ii] = 0; //initializes all data content to zero
        }
        for (int Count = 0; Count < Answer.Row*Answer.Col; Count++)
        {
            colCount = 0;
            colInc = 0;
            for (int rowCount = rowInc; rowCount < mList[MatrixOne].Row + rowInc; rowCount++)
            { //adding and multiplying rows/cols
                 if (rowCount > Mat1Size)
                     break;
                 Answer.Data[Count] += (mList[MatrixOne].Data[rowCount] * mList[MatrixTwo].Data[colCount]);
                 colCount += mList[MatrixTwo].Col;
                 if (colCount > Mat2Size)
                 {
                     colInc++;
                     colCount = colInc;
                 }
            }
            rowInc += mList[MatrixOne].Row;
        }
        matrixOutput(Answer);
    }
    Any help would be greatly appreciated.

  2. #2
    Code Goddess Prelude's Avatar
    Join Date
    Sep 2001
    Posts
    9,796
    >I thought it would be easier than creating an array of pointers that would point to arrays
    Which would be easier? A little discomfort when you create an array of arrays out of a char**, or constant frustration in indexing a single dimension array so that you can fake multidimensional access?
    My best code is written with the delete key.

  3. #3
    Registered User
    Join Date
    Apr 2003
    Posts
    2,662
    "I thought it would be easier than creating an array of pointers that would point to arrays..."

    Apparently not. If you create the pseudo 2d arrays dynamically, then it's very straighforward to multiply two arrays together;
    Code:
    int** matrix1;
    int** matrix2;
    int** matrix3;
    
    for(int i=0; i<rows; i++)
       for(int j=0; j<columns; j++)
          matrix3[i][j] = matrix1[i][j]*matrix2[i][j];
    Last edited by 7stud; 05-25-2003 at 10:38 PM.

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