Convert Array to Matrix

Question

I have an array of the length m*n, that stores a list of double element.

How to convert it into a matrix of m*n?

This is the method signature.

//returns a matrix of [m,n], given arr is of length m*n
static double[,] ConvertMatrix(Array arr, int m, int n)
{
}

Hhm you have to know either n or m. An array with 16 elements could be represented by 16*1, 8*2, 4*4, 2*8 and 1*16. — Lasse Espeholt
– Lasse Espeholt, Commented Oct 2, 2010 at 10:18

Jon Skeet · Accepted Answer · 2010-10-03 11:34:34Z

14

You can use Buffer.BlockCopy to do this very efficiently:

using System;

class Test
{
    static double[,] ConvertMatrix(double[] flat, int m, int n)
    {
        if (flat.Length != m * n)
        {
            throw new ArgumentException("Invalid length");
        }
        double[,] ret = new double[m, n];
        // BlockCopy uses byte lengths: a double is 8 bytes
        Buffer.BlockCopy(flat, 0, ret, 0, flat.Length * sizeof(double));
        return ret;
    }

    static void Main()
    {
        double[] d = { 2, 5, 3, 5, 1, 6 };

        double[,] matrix = ConvertMatrix(d, 3, 2);

        for (int i = 0; i < 3; i++)
        {
            for (int j = 0; j < 2; j++)
            {
                Console.WriteLine("matrix[{0},{1}] = {2}", i, j, matrix[i, j]);
            }
        }
    }
}

edited Oct 3, 2010 at 11:34

answered Oct 2, 2010 at 10:20

Jon Skeet

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2 Comments

Frédéric Hamidi Over a year ago

Using sizeof(double) instead of plain 8 would support implementations where the double type has a different size. I don't know if such a beast exists. Yet.

Jon Skeet Over a year ago

@Frédéric: It's actually guaranteed to be 8 by the specification (see section 18.5.8) - but sizeof(...) would be more self-explanatory anyway.

Eugene Cheverda · Accepted Answer · 2010-10-02 10:21:48Z

private static T[,] create2DimArray<T>(T[] array, int n)
        {
            if (n <= 0)
                throw new ArgumentException("Array N dimension cannot be less or equals zero","n");
            if (array == null)
                throw new ArgumentNullException("array", "Array cannot be null");
            if (array.Length == 0)
                throw new ArgumentException("Array cannot be empty", "array");

            int m = array.Length % n == 0 ? array.Length / n : array.Length / n + 1;
            var newArr = new T[m,n];
            for (int i = 0; i < arr.Length; i++)
            {
                int k = i / n;
                int l = i % n;
                newArr[k, l] = array[i];
            }

            return newArr;
        }

Guffa · Accepted Answer · 2010-10-02 10:22:50Z

3

Create the matrix and loop the items into it:

static double[,] ConvertMatrix(double[] arr, int m, int n) {
  double[,] result = new double[m, n];
  for (int i = 0; i < arr.Length; i++) {
    result[i % m, i / m] = arr[i];
  }
  return result;
}

answered Oct 2, 2010 at 10:22

Guffa

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