在3D矩阵中找到最大值

Question

I have matrix with 3 dimension (n*m*k). I am trying to fined the maximum number for each n and m by searching in k dimension.((I try to find the maximum number in k dimension for each n and m)) and at last i have a 2d matrix (n*m). i have the following code but it is so slow. Is there any new code or any changes to the current code that do this more quickly.

thanks.

my c# code: note: li is the 3 dimension matrix and they are grater or equal to zero.

int[,] array2 = new int[n, m];   
int[,] array = new int[n, m];
 List<Array> li = new List<Array>();             
for(int k = 0; k <'not a specific value, change each time' ; k++)
   { 
     li.Add(array);
     %% changing array
   } %% so li will became a (n*m*k) matrix   

for (int i = 0; i < n; i++)
                for (int j = 0; j < m; j++)
                {
                    int ma = -2;
                    int d = 0;
                    while (d <= k)
                    {
                        ma = Math.Max(ma, Convert.ToInt32(li[d].GetValue(i, j)));
                        d++;
                    }
                    array2[i, j] = ma;
                }

Answer 1

The biggest performance issue is that you use Array objects as elements of your list. This makes it so that every element access using GetValue boxes the value, ie allocates a new tiny object to hold the element value.

Your code will run a lot faster if you replace

List<Array> li = new List<Array>();

with

List<int[,]> li = new List<int[,]>();

and

ma = Math.Max(ma, Convert.ToInt32(li[d].GetValue(i, j)));

with

ma = Math.Max(ma, li[d][i, j];

Since you don't know the 3rd dimension in advance, it is harder to use 3D arrays.

An entirely different approach would be to compute the maximum as you're building the list li . This will help in two ways: 1. You avoid indexing into the list of arrays and 2. as long as m and n aren't too large, you improve locality. That is: the values you're working with are closer together in memory, and more likely to be in the processor cache.

Answer 2

This should do the trick (even though it could be kinda slower than your approach):

// put this at the top of your source file
using System.Linq;

// put this where you calculate the maxima
for(int i = 0; i < array2.GetLength(0); ++i)
for(int j = 0; j < array2.GetLength(1); ++j)
{
    array2[i, j] = Convert.ToInt32(li.Max(x => x.GetValue(i, j)));
}

Answer 3

From an algorithm perspective, there's no more efficient way to evaluate the maximum value for fixed n,m for all k. It will take O(n*m*k) operations.

Now, the only way to improve performance is to find improvements in your implementation, particularly in your storage of the 3D matrix.

Using List<Array> is a major area for improvement. You are prone to boxing issues (conversion of primitive types to objects) and making more function calls than are necessary.

Reduce your 3D matrix to an array of primitives:

int[] my3DArray = new int[n * m * l]; // Note I'm using l where you use k 

Now index into your array at [i, j, k] using the following offset:

int elementAtIJK = my3DArray[i + (n * j) + (m * n * k)];

If you just use arrays of primitives you should see a definite improvement.

Edit:

In fact, in C# (and several other languages) it's very easy to implement 3D arrays directly , eg:

   int[,,] my3DArray = new int[n,m,l];
   int elementAtIJK = my3DArray[i,j,k];

Which is much simpler than I first described (but at the end of the day is internally translated in the 1D form).

What to do if the 3rd dimension varies in size...

Now, it gets more interesting if the size of the 3rd dimension varies significantly. If it has a known maximum and isn't too big, you can simply set it to the maximum and fill the empty values with zeroes. This is simple and may meet your needs.

However, if the 3rd dimension can be very big, all these extra stored zeroes could waste a lot of valuable space and what you need is a Sparse Matrix representation.

There are different storage mechanisms for sparse matrices. For your purposes, you could consider your 3D array to be a 2D matrix, with (n*m) rows and max(k) columns. As the 3rd dimension varies in length, there are lots of empty spaces in your columns. This is called a sparse row and the standard data storage for this is "Compressed Sparse Row". Again, for performance this can be represented just by three primitive arrays, a data array, a row index array and a column pointer array. There are resources elsewhere on the web that describe the CSR implementation better than I can, but hopefully this will point you in the right direction.

Answer 4

You could use a three-dimensional array like this:

int xRange = 10;
int yRange = 10;
int zRange = 10;

int[, ,] matrix = new int[xRange, yRange, zRange];

// set up some dummy values
for (int x = 0; x < xRange; x++)
    for (int y = 0; y < yRange; y++)
        for (int z = 0; z < zRange; z++)
            matrix[x, y, z] = x * y * z;

// calculate maximum values
int[,] maxValues = new int[xRange, yRange];

/* LINQ version of maximum calculation
for (int x = 0; x < xRange; x++)
    for (int y = 0; y < yRange; y++)
        maxValues[x, y] = Enumerable.Range(0, zRange).Select(z => matrix[x, y, z]).Max();
*/

// longhand version of maximum calculation
for (int x = 0; x < xRange; x++)
    for (int y = 0; y < yRange; y++)
        for (int z = 0; z < zRange; z++)
            maxValues[x, y] = Math.Max(maxValues[x, y], matrix[x, y, z]);

// display results
for (int x = 0; x < xRange; x++)
{
    for (int y = 0; y < yRange; y++)
        Console.Write("{0}\t", maxValues[x, y]);
    Console.WriteLine();
}