C# Cookbook

9.3. Reversing a Two-Dimensional Array

Problem

You need to reverse each row in a two-dimensional array. The Array.Reverse method does not support this.

Solution

Use two loops; one to iterate over rows, the other to iterate over columns:

public static void Reverse2DimArray(int[,] theArray)
{
    for (int rowIndex = 0; 
         rowIndex <= (theArray.GetUpperBound(0)); rowIndex++)
    {
        for (int colIndex = 0; 
             colIndex <= (theArray.GetUpperBound(1) / 2); colIndex++)
        {
            int tempHolder = theArray[rowIndex, colIndex];                        
            theArray[rowIndex, colIndex] = 
              theArray[rowIndex, theArray.GetUpperBound(1) - colIndex];   
            theArray[rowIndex, theArray.GetUpperBound(1) - colIndex] = 
              tempHolder;      
        }
    }
}

Discussion

The following TestReverse2DimArray method shows how the Reverse2DimArray method is used:

public static void TestReverse2DimArray( )
{
    int[,] someArray = 
      new int[5,3] {{1,2,3},{4,5,6},{7,8,9},{10,11,12},{13,14,15}};

    // Display the original array
    foreach (int i in someArray)
    {
        Console.WriteLine(i);
    }
    Console.WriteLine( );

    Reverse2DimArray(someArray);

    // Display the reversed array
    foreach (int i in someArray)
    {
        Console.WriteLine(i);
    }
}

This method displays the following:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

3    ← Note that each row of 3 elements are reversed
2
1
6    ← This is the start of the next row
5
4
9
8
7
12
11
10
15
14
13

The Reverse2DimArray method uses the same logic presented in the previous recipe to reverse the array; however, a nested for loop is used instead of a single for loop. The outer for loop iterates over each ...