How can I delete rows and columns that only contains zero's in an 2d array?

Question

I have an 2d array and I have no idea how I can delete the rows and columns from it in which are only zero's.

int [,] test = {   {0, 0, 0, 0},
                   {0, 0, 1, 1},
                   {0, 1, 1, 0},
                   {0, 0, 0, 0}  }

 int row = pieces.GetLength(0);
        int col = pieces.GetLength(1);
        for (int y = 0; y < row; y++)
        {
            for (int x = 0; x < col; x++)
            {
                if (pieces[y,x] == 0)
                {
                }
            }
        }

So I want it to be this:

int [,] test = { {0, 1, 1},
                 {1, 1, 0}}

Copy only the array elements that you want from the old array to a new array. — Robert Harvey
– Robert Harvey, Commented May 18, 2022 at 17:44
But can I make a program so the computer will do it automatically? Because I want this for other arrays too. — MajorAnonymous
– MajorAnonymous, Commented May 18, 2022 at 17:48
You are not deleting zeros (since there are sill some left in the desired output), but rather omitting rows and columns that contain only zeros. Please edit the post and fix the title so it is clear what you want. — John Alexiou
– John Alexiou, Commented May 18, 2022 at 17:48
what happens when you have such row/column in middle? do you delete it? — eocron
– eocron, Commented May 18, 2022 at 17:52
I have never such a row/column in the middle. To be clear, this is a figure of tetris. — MajorAnonymous
– MajorAnonymous, Commented May 18, 2022 at 17:53

AMunim · Accepted Answer · 2022-05-18 18:23:32Z

2

Here this will remove every Column/Row with only zero

int[][] test = {   new int[]{0, 0, 0, 0},
                   new int[]{0, 0, 1, 1},
                   new int[]{0, 1, 1, 0},
                   new int[]{0, 0, 0, 0}  };
var result = 
    Transpose(Transpose(test.Where(row => !row.All(item => item == 0)).ToArray()).Where(row => !row.All(item => item == 0)).ToArray())
    .ToArray();

This basically selects the rows where there isn't a single 0. Now transpose the matrix(its easier to not select rows than not selecting columns) and again select every row where there isn't any 0(meaning there is at least a 1 we need), now transpose back to original order.

here are the helper methods


static int[][] Transpose(int[][] originalArray)
{
    int w = originalArray.Length;
    int h = originalArray[0].Length;

    int[][] result = CreateJaggedArray<int[][]>(h, w);

    for (int i = 0; i < originalArray.Length; i++)
        for (int j = 0; j < originalArray[i].Length; j++)
        {
            result[j][i] = originalArray[i][j];
        }

    return result;
}

static T CreateJaggedArray<T>(params int[] lengths)
{
    return (T)InitializeJaggedArray(typeof(T).GetElementType(), 0, lengths);
}

static object InitializeJaggedArray(Type type, int index, int[] lengths)
{
    Array array = Array.CreateInstance(type, lengths[index]);
    Type elementType = type.GetElementType();

    if (elementType != null)
    {
        for (int i = 0; i < lengths[index]; i++)
        {
            array.SetValue(
                InitializeJaggedArray(elementType, index + 1, lengths), i);
        }
    }

    return array;
}

you can convert them to extensions if you want since they are static anyway.

Check out the complete example at dotnetfiddle

answered May 18, 2022 at 18:23

AMunim

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3 Comments

MajorAnonymous Over a year ago

Appreciate it man!

Salman Arshad Over a year ago

A comment below OP allows for a simpler solution: iterate the matrix top to bottom, left to right, and keep a track of smallest and largest row and smallest and largest column that contain a non zero value. Then extract that "rectangle" from the matrix. All this if there is no blank row/column in the middle.

AMunim Over a year ago

@SalmanA Nice idea, hats off to my senior for always finding a solution better than mine XD

user13770736 · Accepted Answer · 2022-05-18 18:47:41Z

Ok i created a code, that will first create List<int> out of each row and column, and add its indexes to another List<int> that will mark columns and rows at which '1' does appear. This code uses Linq, to check .Contains() if 1 does exists. Rest should be fairly easy to read.

int[,] test = { {0, 0, 0, 0},
                {0, 0, 1, 1},
                {0, 1, 1, 0},
                {0, 0, 0, 0}};

//get size
int columns = test.GetLength(0);
int rows = test.Length / columns;
//create lists
List<int> nonZeroRows = new List<int>();
List<int> nonZeroColumns = new List<int>();
for (int row = 0; row < rows; row++)
{
    List<int> rowAsList = new List<int>();
    List<int> columnAsList = new List<int>();
    for (int col = 0; col < columns; col++)
    {
        rowAsList.Add(test[row, col]);
        columnAsList.Add(test[col, row]);
    }
    //check if row & column contains 1
    if(rowAsList.Contains(1))
    {
        nonZeroRows.Add(row);
    }
    if (columnAsList.Contains(1))
    {
        nonZeroColumns.Add(row);
    }
}

//create new array
int[,] result = new int[nonZeroRows.Count, nonZeroColumns.Count];
int irow = 0;
foreach(int row_id in nonZeroRows)
{
    int icol = 0;
    foreach (int col_id in nonZeroColumns)
    {
        result[irow,icol] = test[row_id, col_id];
        icol++;
    }
    irow++;
}

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