C# Using Linq to get column from jagged array

Question

How do I get elements of a column from a jagged array as a flat array using Linq ????

public class Matrix<T>
{
    private readonly T[][] _matrix;

    public Matrix(int rows, int cols)
    {
        _matrix = new T[rows][];
        for (int r = 0; r < rows; r++)
        {
            _matrix[r] = new T[cols];
        }
    }

    public T this[int x, int y]
    {
        get { return _matrix[x][y]; }
        set { _matrix[x][y] = value; }
    }

    // Given a column number return the elements
    public IEnumerable<T> this[int x]
    {
        get
        {
        }
    }
}

Matrix<double> matrix = new Matrix<double>(6,2);
matrix[0, 0] = 0;
matrix[0, 1] = 0;
matrix[1, 0] = 16.0;
matrix[1, 1] = 4.0;
matrix[2, 0] = 1.0;
matrix[2, 1] = 6.0;
matrix[3, 0] = 5.0;
matrix[3, 1] = 7.0;
matrix[4, 0] = 1.3;
matrix[4, 1] = 1.0;
matrix[5, 0] = 1.5;
matrix[5, 1] = 4.5;

Mmh... could you provide an example of the input data and the desired output ? — digEmAll
– digEmAll, Commented Oct 12, 2011 at 10:48

Community · Accepted Answer · 2017-05-23 12:00:16Z

14

It's just:

public IEnumerable<T> this[int x]
{
    get
    {
       return _matrix.Select(row => row[x]);
    }
}

Of course it's better to check if x is not out of the range before the LINQ query.

Anyway, given that you're working on a matrix, for more clarity you could switch to a bidimensional array instead of a jagged array (so no doubt about the size of the 2 dimensions).

Instead, if performance really really matters for you, keep using jagged arrays which seem to be a bit faster than bidimensional arrays (e.g LINK1, LINK2).

edited May 23, 2017 at 12:00

CommunityBot

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answered Oct 12, 2011 at 11:02

digEmAll

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4 Comments

IUnknown Over a year ago

In my opinion this would be better: _matrix.Where(array => array.Length > index).Select(array => array[index])

digEmAll Over a year ago

@YuriyRozhovetskiy: actually this control can be made before the query, because this jagged array has always the same number of columns by construction.

day0ops Over a year ago

@digEmAll i see your point. is it because jaggeds can have mixed sizes ?

digEmAll Over a year ago

@kuzyt: yes, but it's just a matter of syntax/clarity. If performance really matters for you, jagged arrays seems to be a bit faster than multidimensional array (check my edit).

IUnknown · Accepted Answer · 2011-10-12 10:59:41Z

2

var source = new int[3][] {
    Enumerable.Range(1,3).ToArray(),
    Enumerable.Range(10,5).ToArray(),
    Enumerable.Range(100,10).ToArray()
};

int index = 0;

var result = (from array in source
                from item in array
                group item by Array.IndexOf(array, item) into g
                where g.Key == index
                select g.ToArray()).FirstOrDefault();

answered Oct 12, 2011 at 10:59

IUnknown

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Comments

Massimiliano Peluso · Accepted Answer · 2011-10-12 10:50:15Z

0

var q = from row in jagged 
    from value in row 
    where value == anyvalue 
    select value;

anyway why do you want to use LINQ? using a classic for it will improve the performance and make easier debugging

answered Oct 12, 2011 at 10:50

Massimiliano Peluso

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2 Comments

digEmAll Over a year ago

Your where clause works on the value, but in order to select always the same column, I think it should work on the index...

Massimiliano Peluso Over a year ago

In general I would use a classic for which is faster and easier to debug. You should use LINQ to object only to improve readability as in general the classic for is much faster

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