I need to create 2D array in C#

Question

I need to create 2D jagged array. Think of a matrix. The number of rows is known, the number of columns is not known. For example I need to create array of 10 elements, where type of each element string[]. Why do I need that? The number of columns is not known - this function must simply do the allocation and pass array to some other function.

string[][] CreateMatrix(int numRows)
{
 // this function must create string[][] where numRows is the first dimension.
}

UPDATE

I have C++ background. In C++ I would write the following (nevermind the syntax)

double ** CreateArray()
{
 double **pArray = new *double[10]() // create 10 rows first
}

UPDATE 2

I was considering using List, but I need to have indexed-access to both rows and columns.

Lists give you indexed access to both. So if you use new List<List<string>> you can access it like myList[0][2] — Jamiec
– Jamiec, Commented Oct 7, 2010 at 12:45
Thanks Jamiec, will give it a try. I didn't know List<> provides indexed access. — Captain Comic
– Captain Comic, Commented Oct 7, 2010 at 12:50

Itay Karo · Accepted Answer · 2010-10-07 12:40:39Z

8

return new string[numRows][];

answered Oct 7, 2010 at 12:40

Itay Karo

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2 Comments

CesarGon Over a year ago

Compiles OK for me! Using Visual Studio 2008 and .NET 3.5.

BrokenGlass Over a year ago

This is indeed what the OP asked for and the right solution imo. +1 (any yes it compiles)

Richard J. Ross III · Accepted Answer · 2010-10-07 12:40:23Z

5

Cannot be done. However you could do something like this:

List<List<string>> createMatrix(int numRows)
{
     return new List<List<string>>(numRows);
}

That allows you to be able to have a flexible amount of objects in the second dimesion.

answered Oct 7, 2010 at 12:40

Richard J. Ross III

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2 Comments

Jamiec Over a year ago

Care to explain why you think it cant be done? .NET supports an array of arrays, which is what the OP has asked about.

Richard J. Ross III Over a year ago

Compile time error, try it.. The second boundary must be supplied

CesarGon · Accepted Answer · 2010-10-07 12:51:46Z

2

You can write:

string[][] matrix = new string[numRows][];

and that will produce a 2D array of null elements. If you want to fill the array with non-null items, you need to write:

for (int row = 0; row < numRows; row++)
{
    matrix[row] = new string[/* col count for this row */];
}

You need to specify the column count for each row of the matrix. If you don't know it in advance, you can either.

Leave the matrix items unintialised, and populate them as you know their size.
Use a fixed number that will give you room enough for the maximum possible size.
Avoid arrays and use List<> as suggested by others here.

Hope this helps.

answered Oct 7, 2010 at 12:51

CesarGon

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1 Comment

Captain Comic Over a year ago

Thanks you Cesar, so I have two options for System.Array reference - either keep is set to null or set it some System.Array of known size.

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