C# - float[][] declaration

Question

I'm trying to declare this kind of variable:

float[][]

Things that didn't work for me (wouldn't compile) -

float[][] inputs = new float[10][5];
float[][] inputs = new float[10, 5];

When trying to declare the array like this -

int a = 3;
int b = 2;

float[][] inputs = new float[][]
{
   new float[a],
   new float[b]
};

I get a multidimensional array with two float arrays instead of an array that has 3 arrays and every array size is 2.

This is explained in a lot of C# starters. E.g.: https://msdn.microsoft.com/en-us/library/2yd9wwz4.aspx and https://msdn.microsoft.com/en-us/library/2s05feca.aspx — JHBonarius, Mar 01 '17 at 10:00

Dmitry Bychenko · Accepted Answer · 2017-03-01T09:32:55.667

7

Well, there are two different types:

Array of array (jagged array):

float[][] sample = new float[][] {
  new float[] {1, 2, 3},
  new float[] {4, 5}, // notice that lines are not necessary of the same length
};

2d array:

float[,] sample2 = new float[,] {
  {1, 2, 3},
  {4, 5, 6},
};

Edit: your code amended:

  // jagged array (10 arrays each of which has 5 items)
  float[][] inputs = new float[10][] {
    new float[5],
    new float[5],
    new float[5],
    new float[5],
    new float[5],
    new float[5],
    new float[5],
    new float[5],
    new float[5],
    new float[5],
  };

You can shorten the declaration with a help of Linq:

  float[][] inputs = Enumerable
    .Range(0, 10)               // 10 items
    .Select(i => new float[5])  // each of which is 5 items array of float
    .ToArray();                 // materialized as array

Or in case of 2d array

  // 2d array 10x5
  float[,] inputs = new float[10,5];

edited Mar 01 '17 at 09:32

answered Mar 01 '17 at 09:15

Dmitry Bychenko

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Related [What are the differences between a multidimensional array and an array of arrays in C#?](http://stackoverflow.com/q/597720/579895) – Pikoh Mar 01 '17 at 09:16
is your (amended) initialization code more efficient then using a loop? E.g. `for (var idx = 0; idx < a; idx++) input[idx] = new float[b];`? – JHBonarius Mar 01 '17 at 09:40
@J.H.Bonarius: If *building* array is allowed then you can come up with a *loop* solution (as you've done), however, when *declaration* required, e.g. `private float[][] inputs = ?` you can't use loops. – Dmitry Bychenko Mar 01 '17 at 09:42
Could you elaborate on "building arrays"? I don't know the concept... I've been using the loop a lot in my code, and as its a high performance mathematical application, I would like to optimize. edit: maybe I should just ask a question in [cr] – JHBonarius Mar 01 '17 at 09:46
@:J.H.Bonarius you can't *declare* an array (say, a `field` within a class) with a help of loops: `private float[][] inputs = /* no loops here*/`. However, if you're allowed to declare the field as `float[][] inputs = new float[10][];` and only after that (in constructor) to build the array: `public MyClass() {/* you can use loops within a method/constructor */}` loop is a good solution. – Dmitry Bychenko Mar 01 '17 at 09:53

C# - float[][] declaration

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