CODEWITHSUNDEEP
c#arrays
Dork
Dork

Reputation: 1886

initializing array of arrays populated with same element

var board = Enumerable.Repeat(Enumerable.Repeat('.', 3).ToArray(), 3).ToArray();
board[0][0] = 'Q';

Here I'm trying to create array of arrays (3*3) populated with same element. But when I try to change one element ([0,0]) it changes first element of all three arrays. Why is that and how can this behavior be fixed without having 2 loops to initiate arrays?

Upvotes: 1

Views: 118

Answers (2)

15ee8f99-57ff-4f92-890c-b56153
15ee8f99-57ff-4f92-890c-b56153

Reputation: 37060

Enumerable.Repeat() returns the thing you give it. It doesn't create a new copy of it each time.

The inner Enumerable.Repeat() returns the character '.' three times. Chars are a "value type", meaning that this creates a new copy of the character 'y':

char x = 'y';

You can't not create a copy of a value type when you assign it.

But arrays are a reference type, and the rules are different. The outer Enumerable.Repeat() returns the same array object three times. You're not passing in an instruction to create a new array three times. You're passing in a reference to an existing array, and saying "give me that thing, three times". And unlike a character, giving you the same array again doesn't create a copy of it.

Arrays are a "reference type", meaning that the second line below creates a new reference to the same object:

char[] a = new [] { 'a', 'b', 'c' };
var y = a;

a and y are the same object. If you set a[0] = 'd';, you can use the reference y to look at that same array object, and you'll see the same letter 'd' in [0].

Enumerable.Repeat() returns the same thing x number of times. It's not what you want if you're trying to create multiple instances of a value type.

This is what you want, if you want Linq:

var board = Enumerable.Range(0, 3)
            .Select(n => Enumerable.Repeat('.', 3).ToArray())
            .ToArray();

Select() executes code each time, so the code you execute can create a new array each time. Repeat() just keeps returning whatever value you gave it.

Upvotes: 1

Backs
Backs

Reputation: 24903

Because you created one array Enumerable.Repeat('.', 3).ToArray() and cloned reference to this array 3 times. So, you have 3 reference onto one value. You need to initialize every row:

var board = Enumerable.Repeat((char[])null, 3).ToArray();
for (int i = 0; i < 3; i++)
{
    board[i] = Enumerable.Repeat('.', 3).ToArray();
}
board[0][0] = 'Q';

Upvotes: 2

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