C# Jagged array: outside the bounds of the array issue

Question

I have a jagged array declared as:

char[][] m = new char[10][];

Later it is populated.

System.Text.StringBuilder s = new System.Text.StringBuilder(c);
for (int x = 0; x < 10; x++)
    s.Append('=');

    for (int x = 0; x < 10; x++) m[x] = s.ToString().ToCharArray();

If I perform the below operations, I get an error on the second dimension:

Console.WriteLine(String.Format("width={0}", m.GetLength(0)));
Console.WriteLine(String.Format("height={0}", m.GetLength(1))); <---- ERROR HERE

Any ideas?

It's a jagged array, not a square array (which you declare with char[,] maz). Each sub array is going to have a different length. that you check with maz[index].Length. — Abion47
– Abion47, Commented Feb 23, 2017 at 14:17
Don't use GetLenght(), rather Length of individual item of first array. — Sinatr
– Sinatr, Commented Feb 23, 2017 at 14:17

Abion47 · Accepted Answer · 2017-02-23 16:37:15Z

You are confusing square arrays and jagged arrays. A jagged array is also known as an array of arrays, and it looks like this:

char[][] a = new char[10][];

Each element in a jagged array is an array in its own right, and it can have a completely different length from any of the others:

a[0] = new char[1];
a[1] = new char[100];
a[2] = new char[5];
...
a[9] = new char[999999];

As you can see, GetLength(1) makes no sense on a jagged array. What would it even return? To get the lengths of the second level of arrays, you would have to iterate through them and call their various Length properties:

for (int i = 0; i < a.Length; i++)
{
    int length = a[i].Length;
}

For your purposes, you probably want to use a square array (also known as a 2D array) instead:

char[,] a = new char[10,10];

Unlike a jagged array, the second dimension of a square array is guaranteed to be the same length. That means GetLength(1) can return a value with confidence.

René Vogt · Accepted Answer · 2017-02-23 14:19:23Z

1

Array.GetLength(int dimension) is only for multi-dimensional arrays. Your array 'm' is not multi-dimensional, but jagged.

That means it's an array with element type char[].

So you only have one dimension (with length 10) and GetLength(1) throws the error.

You can get m[i].Length for i from 0 to 9. But only if you initialized the elements:

m[0] = new char[10];
int l = m[0].Length; // 10

answered Feb 23, 2017 at 14:19

René Vogt

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1 Comment

Abion47 Over a year ago

Technically speaking, you can use GetLength for any kind of array. It will just throw an exception if you try to query a dimension that doesn't exist, and for every non-multidimensional array, the only dimension that exists is 0. (There's also the fact that there's literally no reason to use GetLength on a non-multidimensional array since Length is both faster and more convenient.)

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