Fastest way to check if an array is sorted

Question

Considering there is an array returned from a function which is of very large size.

What will be the fastest approach to test if the array is sorted?

Answer 1

The only improvement i can think of is check both ends of the array at the same time, this little change will do it in half time...

public static bool IsSorted(int[] arr)
{
int l = arr.Length;
for (int i = 1; i < l/2 + 1 ; i++)
{
    if (arr[i - 1] > arr[i] || arr[l-i] < arr[l-i-1])
    {
    return false;
    }
}
return true;
}

Answer 2

This is what I came up with and find works better particularly with greater sized arrays. The function is recursive and will be called for the very first time, say in a while loop like this

while( isSorted( yourArray, 0 )

The if statement checks if the bounds of the array have been reached.

The else if statement will call itself recursively and break at any time when the condition becomes false

 public static bool IsSorted(int[] arr, int index)
    {
        if (index >= arr.Length - 1)
        {
            return true;
        }
        else if ((arr[index] <= arr[ index + 1]) && IsSorted(arr, index + 1))
        {
            return true;
        }
        else
        {
            return false;
        }
    }

Answer 3

Here is my version of the function IsSorted

public static bool IsSorted(int[] arr)
{               
    int last = arr.Length - 1;
    if (last < 1) return true;

    int i = 0;

    while(i < last && arr[i] <= arr[i + 1])
        i++;

    return i == last;
}

While this function is a bit faster than in the question, it will do fewer assignments and comparisons than anything has been posted so far. In the worst case, it does 2n+1 comparisons. It still can be improved if you can make a reasonable assumption about the nature of the data like minimum data size or array contains even number of elements.