Binary search of a sorted array

Question

I am trying to search a descending sorted array using this binary search code. However, after I sort it, and try to search, it doesn\'t come back with any result, just a loading

Answer 1

There is a binary search in the Array class:

int index = Array.BinarySearch(mynumbers, target);

For descending order, this can be easily accomplished with a ReverseComparer which is easy to write like:

    public class ReverseComparer<T> : IComparer<T>
    {
        public int Compare(T x, T y)
        {
            return Comparer<T>.Default.Compare(y, x);
        }
    }

Then:

int index = Array.BinarySearch(numbers, 7, new ReverseComparer<int>());

If this is an academic exercise and you must use a custom search, of course, this won't apply. If it's got to be a custom algorithm for a class, then the problems are that you must break out of the loop when found, and the index is at mid, not at mynumbers[mid]:

    //for a sorted array with descending values
    while (first<=last)
    {
        mid = (first + last) / 2;

        if (target < mynumbers[mid])
        {
            first = mid + 1;
        }

        if (target > mynumbers[mid])
        {
            last = mid - 1;
        }

        else
        {
            // the index is mid, not mynumbers[mid], and you need to break here
            // once found or it's an infinite loop once it finds it.
            Label11.Text = "Target " + item + " was found at index " + mid;
            break;
        }
    }

And actually, I'd probably set a bool flag instead to keep the algorithm pure and not mix the find with the output concerns, this will also make it easier to tell what happened if you exit the loop with not found:

    bool found = false;

    //for a sorted array with descending values
    while (!found && first<=last)
    {
        mid = (first + last) / 2;

        if (target < mynumbers[mid])
        {
            first = mid + 1;
        }

        if (target > mynumbers[mid])
        {
            last = mid - 1;
        }

        else
        {
            // You need to stop here once found or it's an infinite loop once it finds it.
            found = true;
        }
    }

    Label11.Text = found 
        ? "Item " + item + " was found at position " + mid
        : "Item " + item + " was not found";

Answer 2

Its worked for me

public static int search(int[] arr, int value)
{
    Debug.Assert(arr.Length>0);
    var left = 0;
    var right = arr.Length - 1;

    while (((right - left)/2) > 0)
    {
        var middle = left + (right - left)/2;
        if (arr[middle] < value)
            left = middle + 1;
        else
            right = middle;
    }
    return arr[left] >= value ? left : right;
}

Answer 3

This is one correct :

if target< mynumbers[mid] then you have to take last to mid-1, because we have to search in lower range i.e. first to mid-1

while (first<=last)
        {
            mid = (first + last) / 2;
            if (target == mynumbers[mid])
            Label11.Text = "Target " + item + " was found at index " + mynumbers[mid];

            else if (target < mynumbers[mid])
                last = mid - 1;
            else (target > mynumbers[mid])
                first = mid + 1;

            }

Answer 4

Shot in the dark:

if (target < mynumbers[mid]) 
   first = mid + 1; 
else if (target > mynumbers[mid]) 
   last = mid - 1; 
else 
{
    ....
    break;
}

I suspect you're bouncing back and forth between mid+1 and mid-1

Answer 5

//this works fine with these Test cases    
// has to check if (target == mynumbers[mid])    
// this is for an array with ascending order.
class Program
{

    static void Main(string[] args)
    {
        // TEST cases:
        // for 8: item 8 was not found
        // for 4: item 4 found at Position 3
        // for 1: item 1 found at position 0
        // for 0: item 0 was not found


        int target =8;
        int searchkey = target;

        int[] mynumbers = { 1, 2, 3, 4, 5 };

        int mid=0, first = 0, last = mynumbers.Length - 1;

        bool found = false;

        //for a sorted array with ascending values
        while (!found && first <= last)
        {
            mid = (first + last) / 2;

            if (target == mynumbers[mid])
                found = true;
            else
            {

                if (target > mynumbers[mid])
                {
                    first = mid + 1;
                }

                if (target < mynumbers[mid])
                {
                    last = mid - 1;
                }

            }

        }

        String foundmsg = found
            ? "Item " + searchkey + " was found at position " + mid
            : "Item " + searchkey + " was not found";
        Console.WriteLine(foundmsg);
     }
}

Answer 6

public static object BinnarySearch(int[] array,int SearchKey)
    {
        int min = 0;
        int max = array.Length - 1;
        while (min <= max)
        {
            int mid = (min + max) / 2;
            if (SearchKey == array[mid])
            {
                return mid;
            }
            else if (SearchKey < array[mid])
            {
                max = mid - 1;
            }
            else
                min = mid + 1;
        }
        return "Not Found";
    }