Binary search with duplicates in a sorted array

Question

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How can I use the Binary search algorithm to display duplicates of a sorted array, with all their indexes. I have a working Binary search algorithm but cannot display all duplicates. Could anyone help and explain how this can be achieved?
Thank You.

What I have tried:

Searching Algorithm:

C#

static int BinarySearch(int[] arr, int key)
        {
            int minNum = 0;
            int maxNum = arr.Length - 1;

            while (minNum <= maxNum)
            {
                int mid = (minNum + maxNum) / 2;
                if (key == arr[mid])
                {
                    return (++mid);
                }
                else if (key > arr[mid])
                {
                    minNum = mid + 1;
                }
                else
                {
                    maxNum = mid - 1;
                }
            }
            return -1;
        }

Posted 11-Mar-20 16:50pm

User 14735579

Updated 18-May-23 5:25am

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2 solutions

Solution 6

I did it as follows :
int arr[]= {1,2,5,5,5,6,8,9,10}; , number =5;
find first occurence and last occurence of given `number`

Java

helper_rec(arr, number, firstindex , lastindex){

	int mid = firstindex + lastindex/2;
	//base case
	if(arr[mid] == number){
		int tempmid=mid;
		while(arr[tempmid]==number && mid >=0){
			tempmid--;//2
		}
		if(arr[tempmid+1]==number{
			first = arr[tempmid+1];
		}
		tempmid =mid;//0
		while(arr[tempmid]==number && mid< arr.length){
			tempmid++;
		}
		if(arr[tempmid]==number){
			second =arr[tempmid-1];
		}
		return;
	}
	
	//recursive
	
	if(mid< number ){
		helper_rec(arr, number, mid+1, lastindex);
	}
	else{
		helper_rec(arr, number , 0 , mid);
	}
	return;
}

Posted 18-May-23 5:25am

RRohra

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Patrice T · Accepted Answer · 2020-03-11T19:46:00

Quote:
How can I use the Binary search algorithm to display duplicates of a sorted array, with all their indexes.

Short answer: you don't, it is not its purpose.
A binary search only gives you the position of the value you want, or the position of 1 of them if duplicated.
To display all duplicates and indexes, you need to do a secondary search around the position returned by binary search routine.

C++

return (++mid);

By the way, why are you adding 1 to the position of matching element ?