Implementation of C lower_bound

lower_bound is almost like doing a usual binary search, except:

1. If the element isn't found, you return your current place in the search, rather than returning some null value.
2. If the element is found, you search leftward until you find a non-matching element. Then you return a pointer/iterator to the first matching element.

Yes, it's really that simple. :-)

Here are the equivalent implementations of upper_bound and lower_bound. This algorithm is O(log(n)) in the worst case, unlike the accepted answer which gets to O(n) in the worst case.

Note that here high index is set to n instead of n - 1. These functions can return an index which is one beyond the bounds of the array. I.e., it will return the size of the array if the search key is not found and it is greater than all the array elements.

int bs_upper_bound(int a[], int n, int x) {
    int l = 0;
    int h = n; // Not n - 1
    while (l < h) {
        int mid =  l + (h - l) / 2;
        if (x >= a[mid]) {
            l = mid + 1;
        } else {
            h = mid;
        }
    }
    return l;
}

int bs_lower_bound(int a[], int n, int x) {
    int l = 0;
    int h = n; // Not n - 1
    while (l < h) {
        int mid =  l + (h - l) / 2;
        if (x <= a[mid]) {
            h = mid;
        } else {
            l = mid + 1;
        }
    }
    return l;
}

The actual C++ implementation works for all containers. You can find it here.

I know that this is a very old post. However, I was working on a problem and I came across this post. I would like to add my iterative version for the problem which is an extension of the last answer. I checked this with the test cases I could think of. I've attached my code in C#.

This code was working for all ranges. However, the range should be within the first index to the last index+1. If the array is of size N and considering range as [0,N] the search space will be within [0,N). I know that's pretty obvious but it helped me checking some edge cases.

        static int lower_bound(int[] a, int lo,int hi, int x)
        {
            while (lo < hi) 
            {
                int mid = lo + (hi-lo) / 2;
                if(a[mid]==x)
                {
                    /*when there is a match, we should keep on searching
                    for the next same element. If the same element is not                                                         
                    found, mid is considered as the answer and added to 'hi'
                    Finally 'hi' is returned*/
                    if(a[mid-1]!=x)
                    {
                        hi=mid;
                        break;
                    }
                    else
                        hi=mid-1; 
                }
                else if(a[mid]>x)
                    hi=mid-1;
                else
                    lo=mid+1;
            }
            //if element is not found, -1 will be returned   
            if(a[hi]!=x)
                return -1;
            return hi;
        }
        static int upper_bound(int[] a, int lo,int hi, int x)
        {
            int temp=hi;
            while (lo < hi) 
            {
                int mid = lo + (hi-lo) / 2;
                if(a[mid]==x)
                {
                    /*this section make sure that program runs within        
                    range [start,end)*/
                    if(mid+1==hi)
                    {   
                        lo=mid;
                        break;
                    }
                    /*when there is a match, we should keep on searching
                      for the next same element. If the same element is not                                                         
                      found, mid is considered as the answer and added to
                      'lo'. Finally 'lo' is returned*/ 
                    if(a[mid+1]!=x)
                    {
                        lo=mid;
                        break;
                    }
                    else
                        lo=mid+1;
                }


         else if(a[mid]>x)
             hi=mid-1;
         else
             lo=mid+1;
    }
    //if element is not found, -1 will be returned
    if(a[lo]!=x)
            return -1;
        return lo;
    }

Here is a test case that I used:

Array(a) : 1 2 2 2 2 5 5 5 5
size of the array(a) : 9

Considering search element as 2:

upper_bound(a,0,9,2)=4, lower_bound(a,0,9,2)=1

Considering search element as 5:

upper_bound(a,0,9,2)=8, lower_bound(a,0,9,2)=5

Considering search element as 1:

upper_bound(a,0,9,2)=0, lower_bound(a,0,9,2)=0

Considering search element as 5:

upper_bound(a,5,9,2)=8, lower_bound(a,5,9,2)=5

The lower_bound and upper_bound functions in python would be implemented as follows:

def binLowerBound(a, lo, hi, x):
  if (lo > hi):
    return hi
  mid = (lo + hi) / 2;
  if (a[mid] == x):
    return binLowerBound(a, lo, mid-1, x)
  elif (a[mid] > x):
    return binLowerBound(a, lo, mid-1, x)
  else:
    return binLowerBound(a, mid+1, hi, x)

def binHigherBound(a, lo, hi, x):
  if (lo > hi):
    return lo
  mid = (lo + hi) / 2;
  if (a[mid] == x):
    return binHigherBound(a, mid+1, hi, x)
  elif (a[mid] > x):
    return binHigherBound(a, lo, mid-1, x)
  else:
    return binHigherBound(a, mid+1, hi, x)