Implementing upper_bound() and lower_bound() in C

Given a sorted array arr[] of N integers and a number K, the task is to write the C program to find the upper_bound() and lower_bound() of K in the given array.
Examples:

Input: arr[] = {4, 6, 10, 12, 18, 20}, K = 6 
Output:
Lower bound of 6 is 6 at index 1 
Upper bound of 6 is 10 at index 2
Input: arr[] = {4, 6, 10, 12, 18, 20}, K = 20 
Output:
Lower bound of 20 is 20 at index 5 
Upper bound doesn’t exist 

Approach: The idea is to use Binary Search. Below are the steps: 

• For lower_bound():
  1. Initialise the startIndex as 0 and endIndex as N – 1.
  2. Compare K with the middle element(say arr[mid]) of the array.
  3. If the middle element is greater equals to K then update the endIndex as a middle index(mid).
  4. Else Update startIndex as mid + 1.
  5. Repeat the above steps until startIndex is less than endIndex.
  6. After all the above steps the startIndex is the lower_bound of K in the given array.
• For upper_bound():
  1. Initialise the startIndex as 0 and endIndex as N – 1.
  2. Compare K with the middle element(say arr[mid]) of the array.
  3. If the middle element is less than equals to K then update the startIndex as middle index + 1(mid + 1).
  4. Else Update endIndex as mid.
  5. Repeat the above steps until startIndex is less than endIndex.
  6. After all the above steps the startIndex is the upper_bound of K in the given array.

Below is the iterative and recursive implementation of the above approach:

Iterative Solution

// C program for iterative implementation
// of the above approach

#include <stdio.h>

// Function to implement lower_bound
int lower_bound(int arr[], int N, int X)
{
    int mid;

    // Initialise starting index and
    // ending index
    int low = 0;
    int high = N;

    // Till low is less than high
    while (low < high) {
        mid = low + (high - low) / 2;

        // If X is less than or equal
        // to arr[mid], then find in
        // left subarray
        if (X <= arr[mid]) {
            high = mid;
        }

        // If X is greater arr[mid]
        // then find in right subarray
        else {
            low = mid + 1;
        }
    }

    // if X is greater than arr[n-1]
    if(low < N && arr[low] < X) {
       low++;
    }

    // Return the lower_bound index
    return low;
}

// Function to implement upper_bound
int upper_bound(int arr[], int N, int X)
{
    int mid;

    // Initialise starting index and
    // ending index
    int low = 0;
    int high = N;

    // Till low is less than high
    while (low < high) {
        // Find the middle index
        mid = low + (high - low) / 2;

        // If X is greater than or equal
        // to arr[mid] then find
        // in right subarray
        if (X >= arr[mid]) {
            low = mid + 1;
        }

        // If X is less than arr[mid]
        // then find in left subarray
        else {
            high = mid;
        }
    }

    // if X is greater than arr[n-1]
    if(low < N && arr[low] <= X) {
       low++;
    }

    // Return the upper_bound index
    return low;
}

// Function to implement lower_bound
// and upper_bound of X
void printBound(int arr[], int N, int X)
{
    int idx;

    // If lower_bound doesn't exists
    if (lower_bound(arr, N, X) == N) {
        printf("Lower bound doesn't exist");
    }
    else {

        // Find lower_bound
        idx = lower_bound(arr, N, X);
        printf("Lower bound of %d is"
               "% d at index % d\n ",
               X,
               arr[idx], idx);
    }

    // If upper_bound doesn't exists
    if (upper_bound(arr, N, X) == N) {
        printf("Upper bound doesn't exist");
    }
    else {

        // Find upper_bound
        idx = upper_bound(arr, N, X);
        printf("Upper bound of %d is"
               "% d at index % d\n ",
               X,
               arr[idx], idx);
    }
}

// Driver Code
int main()
{
    // Given array
    int arr[] = { 4, 6, 10, 12, 18, 20 };
    int N = sizeof(arr) / sizeof(arr[0]);

    // Element whose lower bound and
    // upper bound to be found
    int X = 6;

    // Function Call
    printBound(arr, N, X);
    return 0;
}

Recursive Solution

// C program for recursive implementation
// of the above approach

#include <stdio.h>

// Recursive implementation of
// lower_bound
int lower_bound(int arr[], int low,
                int high, int X)
{

    // Base Case
    if (low > high) {
        return low;
    }

    // Find the middle index
    int mid = low + (high - low) / 2;

    // If arr[mid] is greater than
    // or equal to X then search
    // in left subarray
    if (arr[mid] >= X) {
        return lower_bound(arr, low,
                           mid - 1, X);
    }

    // If arr[mid] is less than X
    // then search in right subarray
    return lower_bound(arr, mid + 1,
                       high, X);
}

// Recursive implementation of
// upper_bound
int upper_bound(int arr[], int low,
                int high, int X)
{

    // Base Case
    if (low > high)
        return low;

    // Find the middle index
    int mid = low + (high - low) / 2;

    // If arr[mid] is less than
    // or equal to X search in
    // right subarray
    if (arr[mid] <= X) {
        return upper_bound(arr, mid + 1,
                           high, X);
    }

    // If arr[mid] is greater than X
    // then search in left subarray
    return upper_bound(arr, low,
                       mid - 1, X);
}

// Function to implement lower_bound
// and upper_bound of X
void printBound(int arr[], int N, int X)
{
    int idx;

    // If lower_bound doesn't exists
    if (lower_bound(arr, 0, N, X) == N) {
        printf("Lower bound doesn't exist");
    }
    else {

        // Find lower_bound
        idx = lower_bound(arr, 0, N, X);
        printf("Lower bound of %d "
               "is %d at index %d\n",
               X, arr[idx], idx);
    }

    // If upper_bound doesn't exists
    if (upper_bound(arr, 0, N, X) == N) {
        printf("Upper bound doesn't exist");
    }
    else {

        // Find upper_bound
        idx = upper_bound(arr, 0, N, X);
        printf("Upper bound of %d is"
               "% d at index % d\n ",
               X,
               arr[idx], idx);
    }
}

// Driver Code
int main()
{
    // Given array
    int arr[] = { 4, 6, 10, 12, 18, 20 };
    int N = sizeof(arr) / sizeof(arr[0]);

    // Element whose lower bound and
    // upper bound to be found
    int X = 6;

    // Function Call
    printBound(arr, N, X);
    return 0;
}

Output:

Lower bound of 6 is 6 at index  1
 Upper bound of 6 is 10 at index  2

Time Complexity: O(log₂ N), where N is the number of elements in the array.
Auxiliary Space: O(1).