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C / C++ Program for Median of two sorted arrays of same size

  • Last Updated : 11 Dec, 2018

There are 2 sorted arrays A and B of size n each. Write an algorithm to find the median of the array obtained merging the above 2 arrays(i.e. array of length 2n). The complexity should be O(log(n)).

median-of-two-arrays

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

Note : Since size of the set for which we are looking for median is even (2n), we need take average of middle two numbers and return floor of the average.

Method 1 (Simply count while Merging)
Use merge procedure of merge sort. Keep track of count while comparing elements of two arrays. If count becomes n(For 2n elements), we have reached the median. Take the average of the elements at indexes n-1 and n in the merged array. See the below implementation.

C++




// A Simple Merge based O(n)
// solution to find median of
// two sorted arrays
#include <bits/stdc++.h>
using namespace std;
  
/* This function returns 
median of ar1[] and ar2[].
Assumptions in this function:
Both ar1[] and ar2[] 
are sorted arrays
Both have n elements */
int getMedian(int ar1[],
              int ar2[], int n)
{
    int i = 0; /* Current index of 
                  i/p array ar1[] */
    int j = 0; /* Current index of 
                  i/p array ar2[] */
    int count;
    int m1 = -1, m2 = -1;
  
    /* Since there are 2n elements, 
    median will be average of elements 
    at index n-1 and n in the array 
    obtained after merging ar1 and ar2 */
    for (count = 0; count <= n; count++) {
        /* Below is to handle case where 
           all elements of ar1[] are
           smaller than smallest(or first)
           element of ar2[]*/
        if (i == n) {
            m1 = m2;
            m2 = ar2[0];
            break;
        }
  
        /*Below is to handle case where 
          all elements of ar2[] are
          smaller than smallest(or first)
          element of ar1[]*/
        else if (j == n) {
            m1 = m2;
            m2 = ar1[0];
            break;
        }
  
        if (ar1[i] < ar2[j]) {
            /* Store the prev median */
            m1 = m2;
            m2 = ar1[i];
            i++;
        }
        else {
            /* Store the prev median */
            m1 = m2;
            m2 = ar2[j];
            j++;
        }
    }
  
    return (m1 + m2) / 2;
}
  
// Driver Code
int main()
{
    int ar1[] = { 1, 12, 15, 26, 38 };
    int ar2[] = { 2, 13, 17, 30, 45 };
  
    int n1 = sizeof(ar1) / sizeof(ar1[0]);
    int n2 = sizeof(ar2) / sizeof(ar2[0]);
    if (n1 == n2)
        cout << "Median is "
             << getMedian(ar1, ar2, n1);
    else
        cout << "Doesn't work for arrays"
             << " of unequal size";
    getchar();
    return 0;
}
  
// This code is contributed
// by Shivi_Aggarwal

C




// A Simple Merge based O(n) solution to find median of
// two sorted arrays
#include <stdio.h>
  
/* This function returns median of ar1[] and ar2[].
   Assumptions in this function:
   Both ar1[] and ar2[] are sorted arrays
   Both have n elements */
int getMedian(int ar1[], int ar2[], int n)
{
    int i = 0; /* Current index of i/p array ar1[] */
    int j = 0; /* Current index of i/p array ar2[] */
    int count;
    int m1 = -1, m2 = -1;
  
    /* Since there are 2n elements, median will be average
     of elements at index n-1 and n in the array obtained after
     merging ar1 and ar2 */
    for (count = 0; count <= n; count++) {
        /*Below is to handle case where all elements of ar1[] are
          smaller than smallest(or first) element of ar2[]*/
        if (i == n) {
            m1 = m2;
            m2 = ar2[0];
            break;
        }
  
        /*Below is to handle case where all elements of ar2[] are
          smaller than smallest(or first) element of ar1[]*/
        else if (j == n) {
            m1 = m2;
            m2 = ar1[0];
            break;
        }
  
        if (ar1[i] < ar2[j]) {
            m1 = m2; /* Store the prev median */
            m2 = ar1[i];
            i++;
        }
        else {
            m1 = m2; /* Store the prev median */
            m2 = ar2[j];
            j++;
        }
    }
  
    return (m1 + m2) / 2;
}
  
/* Driver program to test above function */
int main()
{
    int ar1[] = { 1, 12, 15, 26, 38 };
    int ar2[] = { 2, 13, 17, 30, 45 };
  
    int n1 = sizeof(ar1) / sizeof(ar1[0]);
    int n2 = sizeof(ar2) / sizeof(ar2[0]);
    if (n1 == n2)
        printf("Median is %d", getMedian(ar1, ar2, n1));
    else
        printf("Doesn't work for arrays of unequal size");
    getchar();
    return 0;
}
Output:



Median is 16


Method 2 (By comparing the medians of two arrays)

This method works by first getting medians of the two sorted arrays and then comparing them.

C++




// A divide and conquer based
// efficient solution to find
// median of two sorted arrays
// of same size.
#include <bits/stdc++.h>
using namespace std;
  
/* to get median of a
   sorted array */
int median(int[], int);
  
/* This function returns median 
   of ar1[] and ar2[].
Assumptions in this function:
    Both ar1[] and ar2[] are 
    sorted arrays
    Both have n elements */
int getMedian(int ar1[],
              int ar2[], int n)
{
    /* return -1 for 
       invalid input */
    if (n <= 0)
        return -1;
    if (n == 1)
        return (ar1[0] + ar2[0]) / 2;
    if (n == 2)
        return (max(ar1[0], ar2[0]) + min(ar1[1], ar2[1])) / 2;
  
    /* get the median of 
       the first array */
    int m1 = median(ar1, n);
  
    /* get the median of 
       the second array */
    int m2 = median(ar2, n);
  
    /* If medians are equal then 
       return either m1 or m2 */
    if (m1 == m2)
        return m1;
  
    /* if m1 < m2 then median must 
       exist in ar1[m1....] and
                ar2[....m2] */
    if (m1 < m2) {
        if (n % 2 == 0)
            return getMedian(ar1 + n / 2 - 1,
                             ar2, n - n / 2 + 1);
        return getMedian(ar1 + n / 2,
                         ar2, n - n / 2);
    }
  
    /* if m1 > m2 then median must 
       exist in ar1[....m1] and 
                ar2[m2...] */
    if (n % 2 == 0)
        return getMedian(ar2 + n / 2 - 1,
                         ar1, n - n / 2 + 1);
    return getMedian(ar2 + n / 2,
                     ar1, n - n / 2);
}
  
/* Function to get median 
   of a sorted array */
int median(int arr[], int n)
{
    if (n % 2 == 0)
        return (arr[n / 2] + arr[n / 2 - 1]) / 2;
    else
        return arr[n / 2];
}
  
// Driver code
int main()
{
    int ar1[] = { 1, 2, 3, 6 };
    int ar2[] = { 4, 6, 8, 10 };
    int n1 = sizeof(ar1) / sizeof(ar1[0]);
    int n2 = sizeof(ar2) / sizeof(ar2[0]);
    if (n1 == n2)
        cout << "Median is "
             << getMedian(ar1, ar2, n1);
    else
        cout << "Doesn't work for arrays "
             << "of unequal size";
    return 0;
}
  
// This code is contributed
// by Shivi_Aggarwal

C




// A divide and conquer based efficient solution to find median
// of two sorted arrays of same size.
#include <bits/stdc++.h>
using namespace std;
  
int median(int[], int); /* to get median of a sorted array */
  
/* This function returns median of ar1[] and ar2[].
   Assumptions in this function:
   Both ar1[] and ar2[] are sorted arrays
   Both have n elements */
int getMedian(int ar1[], int ar2[], int n)
{
    /* return -1  for invalid input */
    if (n <= 0)
        return -1;
    if (n == 1)
        return (ar1[0] + ar2[0]) / 2;
    if (n == 2)
        return (max(ar1[0], ar2[0]) + min(ar1[1], ar2[1])) / 2;
  
    int m1 = median(ar1, n); /* get the median of the first array */
    int m2 = median(ar2, n); /* get the median of the second array */
  
    /* If medians are equal then return either m1 or m2 */
    if (m1 == m2)
        return m1;
  
    /* if m1 < m2 then median must exist in ar1[m1....] and
        ar2[....m2] */
    if (m1 < m2) {
        if (n % 2 == 0)
            return getMedian(ar1 + n / 2 - 1, ar2, n - n / 2 + 1);
        return getMedian(ar1 + n / 2, ar2, n - n / 2);
    }
  
    /* if m1 > m2 then median must exist in ar1[....m1] and
        ar2[m2...] */
    if (n % 2 == 0)
        return getMedian(ar2 + n / 2 - 1, ar1, n - n / 2 + 1);
    return getMedian(ar2 + n / 2, ar1, n - n / 2);
}
  
/* Function to get median of a sorted array */
int median(int arr[], int n)
{
    if (n % 2 == 0)
        return (arr[n / 2] + arr[n / 2 - 1]) / 2;
    else
        return arr[n / 2];
}
  
/* Driver program to test above function */
int main()
{
    int ar1[] = { 1, 2, 3, 6 };
    int ar2[] = { 4, 6, 8, 10 };
    int n1 = sizeof(ar1) / sizeof(ar1[0]);
    int n2 = sizeof(ar2) / sizeof(ar2[0]);
    if (n1 == n2)
        printf("Median is %d", getMedian(ar1, ar2, n1));
    else
        printf("Doesn't work for arrays of unequal size");
    return 0;
}
Output:
Median is 5

Please refer complete article on Median of two sorted arrays of same size for more details!

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