median_search.cpp

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#include <iostream>
#include <algorithm>
#include <vector>
#include <cassert>

namespace search {
namespace median_search {  
int median_of_medians(const std::vector<int>& A,  const int& idx) {
    int pivot = 0;                  // initialized with zero
    std::vector<int> a(A.begin(), A.end());
    std::vector<int> m;
    int r = a.size();
    for(int i = 0; i < r; i += 5){
        std::sort(a.begin() + i, a.begin() + std::min(r, i + 5));
        int mid = (i + std::min(r, i + 5)) / 2;
        m.push_back(a[mid]);
    }
    int sz = int(m.size());
    if(sz <= 5){
        std::sort(m.begin(), m.end());
        pivot = m[(sz- 1) / 2];
    }
    else{
        pivot = median_of_medians(m, idx);
    }
    std::vector<int> low;
    std::vector<int> high;
    for(int i = 0; i < r; i++){
        if(a[i] < pivot){
            low.push_back(a[i]);
        }
        else if(a[i] > pivot){
            high.push_back(a[i]);
        }
    }
    int k = int(low.size());
    if(idx < k){
        return median_of_medians(low, idx);
    }
    else if(idx > k){
        return median_of_medians(high, idx-k-1);
    }
    else{
        return pivot;
    }
}
}  // namespace median_search
}  // namespace search

void test(){
    std::vector<int> A{25,21,98,100,76,22,43,60,89,87};
    int i = 3;
    assert(A[6] == search::median_search::median_of_medians(A, i));     // A[6]  = 43, is the fourth smallest element.
    std::cout << "test case:1 passed\n";
    
    std::vector<int> B{1,2,3,4,5,6};
    int j = 4;
    assert(B[4] == search::median_search::median_of_medians(B, j));     // B[4] = 5, is the fifth smallest element.
    std::cout << "test case:2 passed\n";
    
    std::vector<int> C{1,2,3,4,5,1000,8,9,99};
    int k = 3;
    assert(C[3] == search::median_search::median_of_medians(C, k));     // C[3] = 4, is the fourth smallest element.
    std::cout << "test case:3 passed\n";
    std::cout << "--All tests passed--\n";
}

int main()
{
    test();
    int n = 0;
    std::cout << "Enter Size of Array: ";
    std::cin >> n;
    std::vector<int> a(n);
    std::cout << "Enter Array: ";
    for(int i = 0; i < n; i++){
        std::cin >> a[i];
    }
    std::cout << "Median: ";            // Median defination: https://en.wikipedia.org/wiki/Median
    int x = search::median_search::median_of_medians(a,  (n - 1) / 2);
    if(n % 2 == 0){
        int y = search::median_search::median_of_medians(a, n / 2);
        std::cout << (float(x) + float(y))/2.0;
    }
    else{
        std::cout << x;
    }
    std::cout << "\nTo find i-th smallest element ";
        std::cout << "\nEnter i: ";
    int idx = 0;
    std::cin >> idx;
    idx--;
    std::cout << idx + 1<< "-th smallest element: " << search::median_search::median_of_medians(a, idx) << '\n';
    return 0;
}