find_by_order() in C++
Last Updated :
14 Sep, 2023
The find_by_order() is a built-in function of Ordered Set which is a Policy Based Data Structure in C++. Policy-based data structures are not part of the C++ Standard Template Library but the g++ compiler supports them.
Ordered Set is a policy-based data structure in g++ that maintains unique elements in sorted order. It performs all the operations as performed by Set in STL in O(logN) complexity.
In addition to that, the following two operations are also performed in O(logN) complexity:
- order_of_key (K): Number of items strictly smaller than K.
- find_by_order(k): Kth element in a Set (counting from zero).
The find_by_order() function accepts a key, say K, as an argument and returns the iterator to the Kth (starting from zero) element in the Set.
Note: The Ordered Set container generally keeps the data in ordered manner, so Kth element may be the Kth smallest or largest element.
Examples:
Considering a Set S = {1, 5, 6, 17, 88},
s.find_by_order(0): Returns the 0th element, i.e. the minimum element, i.e. 1.
s.find_by_order(2): Returns the 2nd element, i.e. 6.
Note: If K >= N, where N is the size of the set, then the function returns either 0 or in some compilers, the iterator to the smallest element.
Below is the implementation of find_by_order() function in C++:
C++14
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag,
tree_order_statistics_node_update>
pbds;
int main()
{
int arr[] = {1, 5, 6, 17, 88};
int n = sizeof(arr)/sizeof(arr[0]);
pbds S;
for (int i = 0; i < n; i++) {
S.insert(arr[i]);
}
cout << *S.find_by_order(0) << " ";
cout << *S.find_by_order(2);
return 0;
}
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Time Complexity: O(n*log(n))
Auxiliary Space: O(n)
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