How to Access Elements in Set by Index in C++?
In C++, elements of a set cannot be accessed directly by index or position. However, we can work around this limitation using iterators. In this article, we will learn how to access the elements in set by index in C++.
The most efficient way to access a set element by index is to use the std::next() function. Let's take a look at a simple example:
#include <bits/stdc++.h>
using namespace std;
int main() {
set<int> s = {1, 4, 6, 9};
int i = 2;
// Accesing the element at index i
cout << *next(s.begin(), i);
return 0;
}
using namespace std;int main() { set<int> s = {1, 4, 6, 9}; int i = 2; // Accesing the element at index i cout << *next(s.begin(), i); return 0;}Output
6
Explanation: next() function returns the set::begin() iterator incremented by i times. We have dereferenced this iterator to access the value.
Note: The next() function does not modify the given iterator. It returns the incremented copy of the iterator.
There are also some other methods in C++ to access elements in a set by index, but they all use same concept.
Table of Content
Using advance() Function
The std::advance() can also be used to access the set element at some given index in the same way as next() function, but this function modifies the given iterator.
#include <bits/stdc++.h>
using namespace std;
int main() {
set<int> s = {1, 4, 6, 9};
int i = 2;
auto it = s.begin();
// Accesing the element at index 2
advance(it, i);
cout << *it;
return 0;
}
using namespace std;int main() { set<int> s = {1, 4, 6, 9}; int i = 2; auto it = s.begin(); // Accesing the element at index 2 advance(it, i); cout << *it; return 0;}Output
6
Incrementing Using Loops
Increment the set::begin() iterator i number of times using a loop. This will make it point to the element at the given index.
#include <bits/stdc++.h>
using namespace std;
set<int>::iterator get(set<int>& s, int i) {
auto it = s.begin();
// Incrementing it i times one by one
for (int j = 0; j < i; j++)
it++;
return it;
}
int main() {
set<int> s = {1, 4, 6, 9};
int i = 2;
cout << *get(s, i);
return 0;
}
using namespace std;set<int>::iterator get(set<int>& s, int i) { auto it = s.begin(); // Incrementing it i times one by one for (int j = 0; j < i; j++) it++; return it;}int main() { set<int> s = {1, 4, 6, 9}; int i = 2; cout << *get(s, i); return 0;}Output
6
This is how both the next() and advance() functions are internally implemented with some minor changes.
