Get element from arbitrary index in set - c++

Question

I have a set of type set<int> and I want to get an iterator to someplace that is not the beginning.
I am doing the following:
set<int>::iterator it = myset.begin() + 5;
I am curious why this is not working and what is the correct way to get an iterator to where I want it.

Answer 1

myset.begin() + 5; only works for random access iterators, which the iterators from std::set are not.
For input iterators, there's the function std::advance:
set<int>::iterator it = myset.begin();
std::advance(it, 5); // now it is advanced by five
In C++11, there's also std::next which is similar but doesn't change its argument:
auto it = std::next(myset.begin(), 5);
std::next requires a forward iterator. But since std::set<int>::iterator is a bidirectional iterator, both advance and next will work.

Answer 2

The operator+ doesn’t define for this structure and only It make sense for random access iterators.
First solution:
You can use std::advance, the function uses repeatedly the increase or decrease operator (operator++ or operator--) until n elements have been advanced.
set<int>::iterator it = myset.begin();
std::advance(it, 5);
std::out << *it << std::endl; // == it + 5
Second solution:
Use std::next or std::prev functions,The performance same as the old one because uses repeatedly the increase or decrease operator (operator++ or operator--)until n element have been advanced.
Note: If it is a random access iterator, the function just uses just
once operator+ or operator-.
set<int>::iterator it1 = myset.begin();
std::next(it1, 5); // == it1 + 5
std::out << *it1 << std::endl; // == it1 + 5
set<int>::iterator it2 = myset.end();
std::prev(it2, 5); // == it2 - 5
std::out << *it2 << std::endl; // == it2 - 5
Note: If you want to access, vectors are very efficient accessing its elements (just like arrays) and relatively efficient adding or removing elements from its end.

Answer 3

Get element at index from C++11 std::set
std::set in C++ has no getter by index so you'll have to roll your own by iterating the list yourself and copying into an array then indexing that.
For example:
#include<iostream>
#include<set>
using namespace std;
int main(){
set<int> uniqueItems; //instantiate a new empty set of integers
uniqueItems.insert(10);
uniqueItems.insert(20); //insert three values into the set
uniqueItems.insert(30);
int myarray[uniqueItems.size()]; //create an int array of same size as the
//set<int> to accomodate elements
int i = 0;
for (const int &num : uniqueItems){ //iterate over the set
myarray[i] = num; //assign it to the appropriate array
i++; //element and increment
}
cout << myarray[0] << endl; //get index at zero, prints 10
cout << myarray[1] << endl; //get index at one, prints 20
cout << myarray[2] << endl; //get index at two, prints 30
}
Or a handy dandy function to step through then return the right one:
int getSetAtIndex(set<int> myset, int index){
int i = 0;
for (const int &num : myset){ //iterate over the set
if (i++ == index){
return num;
}
}
string msg = "index " + to_string(index) + \
"is out of range";
cout << msg;
exit(8);
}
int main(){
set<int> uniqueItems; //instantiate a new empty set of integers
uniqueItems.insert(10);
uniqueItems.insert(20); //insert three values into the set
uniqueItems.insert(30);
cout << getSetAtIndex(uniqueItems, 1);
}