how do you find the summation of elements in a set?

Question

#include <iostream>
#include<algorithm>
#include<vector>
#include<set>
using namespace std;

int main() {
    int i,sum,sum1,n;
    cin>>n;
    vector<int>pre;
    set<int>s1;
    set<int>s2;
    int temp1[n];
    int temp2[n];
    for(i=0;i<n;++i)
        cin>>temp1[i];
    for(i=0;i<n;++i)
        cin>>temp2[i];
    for(i=0;i<sizeof(temp1);++i)
        s1.insert(temp1[i]);    
    for(i=0;i<sizeof(temp2);++i)
        s1.insert(temp2[i]);
    for(i=n;i>=0;--i)
        sum+=i;

    for(i=0;i<s1.size();++i)
        sum1+=s1[i];
    if(sum==sum1)
        cout<<"I become the guy.";
    else
         cout<<"Oh, my keyboard!";
    return 0;
}

ideone link:https://ideone.com/trfOz0 i am trying to add the elements in the set s1 but it seems like this way is not working,how can i do it?

Answer 1

You can't index a std::set. It's not built that way. You can check for membership, but you can't just pick the nth element.

In order to take the sum, you'd have to iterate over the set - but not via index, via iterators:

int sum = 0;
for (std::set<int>::iterator it = s1.begin(); it != s1.end(); ++it) {
    sum += *it;
}

Or using std::accumulate:

int sum = std::accumulate(s1.begin(), s1.end(), 0);

Or in C++11:

int sum = 0;
for (int i : s1) {
    sum += i;
}

Answer 2

1. You don't initialise your variables sum, and sum1. Using an uninitialised variable is undefined behaviour in C++.

2. std::set does not define operator[]. So your code will not compile.

The simplest way to sum an STL container like a set is to use

std::accumulate(s1.begin(), s1.end(), 0);

Where the 0 (which must be of the same type as the contains type in the set) is the value you want if the set doesn't contain any elements.