Fetch first two elements from map

Question

I have to read only the first two elements from an std::map .

Here's my code:

#include<iostream>
#include<map>
#include<iterator>
using namespace std;
int main() {
    map<int,int> a;
    map<int,int>:: iterator itr;
    itr = a.begin();
    cout<<itr->first<<" "<<itr->second<<endl;
    next(itr);
    cout<<itr->first<<" "<<itr->second<<endl;
    return 0;
}

I'm getting this error:

next was not declared in the scope

what am I missing here and if there is a better way to do it?

Answer 1

For using std::next , you need to have at least C++11 compliant compiler.

std::next returns a new incremented iterator. So, you need to use its return value to get the incremented iterator ie:

itr = next( itr );

Right now, itr is pointing to the same element because the return value is not used.

If you meant to increment the itr without a new iterator then std::advance is a better candidate here ie:

std::advance( itr, 1 );

If you're using a pre-C++11 compiler then you can use increment operator like this:

itr++;

Or,

++itr;

Here's a live demo.

---

In addition, the map is uninitialized/empty and you're trying to access its elements that are not there and this would result in Undefined Behavior .

Relevant read:

• Why is "using namespace std;" considered bad practice?
• C++: "std::endl" vs "\n"

Answer 2

you should use itr++ to move itr forward to point next set of pairs. and you can access them by map->first; and map->second;