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?

Works for me, with gcc 9. Perhaps you are using an old C++ compiler that does not support c++11? — Sam Varshavchik, Apr 18 '20 at 04:03
Yes. There are other ways to increment iterators. See your C++ textbook for more information, and examples. — Sam Varshavchik, Apr 18 '20 at 04:05
You could do itr++. Also make sure to do checks like itr is still valid, ie., `itr!=a.end()` — Insaf K, Apr 18 '20 at 04:10
See if you can add a `-std=c++11` or similar to the compiler command line. — user4581301, Apr 18 '20 at 04:14

score 2 · Answer 1 · answered Apr 18 '20 at 04:22

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 i.e.:

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 i.e.:

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:

score 0 · Answer 2 · answered Apr 18 '20 at 04:41

0

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;

answered Apr 18 '20 at 04:41

Ankit Mishra

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Fetch first two elements from map

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