C++ Loop through first K elements of unordered_map

Question

I have an unordered_map that stores counts of integers. I want to loop through the map, but instead of fetching all the entries, I only wish to get the first K.It is guaranteed that map has more than K entries.

I'm running into issues when I do the following:

  unordered_map<int, int> u_map;
  // Logic to populate the map
  
  for(auto it=u_map.begin(); it!=u_map.begin()+2; it++)
  cout<<it->first<<" "<<it->second<<endl;

The expression u_map.begin()+2 is causing the issue.

So is it possible to get only the first K entries of a map using for_each loop in C++?

You can use `std::advance` or `std::next`. A map doesn't provide random access iterators, so that's why what you have it causing an error. More info: https://stackoverflow.com/a/21626211/920069 — Retired Ninja, Nov 25 '21 at 04:56

score 3 · Answer 1 · edited Dec 09 '21 at 16:10

3

If you can use C++20, then views::take would be a choice.

#include <unordered_map>
#include <ranges>
#include <iostream>

int main() {
  std::unordered_map<int, int> u_map;
  for (auto [key, value] : u_map | std::views::take(2))
    std::cout << key << " " << value << "\n";
}

Alternative for pre-C++20, using std::next:

std::unordered_map<int, int> u_map;
auto end = std::next(u_map.begin(), 2);
for (auto it = u_map.begin(); it != end; ++it)
  std::cout << it->first << " " << it->second << "\n";

edited Dec 09 '21 at 16:10

Rockcat

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answered Nov 25 '21 at 05:02

康桓瑋

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Unfortunately I can't use C++20 in the project. However thanks a lot for this solution. Wasn't aware of this. – Vamsi Krishna Nov 25 '21 at 05:11

Jason · Accepted Answer · 2021-11-25T05:21:31.907

I only wish to get the first K

Note from std::unordered_map documentation

an unordered_map object makes no guarantees on which specific element is considered its first element.

This essentially means that there is no guarantee that you will iterate over the elements in the inserted order.

For iterating over the elements of the map you can use:

int count  = 0;
for (auto& it: u_map) {
    /* some code here like you can keep a count variable that will check if it 
    reaches the number K and then break the loop. But remember that it is 
    **not** guaranteed that the elements you will get will be in inserted order.*/
   
   if(count < K)
   {
    cout<<it.first<<" "<<it.second<<endl;
   }
   else 
   {
      break;
   }
   ++count;
}

Working example

#include <iostream>
#include <unordered_map>
using namespace std;
int main()
{
std::unordered_map<std::string, std::string> u_map = {
        {"RED","#FF0000"},
        {"GREEN","#00FF00"},
        {"BLUE","#0000FF"},{"PURPLE","#0F00FF"},{"WHITE","#0000RF"},{"ORANGE","#F000FF"}
    };
   int K = 3; 
   int count  = 0;
   for (auto& it: u_map) 
   {
       if(count < K)
       {
            cout<<it.first<<" "<<it.second<<endl;
       }
       else 
       {
            break;
        }
        ++count;
    }
return 0;
}

C++ Loop through first K elements of unordered_map

2 Answers2