std::map 用键的索引遍历键

Question

I need to iterate through the keys of a map, but looking ahead to future keys. For example:

map<int, int> m;
vector<int> v;
for(map<int,int>::iterator it = m.begin(); it != m.end(); ++it) {
  cout << it->first << "\n";
  //is the next element equal to 3?
  auto next = it++;
  std::cout << "equals 3" << next==3 << std::endl
}

but sometimes I don't want to see the next element (n+1), maybe I want to see the n+10 element, etc. How do I do this? If my list has 100 elements, and I arrive at element 99, then 99+10 is gonna break evrything. Is there a way to test if my iterator can achieve n+10?

The best solution I thougth of is to keep track of an index i and see if I can call it + 10 (that is, if i+10<mapSize ). Bus is there a more elegant way? Maybe testing if the n+10 iterator exists or something?

Answer 1

Map does not sound like the appropiate data type for your use case. Try switching to a container that supports random access

Answer 2

I think that your are looking for something like std::advance (Please see here ), but with an additional check, if the advance operation was past the end or not.

We can use a small lambda to do this kind of check. Since it uses only an increment operation, it should work for all type of containers.

Please see the following example to illustrate the function:

#include <iostream>
#include <map>
#include <iterator>

using Type = std::map<int, int>;
using TypeIter = Type::iterator;

int main() {
    // Lambda to advance a container iterator and check, if that was possible
    auto advanceAndCheck = [](const Type& t, const TypeIter& ti, size_t advance) -> std::pair<bool, TypeIter>
        { TypeIter i{ ti }; while ((i != t.end()) && (advance--)) ++i;  return { i != t.end(), i }; };


    // Test data
    Type m{ {1,1}, {2,2}, {3,3}, {4,4}, {5,5} , {6,6} };

    // Iterate over container
    for (TypeIter it = m.begin(); it != m.end(); ++it) {

        // Show some values
        std::cout << it->first << "\n";

        // Test
        {
            // Advance and check
            auto [OK, itn] = advanceAndCheck(m, it, 1);
            if (OK && itn->first == 3) std::cout << "The next Element is 3\n";
        }
        {
            // Advance and check
            auto [OK, itn] = advanceAndCheck(m, it, 5);
            if (OK && itn->first == 6) std::cout << "The 5th next Element is 6\n";
        }
    }
}