Json: Keys when iterating over objects

[mlund]

I'm trying to iterate over a json input file containing an unknown number of molecules (see below). For each of these the key is the name which I currently have found no way to extract. Is this possible?

Code:

#include <faunus/json.hpp>
#include <iostream>
#include <fstream>

/*
 * minimal.json:
 * {
 *   "moleculelist" : { 
 *     "salt" : { "atoms":"Na Cl", "atomic":true, "Ninit":1 },
 *     "water" : { "atoms":"H H O", "atomic":false, "Ninit":55 }
 *   }
 * }
 */

using json = nlohmann::json;

int main() {
  std::ifstream f("minimal.json");
  if (f) {
    json j;
    j << f;
    if ( ! j["moleculelist"].empty() )
      for ( auto &mol : j["moleculelist"] )
        if ( mol.is_object() ) {
          // how to get entry keys? (i.e. "salt", "water", etc.)
          for ( auto &i : mol )
            std::cout << i << "\n";
        }
  }
}

[mlund]

Found one solution,

for ( auto &mol : j["moleculelist"].get<json::object_t>() ) { ... }

which returns a std::map where first and second represents name and value. Don't know if this extra complication warrants a name() function in basic_json or not.

[nlohmann]

Unfortunately, there is no way for a JSON value to "know" whether it is stored in an object, and if so under which key. All you can do is to use the json::iterator class which has a key() and value() member function to access the key and the value when you iterate over an object.

For your code:

for (auto it = j["moleculelist"].begin(); it != j["moleculelist"].end(); ++it)
{
    std::cout << it.key() << " | " << it.value() << "\n";
}

(Here, it would be of type json::iterator.) This prints

salt | {"Ninit":1,"atomic":true,"atoms":"Na Cl"}
water | {"Ninit":55,"atomic":false,"atoms":"H H O"}

I hope this helps!

[mlund]

This will work nicely, thanks!

[wpalfi]

for ( auto it: j["moleculelist"].items() )
{
    std::cout << it.key() << " | " << it.value() << "\n";
}