How to access the value from the json file using runtime key in c++

[RanjithaHosur]

I'm new to json and trying to use nlohmann library and I'm quite stuck.

config.json file looks like this

{
    "Version": 1.1,
    "Size": 1024,
    "integer": 600,
    "Map": [{"0": "india"}, {"1": "usa"}, {"2": "china"}, {"2": "japan"}],
    "name": "xxx",
}

I want to get the value of ["Map"]["1"] where "1" is a runtime entity. I have tried several ways but none of them is working.

std::string strRwFilePath = std::string(CONFIG_PATH) + "config.json";
std::ifstream RwFile(strRwFilePath);
int m_iRegionType = 1;
if (RwFile.fail())
{
    std::cout << "[ReadFileContents]RwFile doesnt exist" << std::endl;
}
else 
{
    // parsing input 
    RwFile >> conf_json;

    if(!(conf_json.empty())) 
    {           
        //METHOD 1
        for (auto it = conf_json["Map"].begin(); it != conf_json["Map"].end(); ++it)
        {
            std::cout << it.key() << " | " << it.value() << "\n";
            std::string keyy = std::to_string(m_iRegionType); 
            std::string tempKey = it.key();                         //ERROR- [json.exception.invalid_iterator.207] cannot use key() for non-object iterators
            std::cout << "for loop: tempKey:" << tempKey << " \n";
            if(!strcmp(tempKey.c_str(),(std::to_string(m_iRegionType)).c_str()))
            {
                std::cout << "SUCCESS 222" << std::endl;
                break; 
            }
            else
            {
                std::cout << "FAILURE 222" << std::endl; 
            }
        }

        //METHOD 2
        for(auto& element : conf_json["Map"].items())
        {
            std::cout << element.key() << " | " << element.value() << "\n";
            std::string keyy = std::to_string(m_iRegionType); 
            std::string tempKey = element.key();                //ERROR: [json.exception.type_error.302] type must be string, but is object
            if(!strcmp(tempKey.c_str(),(std::to_string(m_iRegionType)).c_str()))
            {
                std::cout << "SUCCESS 333" << std::endl;
                break; 
            }
            else
            {
                std::cout << "FAILURE 333" << std::endl; 
            }   
        }
        
        //METHOD 3
        std::string strTempKey = std::to_string(m_iRegionType);
        if(!conf_json["Map"][strTempKey.c_str()].is_null())
        {
            std::string strName = conf_json["Map"][strTempKey.c_str()];     //ERROR: [json.exception.type_error.305] cannot use operator[] with a string argument with array
            std::cout << "key found. RegionName: " << strName << '\n';         
        }
        else
        {
            std::cout << "key not found" << std::endl;
        }
        
        //METHOD 4
         std::string strKey = "/Map/" + std::to_string(m_iRegionType);
         // call find
         auto it_two = conf_json.find(strKey.c_str());
        if(true == (it_two != conf_json.end()))
        {
            std::string strRegionName = conf_json["Map"][m_iRegionType];
            std::cout << "strRegionName: " << strRegionName << '\n';
            std::string strRegionName2 = *it_two;            
            std::cout << "strRegionName2: " << strRegionName2 << '\n';
        }
        else
        {
            //getting this as OUTPUT even though key is available
            std::cout << "invalid region type, key not available m_iRegionType: " << m_iRegionType << std::endl;
        }        
    }
    else
    {
        std::cout << "Read Write Json object is empty" << std::endl;
    }               
}

I have also tried using the small json structure where I am not able to access the value using the runtime key.

json j =
     {
     {"integer", 1},
     {"floating", 42.23},
     {"string", "hello world"},
     {"boolean", true},
     {"object", {{"key1", 1}, {"key2", 2}}},
     {"array", {1, 2, 3}}
     };

int avail4 = j.value("/integer"_json_pointer, 444);
std::cout << "avail4 : " << avail4 << std::endl;         //OUTPUT- avail4 : 1

int avail5 = j.value("/object/key2"_json_pointer, 555);
std::cout << "avail5 : " << avail5 << std::endl;         //OUTPUT- avail5 : 2

auto strKey3 = "/object/key2";
int avail6 = j.value(strKey3, 666);
std::cout << "avail6 : " << avail6 << std::endl;         //OUTPUT- avail6 : 666

Could any one help me. I am using C++11.

[nlohmann]

{
    "Version": 1.1,
    "Size": 1024,
    "integer": 600,
    "Map": [{"0": "india"}, {"1": "usa"}, {"2": "china"}, {"2": "japan"}],
    "name": "xxx",
}

Map maps to an array of objects, so you need to access it via

j["Map"][0]["1"]

(assuming the input was parsed to j)

[RanjithaHosur]

Ok, thanks for the reply!
Sorry for incomplete explanation.

std::cout << "Enter the region number \n 0 for india \n 1 for usa \n 2 for china \n 3 for japan \n" ;
std::cin >> m_iRegionType; //user input is 1

basically my question is
How can I access the value from "Map" using m_iRegionType.

[nlohmann]

What about

j["Map"][m_iRegionType]["1"]

?

[RanjithaHosur]

j["Map"][m_iRegionType]["1"] will throw an exception as
[json.exception.type_error.305] cannot use operator[] with a numeric argument with object.

[nlohmann]

This error message does not really fit the JSON value you posted above: The only numeric argument is m_iRegionType. This means that j["Map"] would be an object, but it is an array above.

[nlohmann]

Do you need further assistance with this issue?