Unable to modify one of the values within the JSON file, and save it

[XIAY0009]

Dear Nlohmann,

Thanks so much for this JSON library :)

For my program, I am trying to:
Step 1: Read from a JSON file which I call it test2.json into my C++ program (No problem in doing so)
Step 2: Modify the value of one of the variables (No problem in doing so)
Step 3: Saves this value into the original JSON file (Not sure how to do this)
Step 4: Add a new variable into the original JSON file (Not sure how to do this)

I'm sure there is a way to accomplish step 3 and 4, but I am not sure how, would really appreciate your help

Please see my simple code below. Currently when I change the value of variable "pi" and write into my original file, it seems to overwrite all my other variables ><

Main program

#include <fstream>
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using namespace nlohmann;

int main() {
    std::ifstream in("/LearnDirectory/LearnJSON/test2.json");
    json file = json::parse(in);

    //auto size = file.size();
    //file["size"] = size;
    std::cout << file.at("pi") << '\n';
    std::cout << std::setw(4) << file << '\n';
    float pi_value = file.at("pi");
    std::cout << "Value of pi is: " << pi_value << '\n';
    //Tries o access the object value
    std::cout << file.at("object") << '\n';
    auto& array = file.at("object");
    for (auto&& val:array){
        std::cout << val << '\n';
    }
    std::ofstream o("pretty.json");
    o << std::setw(4) << file << std::endl;

    std::ofstream out("/LearnDirectory/LearnJSON/test2.json");  
    file.at("pi") = 20;  
    out << file.at("pi") << std::endl;  
}   

test2.json

{
    "answer": {
        "everything": 42
    },
    "happy": true,
    "list": [
        1,
        0,
        2
    ],
    "name": "Niels",
    "nothing": null,
    "object": {
        "currency": "USD",
        "value": 42.99
    },
    "pi": 3.142
}

[XIAY0009]

I figured out how to do it :)
For the benefit of others, showing it here:

int main() {
    std::ifstream in("/LearnJSON/test2.json");
    json file = json::parse(in);

    std::cout << file.at("pi") << '\n';
    std::cout << std::setw(4) << file << '\n';
    float pi_value = file.at("pi");
    std::cout << "Value of pi is: " << pi_value << '\n';
    //Tries o access the object value
    std::cout << file.at("object") << '\n';
    auto& array = file.at("object");
    for (auto&& val:array){
        std::cout << val << '\n';
    }
    std::ofstream o("pretty.json");
    o << std::setw(4) << file << std::endl;

    json j;
    j = file;
    j["unhappy"] = false;

    std::ofstream out("/LearnJSON/test2.json");
    j["pi"] = 3.14159;
    out << std::setw(4) << j << std::endl;
    float test = j["pi"];
    std::cout << "New variable is: " << test << std::endl;

} 

[atulav12]

Where can I get JSON library ?

[nlohmann]

See here: https://github.com/nlohmann/json/releases

[atulav12]

Thank you !!!

[user0921]

Hi, I'm new to c++ and trying to use nlohmann library and I'm quite stuck. I want to modify array of objects from json. json = { "a": "xxxx", "b": [ { "c": "aaa", "d": [{ "e": "yyy" }, { "e": "sss", "f": "fff" } ] } ] } now I want to replace "e" value with "example" from the above following structure. Could some one help me. I tried to loop through the json structure and was able to read the "e" value but can't replace it. Kindly help me. Thanks in advance.