access json array values in c++

Question

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" in the above 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. I tried: `

std::vector<std::string> arr_value;
std::ifstream in("test.json");
json file = json::parse(in);

for (auto& td : file["b"])
    for (auto& prop : td["d"])
        arr_value.push_back(prop["e"]);
        //std::cout<<"prop" <<prop["e"]<< std::endl;

for (const auto& x : arr_value)
    std::cout <<"value in vector string= " <<x<< "\n";

for (decltype(arr_value.size()) i = 0; i <= arr_value.size() - 1; i++)
{
    std::string s = arr_value[i]+ "emp";
    std::cout <<"changed value= " <<s << std::endl;
    json js ;
    js = file;
    std::ofstream out("test.json");
    js["e"]= s;
    out << std::setw(4) << js << std::endl;

}

you are creating a new js and set "e" to s on that new json object. What happens when you try to modify the original json file ? — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Jan 29, 2020 at 15:46
yes, you are right It only creates new "e" value and Im not able to modify the original json file, which is what I want to do. — Amy
– Amy, Commented Jan 29, 2020 at 15:48

Botje · Accepted Answer · 2020-01-29 16:25:27Z

5

The following appends MODIFIED to every "e"-value and writes the result to test_out.json:

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

using json = nlohmann::json;

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

        for (auto& td : file["b"])
                for (auto& prop : td["d"]) {
                        prop["e"] = prop["e"].get<std::string>() + std::string(" MODIFIED");
                }

        std::ofstream out("test_out.json");
        out << std::setw(4) << file << std::endl;
}

The prop["e"] = ... line does three things:

It gets the property with key "e",
Coerces it into a string using .get<std::string>() and appends "modified", and
writes back the result to prop["e"], which is a reference to the object nested in the JSON structure.

answered Jan 29, 2020 at 16:25

Botje

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1 Comment

Amy Over a year ago

Thanks a lot, it worked like charm and thanks for the detailed explanation.

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