Sorting JSON values alphabetically C++

Question

If my json file is this:

[
 {
  "friendName": "Ann",
  "birthday": "1990-04-19",
  "favoriteColor": "Purple",
 },
 {
  "friendName": "Rachel",
  "birthday": "1995-7-05",
  "favoriteColor": "Pink",
 },
 {
  "friendName": "Max",
  "birthday": "1993-10-07",
  "favoriteColor": "Purple",
 },
 {
  "friendName": "Bob",
  "birthday": "1992-02-20",
  "favoriteColor": "Red",
 }
]

How could I obtain the youngest girl's name? (As in sorting the girls' birthday strings in descending order, then grabbing the first object in the list (1993-10-07) and printing their name).

I am using JSON for Modern C++ (https://github.com/nlohmann/json) and Xcode (version 6).

In my project, I will not know the amount of objects I will have. Is there a way to sort these strings by comparison?

The question is confusing to me, why noy just retrieve them as json objects and do the sorting(finding what you want) on those objects yourself? — henryyao, Oct 09 '15 at 20:21
In my project I have over 600 "friends", I want the code to sort it for me. — Melisa K. Savich, Oct 09 '15 at 20:30

maddouri · Accepted Answer · 2015-10-10T16:47:43.223

A nice thing about the nlohmann/json is that it offers STL-like access, which allows for using a nlohmann::json with most of the functions in <algorithm>.

In particular, std::min_element() might come in handy when looking for the min element in a container... given a custom comparison function :)

In the following code, I used the dob_comp() lambda as a comparison function to compare persons by their dates of birth (dob). The youngest person is, therefore, the one with the "minimum date of birth".

[run it online (close main.cpp if it's open, click compile, then execute)]

// compile with: g++ --std=c++11 this_file.cpp -o your_executable_name

#include <algorithm>
#include <ctime>
#include <iomanip>
#include <iostream>

#include "json.hpp"

int main()
{
    // sample json object
    nlohmann::json j = nlohmann::json::parse("["
        "{"
        " \"friendName\": \"Ann\","
        " \"birthday\": \"1990-04-19\","
        " \"favoriteColor\": \"Purple\""
        "},"
        "{"
        " \"friendName\": \"Rachel\","
        " \"birthday\": \"1995-07-05\","
        " \"favoriteColor\": \"Pink\""
        "},"
        "{"
        " \"friendName\": \"Max\","
        " \"birthday\": \"1993-10-07\","
        " \"favoriteColor\": \"Purple\""
        "},"
        "{"
        " \"friendName\": \"Bob\","
        " \"birthday\": \"1992-02-20\","
        " \"favoriteColor\": \"Red\""
        "}"
    "]");

    // converts a date string to a std::tm structure
    // assumes the string is formatted as "YYYY-MM-DD"
    const auto str_to_time = [] (std::string str) {
        std::tm tm;
        // http://stackoverflow.com/a/21021900
        //std::stringstream ss(str);
        //ss >> std::get_time(&tm, "%Y-%m-%d");
        strptime(str.c_str(), "%Y-%m-%d", &tm);
        return tm;
    };

    // simplistic comparison of std::tm structures -- compares only the (year,month,day)
    const auto time_comp = [] (const std::tm& t1, const std::tm& t2) {
        if (t1.tm_year < t2.tm_year)
        {
            return true;
        }
        else if (t1.tm_year > t2.tm_year)
        {
            return false;
        }
        else if (t1.tm_mon < t2.tm_mon)
        {
            return true;
        }
        else if (t1.tm_mon > t2.tm_mon)
        {
            return false;
        }
        else if (t1.tm_mday < t2.tm_mday)
        {
            return true;
        }
        else if (t1.tm_mday > t2.tm_mday)
        {
            return false;
        }
        else
        {
            return true;
        }
    };

    // I didn't have time to read too much of the "json.hpp" header
    // so I used a quick "decltype()" to find the iterator type
    using json_iterator_type = decltype(*j.begin());

    // compares the DatesOfBirth (dob) of two persons
    const auto dob_comp = [&str_to_time, &time_comp] (const json_iterator_type p1, const json_iterator_type p2) {
        std::string dob1 = p1["birthday"];
        std::string dob2 = p2["birthday"];

        auto ttm1 = str_to_time(dob1);
        auto ttm2 = str_to_time(dob2);

        return time_comp(ttm1, ttm2);
    };

    // know your <algorithm>'s :)
    const auto youngest = *std::min_element(j.begin(), j.end(), dob_comp);

    std::cout << "The youngest person is: " << youngest << std::endl;
}

Note: if you want to sort the elements, you can use std::sort() like this:

std::sort(j.begin(), j.end(), dob_comp);

Note2: check out jq if you need a tool for processing json files.

Did you try out the sorting as suggested in your Note? According to [this issue](https://github.com/nlohmann/json/issues/106) the underlying structure of `json` uses a `std::map`, hence `std::sort` would not work. — Patrick, Dec 19 '19 at 11:48

Sorting JSON values alphabetically C++

1 Answers1