Parse elements from array in json file using boost

Question

I have a json file that looks like this:

{
        "type": "2D",
        "data":
        [
            [
                "26",
                "17",
                "1"
            ],
            [
                "13",
                "29",
                "1"
            ],
            [
                "13",
                "30",
                "1"
            ],
....

In data, each array have a meaning so I need to assign a variable to each one (in a loop), like:

int first = 26;
int second = 17;
int third = 1;

I was doing something like this (I defined before v):

BOOST_FOREACH(boost::property_tree::ptree::value_type &v2, v.second.get_child("data")) {

 BOOST_FOREACH (boost::property_tree::ptree::value_type& itemPair, v2.second) {
  cout << itemPair.second.get_value<std::string>() << " ";
      }
  }
 }

Just to print each variable, but I handle only to have them as a set, not each one. Does anyone have an idea how to do it?

thanks in advance!

Community · Accepted Answer · 2017-05-23 12:31:04Z

For JSON arrays, the data node contains multiple child nodes with empty name (docs: c++ How to read XML using boost xml parser and store in map).

So you would just loop through the child nodes (optionally checking that the key == "").

Here's my simplist example. I used a trick with an array to map elements to the local variables one, two, three. Consider using a translator or a "parse" function that parses a tree node into a struct { int first,second, third; } instead (e.g. https://stackoverflow.com/a/35318635/85371)

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#include <boost/property_tree/json_parser.hpp>
#include <iostream>

int main() {
    boost::property_tree::ptree pt;
    read_json("input.txt", pt);

    using namespace std;
    for(auto& array3 : pt.get_child("data")) {
        int first, second, third;
        int* const elements[3] = { &first, &second, &third };
        auto element = begin(elements);

        for (auto& i : array3.second) {
            **element++ = i.second.get_value<int>();

            if (element == end(elements)) break;
        }

        std::cout << "first:" << first << " second:" << second << " third:" << third << "\n";
    }
}

For input {"type":"2D","data":[["26","17","1"],["13","29","1"],["13","30","1"]]} prints:

first:26 second:17 third:1
first:13 second:29 third:1
first:13 second:30 third:1

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