Trying to retrieve data from nested objects

[EtienneBerube]

Hi, I'm trying to map data from a Json file to individual objects in order to sort them and perform other operations on them. However, the data structure required is quite complex and makes it hard to navigate.
Here is a snippet of the Json file:

{
  "items":
  {
    "item":
    [
      {
        "id": "0001",
        "type": "donut",
        "name": "Cake",
        "ppu": 0.55,
        "batters":
        {
          "batter":
          [
            { "id": "1001", "type": "Regular" },
            { "id": "1002", "type": "Chocolate" },
            { "id": "1003", "type": "Blueberry" },
            { "id": "1004", "type": "Devil's Food" }
          ]
        },
        "topping":
        [
          { "id": "5001", "type": "None" },
          { "id": "5002", "type": "Glazed" },
          { "id": "5005", "type": "Sugar" },
          { "id": "5007", "type": "Powdered Sugar" },
          { "id": "5006", "type": "Chocolate with Sprinkles" },
          { "id": "5003", "type": "Chocolate" },
          { "id": "5004", "type": "Maple" }
        ]
      },
      [...] Other items could be there
    ]
  }
}

I cannot change the way it is organised. However, I cannot figure out how to properly get every element and extract it.

My current code:

ifstream input("sample.json");

    json json1;

    input >> json1;

    json json2 = json1["items"]["item"];

    for ( auto& item : json2.get<json::object_t>() ){
        Item *tempItem = new Item();

        tempItem->setId(item.at("id"));
        tempItem->setName(item.at("name"));
        tempItem->setType(item.at("type"));
        tempItem->setId(item.at("ppu"));
        //Trying to go in the array batters
        object_t temp_obj = item.at("batters");

        items.push_back(new Item());
    }

Note: Even then I get an error because json2.get<json::object_t>() returns a pair<>.

Thanks in advance!

[nlohmann]

You could try

for (const auto& item : json1["items"]["item"])
{
    // check if item["batters"]["batter"] exists
    if (item.find("batters") != item.end() and
        item["batters"].find("batter") != item["batters"].end())
    {
        for (const auto& batter : item["batters"]["batter"])
        {
            std::cout << batter["id"] << ", " << batter["type"] << std::endl;
        }
    }
    
    // check if item["topping"] exists
    if (item.find("topping") != item.end())
    {
        for (const auto& batter : item["topping"])
        {
            std::cout << batter["id"] << ", " << batter["type"] << std::endl;
        }
    }
}

[EtienneBerube]

This does answer my initial question, but now I get a parsing error.

    ifstream input;
    input.open(R"(C:\Users\Etienne\Documents\GitHub\Collaboration\assingment 5\sample.json)");

    json json1;

    input >> json1; //Error

    cout<<"after parse";

    json json2 = json1["items"]["item"];

For an unknown reason it doesn't accept the parsing.

[gregmarr]

What is the error?

[EtienneBerube]

It just stops, no error or anything. It tried printing after each statement and it happens at the JSON parsing.

[nlohmann]

Can you check whether you can print the file content? Can you run the code in a debugger?

[EtienneBerube]

After a few hours trying to debug, I finally got it to compile properly (reinstalled MinGW, changed my paths, etc.). The above code that @nlohmann wrote is accurate and works fine!
Thanks for your support and quick answers

[nlohmann]

Thanks for checking back!

[parthbir]

How can one push batter["id"] into another json object or vector.

I'm getting the following error for above query:
error:'batter' was not declared in this scope

[TobeyWins]

Hey, thanks for your example code.
I have one question regarding it.
The following returns 0

    const auto& output = jFile["properties"]["rules"]["output"];
    std::cout << (output.contains("somekey")) << std::endl;

while this returns 1

    for (const auto& output1 : jFile["properties"]["rules"]["output"])
    {
        std::cout << (output1.contains("somekey")) << std::endl;
    }

my debugger is unable to unpack the type so im just wondering, why is the first 0 and the second 1?

[nlohmann]

Can you share the complete code example and JSON file?

[TobeyWins]

thanks for you fast response.
Here is the original json

{
    "title": "Data Transform Configuration",
    "type": "generic",
    "properties": {
  
        "operations":[
            {
                "unique_id": "left_shift_1",
                "type": "operation_shift",
                "direction": "left",
                "bit_start": 0,
                "bit_length": 32,
                "bit_shift": 4
            }
        ],

        "rules": {
            
            "input": {},
            "output": [
                {
                    "lengthNumberType": "byte",
                    "dataLength":12,
                    "descritpion": "format either via references or direct in place, example shows in-place",
                    "format": [
                        {
                            "node_type": "text",
                            "unique_id": "1",
                            "content": "urn:epc:tag:grai-96:"
                        },
                        {
                            "node_type": "data",
                            "unique_id": "2",           
                            "byte": 1,
                            "bit_start": 0,         
                            "bit_length": 3,     
                            "format": "decimal_ascii"
                        }
                    ]   
                }
            ],

            "notify": {

            }
        }
    }
}

the example code did not contain that much more.
Just a read in of the above file

    std::ifstream i(configFilePath);
    i >> jFile;
    std::cout << jFile.dump(4) << std::endl;

where the following went with ok

    for (const auto &output : jFile["properties"]["rules"]["output"])
    {

        if (!(output.contains(Constants::KEY_LENGTH_NUMBER_TYPE)) ||
            !(output.contains(Constants::KEY_RULE_DATA_LENGTH)) ||
            !(output.contains(Constants::KEY_FORMAT)))
        {
            std::cout << "error while parsing. incomplete rule definition" << std::endl;
            return;
        }

    ...
    }

but when i used

const auto &output = jFile["properties"]["rules"]["output"]

instead of the for-loop it just gave 0 for the contains method

[nlohmann]

jFile["properties"]["rules"]["output"] seems to be an array. If you call contains directly on it, it will always return false, see https://json.nlohmann.me/api/basic_json/contains/.

If you iterate it in the for loop, output is a reference to a JSON object. Here, contains does what you expect.