Jsoncpp nested arrays of objects

Question

I need to search for an element in a json file using the jsoncpp library. I can't wrap my head around how to get to the most inner array... Any thoughts?

{
    "key": int,
    "array1": [
        {
            "key": int,
            "array2": [
                {
                    "key": int,
                    "array3": [
                        {
                            "needed_key": int
                        }
                    ]
                }
             ]
          }
     ]
}

Up until now I tried something like this:

    const Json::Value& array1 = root["array1"];
    for (size_t i = 0; i < array1.size(); i++)
    {
        const Json::Value& array2 = array1["array2"];
        for (size_t j = 0; j < array2.size(); j++)
        {
            const Json::Value& array3 = array2["array3"];
            for (size_t k = 0; k < array3.size(); k++)
            {
                std::cout << "Needed Key: " << array3["needed_key"].asInt();
            }
        }
    }

But it throws:

JSONCPP_NORETURN void throwLogicError(String const& msg) {
  throw LogicError(msg);
}

Show what you have tried and the errors and/or unexpected behaviour you get. — super
– super, Commented Apr 26, 2022 at 7:49
I have updated the post. I know what I have by now is rubbish, but I am out of ideas :( — Maria
– Maria, Commented Apr 26, 2022 at 8:21
You iterate through an array with ints. So shouldn't it be something like array2 = array1[i]["array2"] and so on? — super
– super, Commented Apr 26, 2022 at 9:08
do you need to be checking something like array1["key"] == key1 at any level? How do you know which element contains the next level array? — Caleth
– Caleth, Commented Apr 26, 2022 at 9:31
@Caleth, yes. For example if the second key is equal to a certain value, then extract the needed key. But for now that is not my focus. I just want to find a way to get any element from the inner most array. After looking all day , I think that I need to to what is called a deserialization. — Maria
– Maria, Commented Apr 26, 2022 at 12:51

ramsay · Accepted Answer · 2022-04-26 13:29:22Z

1

You can't access array2 with array1["array2"], since array1 contains an array of objects, not an object, so you should get array2 with an index i, array1[i]["array2"] instead.

The following code works for me:

  const Json::Value &array1 = root["array1"];
  for (int i = 0; i < array1.size(); i++) {
    const Json::Value &array2 = array1[i]["array2"];
    for (int j = 0; j < array2.size(); j++) {
      const Json::Value &array3 = array2[j]["array3"];
      for (int k = 0; k < array3.size(); k++) {
        std::cout << "Needed Key: " << array3[k]["needed_key"].asInt();
      }
    }
  }

The output looks like:

Needed Key: 4

answered Apr 26, 2022 at 13:29

ramsay

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