Encoding variable number of nested objects

[orphuz]

Hello and thank you for your great work.
I use your library for building a multisensor platform (based on an Arduino Mega) for indoor air quality and use JSON for communicating with an handheld device. Your library does quite a job in my project. However I still have one problem for which i just can't find a proper solution.

My idea is generate a variable number of nested object within an iteration. In this iteration I try to assign the values of an array of integers and an corresponding array of chararrays to the key-value pairs of each object. I do understand, that I can't build an array of objects, since the objects are references theirself. But I don't know how to deal with that probelm.

Here is what i want to do in pseudo code:

int iLength_of_list = 3;
int list[3];
list[0] = 5;
list[1] = 7;
list[2] = 1;

StaticJsonBuffer<256> jsonBuffer;
JsonObject& joMessage = jsonBuffer.createObject();
joMessage["length of list"] = iLength_of_list;

/* some magic must happen here, similar to JsonObject& nestedObject[iLength_of_list] */

for (int i = 0; i<= iLength_of_list-1;i++) {
    char cBuffer[2];
    sprintf(cBuffer, "%d", i)
    nestedObject[i] = joMessage.createNestedObject(cBuffer);
    nestedObject[i]["en"] = englishTextOfIndex(list[i]);
    nestedObject[i]["de"] = germanTextOfIndex(list[i]);
}
joMessage.printTo(Serial)

I hope you can get an idea of what I am trying to do. I have read through all the issues and think that franklinvv in issue #10 had a similar problem, but the examplecode does not run for me.

Many thanks in advance

Chris

[bblanchon]

Hi Chris,

The problem is the local variable cBuffer.
By using this variable as a name in the JSON object tree, you're implicitly pointing to a variable that doesn't exists outside of the loop.

The solution really depends on your project.

The simpler would be to change joMessage into a JsonArrayinstead of a JsonObject, so as to avoid naming the nested elements.

If you absolutely need to name the elements, you'll need to store the names in a variable that is outside of the loop scope, for instance:

char names[20][2];

for(...) {
    sprintf(names[i], "%d", i);
    ...
}

Regards,
Benoit

[orphuz]

Thank you for your quick response.

Unfortunately, i can't use a JsonArray for this task. I absolutely need to name the elements because there already exists a definition for the serial communication interface I try to develop on the Arduino.
The final result should look like this:

"List": {
    "Length": 5,
    "0": {
        "en": "dog",
        "de": "Hund"
    }, 
    "1": {
        "en": "cat",
        "de": "Katze"
    }, ...
    "2": {
        "en": "mouse",
        "de": "Maus"
    }, ...
}

If you absolutely need to name the elements, you'll need to store the names in a variable that is outside of the loop scope, for instance:

Your are right here, my examplecode is wrong. But that is not the actual problem. I try to generate an Array with a variable number of JSON elements. My aim ist to iterate throguh them:

int iLength_of_list = 5;
...
char * buff_ListItem;
buff_ListItem =  new char[3];
JsonObject& mainObject;
JsonObject&* nestedObject;
for (int i = 0; i<iLength_of_list-1; i++){
    sprintf(buff_ListItem,"%d",i);
    nestedObject[i] = jsonBuffer.createObject();
    nestedObject[i]["en"] = english[i];
    nestedObject[i]["de"] = german[i];
    mainObject.add(buff_ListItem, nestedObject[i]);
}
delete[] buff_ListItem;

As far as I do understand, i can't create an array of JsonObject& and my problem that it does not have a default constructor.

Thank you for your help,

Chris

[bblanchon]

Please try:

int iLength_of_list = 5;
...
char itemNames[MAX_COUNT][2];
for (int i = 0; i<iLength_of_list; i++) {
    sprintf(itemNames[i],"%d",i);
    JsonObject& nestedObject = mainObject.createNestedObject(itemNames[i]);
    nestedObject["en"] = english[i];
    nestedObject["de"] = german[i];
}

[markusand]

Hi! Related to this topic, I'm trying to implement a JSON nested chain similar to this:

{
  "id" : "398274",
  "device" : "Ambient+",
  "version" : "1.2",
  "data" : [{
    "temperature" : "22.5",
    "humidity" : "83",
    "lux" : "4208",
    "CO2" : "465"
  }]
}

Using the code presented, the compiler returns the error:

StaticJsonBuffer<200> jsonBuffer;

  JsonObject& device = jsonBuffer.createObject();
  device["id"] = 398274;
  device["device"] = "Ambient+";
  device["version"] = "1.2";

  JsonObject& data = device.createNestedObject("data");
    data.["temperature"] = "22.5";  
    data.["humidity"] = "83";
    data.["lux"] = "4208";
    data.["CO2"] = "465";

  //root.printTo(Serial);
  device.prettyPrintTo(Serial);

sketch_jul01a:19: error: expected unqualified-id before '[' token
expected unqualified-id before '[' token

[bblanchon]

@markusand just remove the extra ., like

data["temperature"] = "22.5";

[markusand]

I don't even know how I missed that. What a stupid mistake. Thanks.

[bblanchon]

@markusand, you're welcome 😉
Thanks for using ArduinoJson.