Retrieving value from JSON feed

Projects Programming
1

Hi there

I'm trying to retrieve a specific value from a JSON feed but am getting a NaN result when it should return 21.5.

This is the raw JSON as displayed in the Serial Monitor:

==RAW JSON==
[{"LocalObservationDateTime":"2025-08-01T12:06:00+01:00","EpochTime":1754046360,"WeatherText":"Mostly cloudy","WeatherIcon":6,"HasPrecipitation":false,"PrecipitationType":null,"IsDayTime":true,"Temperature":{"Metric":{"Value":21.5,"Unit":"C","UnitType":17},"Imperial":{"Value":71.0,"Unit":"F","UnitType":18}},"MobileLink":"http://www.accuweather.com/en/gb/saltdean/bn2-8/current-weather/2524451?lang=en-us","Link":"http://www.accuweather.com/en/gb/saltdean/bn2-8/current-weather/2524451?lang=en-us"}]
==================

My locate data function in the sketch is:

void showWeather() {
  String json = getWeatherData(); //returns the ==RAW JSON== data

  Serial.println("==RAW JSON==");
  Serial.println(json);
  Serial.println("==================");

  JSONVar root = JSON.parse(json);
  
  float temp_c = (double) root["0"]["Temperature"]["Metric"]["Value"];

  tft.setTextSize(2);
  tft.setTextColor(WHITE);
  tft.setCursor(170, 89);
  tft.print(temp_c, 0);
}

I assume I have a JSON addressing error but I've tried a few variations without luck. Where am I going wrong?

Many thanks

2

It's helpful to format the JSON for us humans

[
  {
    "LocalObservationDateTime": "2025-08-01T12:06:00+01:00",
    "EpochTime": 1754046360,
    "WeatherText": "Mostly cloudy",
    "WeatherIcon": 6,
    "HasPrecipitation": false,
    "PrecipitationType": null,
    "IsDayTime": true,
    "Temperature": {
      "Metric": {
        "Value": 21.5,
        "Unit": "C",
        "UnitType": 17
      },
      "Imperial": {
        "Value": 71,
        "Unit": "F",
        "UnitType": 18
      }
    },
    "MobileLink": "http://www.accuweather.com/en/gb/saltdean/bn2-8/current-weather/2524451?lang=en-us",
    "Link": "http://www.accuweather.com/en/gb/saltdean/bn2-8/current-weather/2524451?lang=en-us"
  }
]
3

Should that zero have quote marks around it? It is an array index, not a label. This is a guess, I'm not that familiar with the Arduino JSON library.

4

Try

void showWeather() {
  String json = getWeatherData(); //returns the ==RAW JSON== data

  Serial.println("==RAW JSON==");
  Serial.println(json);
  Serial.println("==================");

  JSONVar root = JSON.parse(json);

  String temp_c_str = root["0"]["Temperature"]["Metric"]["Value"];

  tft.setTextSize(2);
  tft.setTextColor(WHITE);
  tft.setCursor(170, 89);
  tft.print(temp_c_str);
}

I realise this might not give the formatting you wanted, but does it work at all?

5

Also try

void showWeather() {
  String json = getWeatherData(); //returns the ==RAW JSON== data

  Serial.println("==RAW JSON==");
  Serial.println(json);
  Serial.println("==================");

  JSONVar root = JSON.parse(json);
  
  float temp_c = root["0"]["Temperature"]["Metric"]["Value"].toFloat();

  tft.setTextSize(2);
  tft.setTextColor(WHITE);
  tft.setCursor(170, 89);
  tft.print(temp_c, 0);
}
6

Thanks again PaulRB... much appreciated. I'm sure I tried this before posting but must have got something wrong :frowning:
Sorry also for not formatting the JSON nicely. I WILL do better!

7

I use this site to check if my json string is correct:

“https://jsonviewer.stack.hu/

Look your json at site:

image
image636Ă—432 16.8 KB

8

One nice forum feature: syntax highlighting for things other than "Arduino" or C++. You can choose the language in the WYSIWYG editor, which translates to having the designator after the opening triple-backtick in the Markdown: ```json in this case.

[
  {
    "LocalObservationDateTime": "2025-08-01T12:06:00+01:00",
    "EpochTime": 1754046360,
    "WeatherText": "Mostly cloudy",
    "WeatherIcon": 6,
    "HasPrecipitation": false,
    "PrecipitationType": null,
    "IsDayTime": true,
    "Temperature": {
      "Metric": {
        "Value": 21.5,
        "Unit": "C",
        "UnitType": 17
      },
      "Imperial": {
        "Value": 71,
        "Unit": "F",
        "UnitType": 18
      }
    },
    "MobileLink": "http://www.accuweather.com/en/gb/saltdean/bn2-8/current-weather/2524451?lang=en-us",
    "Link": "http://www.accuweather.com/en/gb/saltdean/bn2-8/current-weather/2524451?lang=en-us"
  }
]

Now there are different colors for the quoted field names versus the field values that are strings; and null is colored the same as true and false (the only three "plain words" allowed in JSON)