JsonFilterExample.ino

Description

This example shows how to use DeserializationOpion::Filter to filter a large input document and save a lot of space in the JsonDocument.

It uses the following extract from the OpenWeatherMap response as the input document:

{
  "cod": "200",
  "message": 0,
  "list": [
    {
      "dt": 1581498000,
      "main": {
        "temp": 3.23,
        "feels_like": -3.63,
        "temp_min": 3.23,
        "temp_max": 4.62,
        "pressure": 1014,
        "sea_level": 1014,
        "grnd_level": 1010,
        "humidity": 58,
        "temp_kf": -1.39
      },
      "weather": [
        {
          "id": 800,
          "main": "Clear",
          "description": "clear sky",
          "icon": "01d"
        }
      ],
      "clouds": {
        "all": 0
      },
      "wind": {
        "speed": 6.19,
        "deg": 266
      },
      "sys": {
        "pod": "d"
      },
      "dt_txt": "2020-02-12 09:00:00"
    },
    {
      "dt": 1581508800,
      "main": {
        "temp": 6.09,
        "feels_like": -1.07,
        "temp_min": 6.09,
        "temp_max": 7.13,
        "pressure": 1015,
        "sea_level": 1015,
        "grnd_level": 1011,
        "humidity": 48,
        "temp_kf": -1.04
      },
      "weather": [
        {
          "id": 800,
          "main": "Clear",
          "description": "clear sky",
          "icon": "01d"
        }
      ],
      "clouds": {
        "all": 9
      },
      "wind": {
        "speed": 6.64,
        "deg": 268
      },
      "sys": {
        "pod": "d"
      },
      "dt_txt": "2020-02-12 12:00:00"
    }
  ],
  "city": {
    "id": 2643743,
    "name": "London",
    "coord": {
      "lat": 51.5085,
      "lon": -0.1257
    },
    "country": "GB",
    "population": 1000000,
    "timezone": 0,
    "sunrise": 1581492085,
    "sunset": 1581527294
  }
}

As you can see the input contains many fields. In this example, we suppose that we are only interested in the following fields:

  • dt, the timestamp,
  • temp, the temperature.

To keep only these fields, we must create the following filter document:

{
  "list": [
    {
      "dt": true,
      "main": {
        "temp": true
      }
    }
  ]
}

After applying this filter, the result should be:

{
  "list": [
    {
      "dt": 1581498000,
      "main": {
        "temp": 3.23,
      }
    },
    {
      "dt": 1581508800,
      "main": {
        "temp": 6.09,
      }
    }
  ]
}

The final document is almost 10 times smaller than the original.

For a complete example using OpenWeatherMap, please see the case studies in Mastering ArduinoJson.

Source code

#include <ArduinoJson.h>

void setup() {
  // Initialize serial port
  Serial.begin(9600);
  while (!Serial) continue;

  // The huge input: an extract from OpenWeatherMap response
  const __FlashStringHelper* input_json = F(
      "{\"cod\":\"200\",\"message\":0,\"list\":[{\"dt\":1581498000,\"main\":{"
      "\"temp\":3.23,\"feels_like\":-3.63,\"temp_min\":3.23,\"temp_max\":4.62,"
      "\"pressure\":1014,\"sea_level\":1014,\"grnd_level\":1010,\"humidity\":"
      "58,\"temp_kf\":-1.39},\"weather\":[{\"id\":800,\"main\":\"Clear\","
      "\"description\":\"clear "
      "sky\",\"icon\":\"01d\"}],\"clouds\":{\"all\":0},\"wind\":{\"speed\":6."
      "19,\"deg\":266},\"sys\":{\"pod\":\"d\"},\"dt_txt\":\"2020-02-12 "
      "09:00:00\"},{\"dt\":1581508800,\"main\":{\"temp\":6.09,\"feels_like\":-"
      "1.07,\"temp_min\":6.09,\"temp_max\":7.13,\"pressure\":1015,\"sea_"
      "level\":1015,\"grnd_level\":1011,\"humidity\":48,\"temp_kf\":-1.04},"
      "\"weather\":[{\"id\":800,\"main\":\"Clear\",\"description\":\"clear "
      "sky\",\"icon\":\"01d\"}],\"clouds\":{\"all\":9},\"wind\":{\"speed\":6."
      "64,\"deg\":268},\"sys\":{\"pod\":\"d\"},\"dt_txt\":\"2020-02-12 "
      "12:00:00\"}],\"city\":{\"id\":2643743,\"name\":\"London\",\"coord\":{"
      "\"lat\":51.5085,\"lon\":-0.1257},\"country\":\"GB\",\"population\":"
      "1000000,\"timezone\":0,\"sunrise\":1581492085,\"sunset\":1581527294}}");

  // The filter: it contains "true" for each value we want to keep
  StaticJsonDocument<200> filter;
  filter["list"][0]["dt"] = true;
  filter["list"][0]["main"]["temp"] = true;

  // Deserialize the document
  StaticJsonDocument<400> doc;
  deserializeJson(doc, input_json, DeserializationOption::Filter(filter));

  // Print the result
  serializeJsonPretty(doc, Serial);
}

void loop() {
  // not used in this example
}

Things used in this example

Classes

Functions

Libraries

See also