How to parse json arrays in arduino IDE

Projects Programming
1

I'm trying to parse a JSON array on a Pi server over wifi. I want to parse the following.

[{"Device": "Curtain", "Value": "1"},{"Device": "Fan", "Value": "1"},{"Device": "Blinds", "Value": "0"},{"Device": "Relay", "Value": "0"}]

These are copied from the web page.

Here is my Arduino code.

#include <ArduinoJson.h>
void setup() {
  Serial.begin(9600);
  while (!Serial) {
    // wait serial port initialization
  }
  StaticJsonBuffer<200> jsonBuffer;    
  char   json[] =
  "[{\"Device\":\"Curtain\",\"Value\":\"1\"},{\"Device\":\"Fan\",\"Value\":\"1\"}]";
 
  JsonArray& root = jsonBuffer.parseArray(json);
  // Test if parsing succeeds.
  if (!root.success()) {
    Serial.println("parseObject() failed");
    return;
  }
  // Fetch values.
  const char* device[] = root["Device"];
          int value[] = root["Value"];
  // Print values.
  Serial.println(device);
  Serial.println(value);
}

Then I get a bunch of errors ......

JsonParseExample.ino: In function 'void setup()':
JsonParseExample:54: error: invalid conversion from 'const char*' to 'size_t {aka unsigned int}' [-fpermissive]
In file included from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/JsonArray.hpp:181:0,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson.h:9,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/ArduinoJson.h:14,
                 from JsonParseExample.ino:8:
/Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/JsonArray.ipp:16:27: error:   initializing argument 1 of 'ArduinoJson::JsonArraySubscript ArduinoJson::JsonArray::operator[](size_t)' [-fpermissive]
 inline JsonArraySubscript JsonArray::operator[](size_t index) {
                           ^
JsonParseExample:54: error: initializer fails to determine size of 'device'
JsonParseExample:54: error: array must be initialized with a brace-enclosed initializer
JsonParseExample:58: error: call of overloaded 'println(const char* [1])' is ambiguous
JsonParseExample.ino:58:24: note: candidates are:
In file included from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/Internals/../Internals/../Arduino/Print.hpp:87:0,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/Internals/../Internals/DummyPrint.hpp:10,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/Internals/../Internals/JsonPrintable.hpp:11,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/Internals/../JsonVariant.hpp:13,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/Internals/../JsonBuffer.hpp:15,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/Internals/BlockJsonBuffer.hpp:10,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson/DynamicJsonBuffer.hpp:10,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/../include/ArduinoJson.h:8,
                 from /Users/Wei0928/Documents/Arduino/libraries/ArduinoJson/src/ArduinoJson.h:14,
                 from JsonParseExample.ino:8:
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:73:12: note: size_t Print::println(char) <near match>
     size_t println(char);
            ^
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:73:12: note:   no known conversion for argument 1 from 'const char* [1]' to 'char'
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:74:12: note: size_t Print::println(unsigned char, int) <near match>
     size_t println(unsigned char, int = DEC);
            ^
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:74:12: note:   no known conversion for argument 1 from 'const char* [1]' to 'unsigned char'
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:75:12: note: size_t Print::println(int, int) <near match>
     size_t println(int, int = DEC);
            ^
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:75:12: note:   no known conversion for argument 1 from 'const char* [1]' to 'int'
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:76:12: note: size_t Print::println(unsigned int, int) <near match>
     size_t println(unsigned int, int = DEC);
            ^
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:76:12: note:   no known conversion for argument 1 from 'const char* [1]' to 'unsigned int'
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:77:12: note: size_t Print::println(long int, int) <near match>
     size_t println(long, int = DEC);
            ^
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:77:12: note:   no known conversion for argument 1 from 'const char* [1]' to 'long int'
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:78:12: note: size_t Print::println(long unsigned int, int) <near match>
     size_t println(unsigned long, int = DEC);
            ^
/Applications/Arduino.app/Contents/Java/hardware/arduino/avr/cores/arduino/Print.h:78:12: note:   no known conversion for argument 1 from 'const char* [1]' to 'long unsigned int'
JsonParseExample:59: error: 'value' was not declared in this scope
invalid conversion from 'const char*' to 'size_t {aka unsigned int}' [-fpermissive]

Most importantly, I haven't figured out how to read the arrays individually.

1 Like
2

Have you tried the Assistant here

3

This project has a json example at the bottom of the page with references to two json libraries
Arduino to Arduino via Serial

4

Not being a Json user, did you mean this
{
"Device": [ "Curtain", { "Value": "1" } ],
"Device": [ "Fan", { "Value": "1" } ]
}
Which I think you can then access by (indices starting at 0)
const char* Device_1 = doc["Device"][1]; // Fan
const char* Device_1_Value = doc["Device"][1]["Value"]; // "1"

6

Thanks for your reply, I think I may know what the problem is.

  const char* device[] = root["Device"];
          int value[] = root["Value"];

Here root is a JsonArray, not a JsonObject, so I can't use a string as a key to access the elements of the array.
I tried changing it to look like this.

JsonObject *firstDeviceObject = root[0];
const char* firstDeviceName = firstDeviceObject["Device"];
7

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