failing building a variable to use in client.publish

Projects Programming
1

I use the PubSubClient im my project to transmit temperature from an DHT22 sensor with an ESP01 via MQTT.

All works fine, all my tests and so on. But I bitterly fail in bulding a variable to use with client.publish.
My cloud cumulocity where I have to transmit the template with the temperature.

e.g. client.publish ("s/us", "211,25") //uses the template 211 for temperature and transmits 25 *C

I've a working function to read the DHT22 but when I try to build the client.publish ("s/us", "211,"+temperatureTemp); with template and the temperature, I'll get the following error.

Temp_Hum_Vcc-ESP01:155: error: invalid operands of types 'const char [5]' and 'char [7]' to binary 'operator+'

     client.publish("s/us", "211,"+temperatureTemp);

                                   ^

exit status 1
invalid operands of types 'const char [5]' and 'char [7]' to binary 'operator+'

This is the code of the function. I can't fugure it out how I can combine them.

now = millis();
  // Publishes new temperature and humidity every 10 seconds
  if (now - lastMeasure > 10000) {
    lastMeasure = now;
    // Sensor readings may also be up to 2 seconds 'old' (its a very slow sensor)
    float h = dht.readHumidity();
    // Read temperature as Celsius (the default)
    float t = dht.readTemperature();
    // Read temperature as Fahrenheit (isFahrenheit = true)
    float f = dht.readTemperature(true);

    // Check if any reads failed and exit early (to try again).
    if (isnan(h) || isnan(t) || isnan(f)) {
      Serial.println("Failed to read from DHT sensor!");
      client.publish("s/us", "211,25");
      return;
    }

    // Computes temperature values in Celsius
    float hic = dht.computeHeatIndex(t, h, false);
    static char temperatureTemp[7];
    dtostrf(hic, 6, 2, temperatureTemp);

    
    // Uncomment to compute temperature values in Fahrenheit 
    // float hif = dht.computeHeatIndex(f, h);
    // static char temperatureTemp[7];
    // dtostrf(hic, 6, 2, temperatureTemp);
    
    static char humidityTemp[7];
    dtostrf(h, 6, 2, humidityTemp);
    
    

    // Publishes Temperature and Humidity values
    client.publish("s/us", "211,"+temperatureTemp);
    //client.publish("s/us", humidityTemp); 
    
    Serial.print("Humidity: ");
    Serial.print(h);
    Serial.print(" %\t Temperature: ");
    Serial.print(t);
    Serial.print(" *C ");
    Serial.print(f);
    Serial.print(" *F\t Heat index: ");
    Serial.print(hic);
    Serial.println(" *C ");

  }

Using client.publish ("s/us", temperatureTemp); works fine, but adding the template "211," before temperature is where I'm failing.

PS: I'm a rookie in programming with Arduino IDE and I'm desperate stranded

2

Take a look at sprintf. Or strcpy and strcat.

3

I've found these while googling but didn't manage to use them or transfer it to my problem.

Solved it this (pretty sure stupid) way:

String stringOne, stringTwo;
stringOne = String("211,");
Serial.println(stringOne);
stringTwo = String(temperatureTemp);
Serial.println(stringTwo);
stringTwo.trim();
stringOne += stringTwo;
Serial.println(stringOne);
char Message[20];
stringOne.toCharArray(Message,20);