Rounding sensor values to 2 decimal places

General
1

I’m reading sensor values from a DHT11 sensor for Temperature and Humidity. Everything works as expected but I’m having trouble rounding the values to 2 places. I used the round function for the sensed values before sending it to the lcd.print function but I can’t get it to work. Any suggestions? Code snippet is below…

// Reading temperature or humidity takes about 250 milliseconds!
// Sensor readings may also be up to 2 seconds 'old' (its a 
// very slow sensor)
	float h = dht.getHumidity();

// Read temperature as Celsius
	float t = dht.getTempCelcius();
// Read temperature as Farenheit
	float f = dht.getTempFarenheit();
  
// 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!");
		return;
	}

// Compute heat index
// Must send in temp in Fahrenheit!
	float hi = dht.getHeatIndex();
	float dp = dht.getDewPoint();
	float k = dht.getTempKelvin();

       **f = round(f*10)/10; //set precision to 1 decimal**
       **h = round(h*10)/10; //set precision to 1 decimal**
	Serial.print("Humidity: "); 
	Serial.print(h);
	lcd.setCursor(0, 0);
	lcd.print("Humidity " + String(h) + "%");
	lcd.setCursor(0, 1);
	lcd.print("Temp " + (String(f) + (char(223)) + "F")); //char 223 is the Degree symbol
2

There is a difference between rounding the raw value and converting a float value to string.
Floating point datatypes cannot be rounded perfectly to any arbitrary value and/or precision due to the way how these values are stored in binary form.

Since we always recommond to stay away from String objects the widely preferred way to do what you want could look like this

  char str[32]
  snprintf(str, sizeof(str), "Humidity %.2f%%", h);
  lcd.print(str);
  snprintf(str, sizeof(str), "Temp %.2f%cF", f, 223);
  lcd.print(str);
2 Likes
3

Thank you. That worked perfectly!

1 Like
4

A follow up question, I’m trying to get the LCD messages to blink and this might be a really “hack” way to do it and I thought the following code would work but I’m getting a “invalid use of non-static member function” error on the lcd.clear function. Thoughts?

if (f >= 75)
      {
        lcd.clear;
        lcd.setCursor(0, 1);
        lcd.print("Temp Excursion!!!!!!");
        delay(500);
        lcd.clear;
        lcd.print("Temp Excursion!!!!!!");
        delay(500);
        lcd.clear;
        lcd.print("Temp Excursion!!!!!!");
        delay(500);
        lcd.clear;
      }

if (h >= 90)
    {
        lcd.clear;
        lcd.setCursor(0, 2);
        lcd.print("Humid Excursion!!!!!!");
        delay(500);
        lcd.clear;
        lcd.print("Humid Excursion!!!!!!");
        delay(500);
        lcd.clear;
        lcd.print("Humid Excursion!!!!!!");
        delay(500);
        lcd.clear;
    }
5

I’m an idiot. I forgot the () after lcd.clear. It should be lcd.clear();

It now works but I’m surprised there’s no function for lcd.blink ?