Comparing a text string to text output (DHT22)

Question

Lately I've received a DHT22 sensor. I've started to create a small temperature and humidity monitor station. One of the things I noticed was if you pulled out the sensor while in use, it displayed "nan". What I wanted to do was if the variable "temp" (for the temperature) equaled "nan", my LCD would display a message. But I keep getting a scope, conversion, and declaring errors. I've tried every variable and a lot of online forums, documentations, and the official Arduino reference site. My code looks like this:

#include <Adafruit_Sensor.h>
#include <DHT.h>
#include <LiquidCrystal.h>
const int rs = 53, en = 51, d4 = 49, d5 = 52, d6 = 48, d7 = 47;
LiquidCrystal lcd(rs, en, d4, d5, d6, d7);
#define DHTPIN 27 
#define DHTTYPE DHT22 
DHT dht(DHTPIN, DHTTYPE); 
int chk;
float hum; 
float temp; 
byte bigdot[8] = {
 B00000,
 B00000,
 B00000,
 B01110,
 B01110,
 B01110,
 B00000,
 B00000
};
byte circle[8] = {
 B00110,
 B01001,
 B01001,
 B00110,
 B00000,
 B00000,
 B00000,
 B00000
};
void setup() {
 lcd.createChar(0, bigdot);
 lcd.createChar(1, circle);
 lcd.begin(16, 2);
 dht.begin();
 do {
 lcd.setCursor(5,1);
 lcd.write(byte(2));
 lcd.setCursor(4,0);
 lcd.print("Loading");
 delay(500);
 lcd.clear();
 lcd.setCursor(7,1);
 lcd.write(byte(2));
 lcd.setCursor(4,0);
 lcd.print("Loading");
 delay(500);
 lcd.clear();
 lcd.setCursor(9,1);
 lcd.write(byte(2));
 lcd.setCursor(4,0);
 lcd.print("Loading");
 delay(500);
 lcd.clear();
 dht.readHumidity();
 dht.readTemperature();
 } while (hum > 90 || temp > 50.00);
 Serial.begin(9600);
}
void loop() {
 hum = dht.readHumidity();
 temp = dht.readTemperature();
 lcd.clear();
 lcd.setCursor(0,0);
 lcd.print("Temp: ");
 lcd.print(temp);
 lcd.write(byte(1));
 lcd.print("C");
 lcd.setCursor(0,1);
 lcd.print("RH: ");
 lcd.print(hum);
 lcd.print("%");
 if (temp == a) {
 lcd.clear();
 lcd.home();
 lcd.print("Whoops! An");
 lcd.setCursor(0,1);
 lcd.print("error occured!");
 };
 Serial.print("Humidity: ");
 Serial.print(hum);
 Serial.print(" %, Temp: ");
 Serial.print(temp);
 Serial.println(" Celsius");
 delay(2000); //Delay 2 sec.
 }

Thanks in advance!

• jsotola: please add the full error message to your post

Answer 1

A "float" number has multiple possible states, besides just being a number (It never, though, can be compared to a string):

• A real floating point number
• Infinity
• Something that isn't a valid number

The latter is what you want - that is, the internal data of the float variable doesn't represent a valid number.

There are two functions in the standard math library that deal with this for you.

• Is the number infinity? ininf(x)
• Is the number invalid? isnan(x)

To use it:

if (isnan(temp)) {
 lcd.print("Invalid");
}