Reading a string from the serial port and comparing to another string

Question

I'm using an Arduino Mega 2560 and writing a really simple piece of code that doesn't work.

String a, letra;
#define led 13

void setup() {
  Serial.begin(9600); // opens serial port, sets data rate to 9600 bps
  pinMode(led, OUTPUT);
}

void loop() {
  while(Serial.available()) {
    letra= Serial.readStringUntil('\n');
    letra.trim();
    Serial.println(letra);

    if(letra.equals("aa\r\n")){
      Serial.println("ABRE 1");
    }
    if(letra == "aa\r\n"){
      Serial.println("ABRE 2");
    }
    if(letra.compareTo("aa\r\n") == 0){
      Serial.println("ABRE 3");
    }
  }
}

It's returning what I typed, but it's not entering any of the if conditions... Can someone help me with that ?

Answer 1

Trim removes whitespace, and you are comparing for \r and \n to be there. Since they are whitespace naturally the compare fails.

See here for one definition of whitespace for C.