Question

I’m working on an Arduino and String.substring does not appear to be operating correctly, so I’m wondering what I may be doing wrong….

My function is as follows:

boolean processSerial()
{
     String buf;
     int iter = 0;
     char thisChar; 

     while(iter < 1000){
     if (Serial.available()) {
       #if ARDUINO >= 100 //For Arduino v1.0+
       {
         thisChar = Serial.read();
         buf += thisChar;
         //if(DEBUG){Serial.print(thisChar);}
       }  
       #else //For Arduino v0023 or earlier
         thisChar = Serial.read();
       #endif
     }
     iter++;
     }
     Serial.print(buf);
     Serial.print(buf.substring(0,10));

     if(buf.substring(1) == "GPGGA"){
        Serial.println("FOUND IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");
        }
        Serial.println("And we're done");

}

The pattern “GPGGA” is coming up but substring is not returning true.

The output from my Serial is as follows:

$GPGGA,053540.000,3804.1237,N,07615.5232,W,1,7,1.39,117.5,M,-33$GPGGA,053And we're done

Thus buf clearly has ‘GPGGA’ in the string. The last little bit that shows ‘GPGGA’ again is the printing of charcters 0-10 in the string. Why is my if statement not returning true?

Answer 1

buf.substring(1) returns the substring starting at position one and continuing all the way to the end of the string. This will start with GPGGA, but it will not equal GPGGA.

If you know the string you compare to is 5 characters in length, you could use

if (buf.substring(1,1+5) == "GPGGA")
  /* ... */

Also note that it’s not the substring() function that is expected to return true, but the comparison operator ==.