Why does Serial.print print out the wrong thing?

Question

I am using a flame sensor. I am reading the state and then printing it. Here is my code:

void loop(){
 flameState = digitalRead(flamePin);//variables already defined and are global
existsFire();//calls the void
delay(1000);
}
void existsFire(){
Serial.println("Flame: " + flameState);
}

Before, this printed out the right thing. Today, it was printing out "lame:". The "F" and the flameState were missing. I changed it to:

void existsFire(){
Serial.print("Flame: ");
Serial.println(flameState);
}

Everything worked fine. Is there a problem with my Arduino Mega. What do I need to do to fix this?

Answer 1

You've stumbled across the difference in the C++ language between a String and an "array of characters". What you gave Serial.println() was an array of characters:

Serial.println("Flame: "...);

and then tried to add a number to it.

Serial.println("Flame: " + flameState);

If it had have been a String, then what you typed would have been fine:

String flame = "Flame: ";
Serial.println(flame + flameState);

Adding (literally +) a number to a String adds characters to the String to represent the number. Unfortunately you didn't start with a String, you started with an array of characters:

"Flame: "

Adding a number to THAT changes where the array starts. flameState must have been 1, and "Flame: " + 1 would have been "lame :" - as you found out!

The version that worked did so through a little bit of magic:

Serial.print("Flame: ");
Serial.println(flameState);

The first line called a version of print() that accepted an array of characters and printed each of them out. The second line called a different version of println() that accepted a number and printed out its representation as a series of characters (digits) - followed by a new line (the ln part of the name).