Problem creating a message with variables

Projects Programming
1

Hello,

I've been a little away from programming and I'm actually doing one project in my university which needed me to learn arduino. I've been doing ok, but it seems I have this problem with sending a message. I've been searching but I couldn't find the right way do add a variable into a string (or array of characters). The point of this function is to create a message in which there is the name of a certain problem detected and the value encountered. I'm not sure how to add strings in arduino so I made it this way, however it seems it does not accept it and I don't know how to put the value of 'value' inside the string.
The code I have is the following:

char *createMessage(byte ID, float value){
  String messageF = "Automatic Message from Cryo Lab: There seems to be a problem with ";
  if (ID == 1){
    messageF = messageF + "Electricity ";
  }
    Serial.println(messageF);

  messageF = messageF + "which was found with the value of ";
    Serial.println(messageF);

  messageF = messageF + value;
  Serial.println(messageF);
  char message[200];
  messageF.toCharArray(message, 200);
  return message;
}

Could you please give me some advice?

Best regards,
Rhen

2 
char message[200];
  messageF.toCharArray(message, 200);
  return message;

Never return a reference to an automatic variable.
Pass the char buffer into the function, if necessary.
"sprintf" may be a lot easier to use than the String class

3

Or strcat maybe.

To use sprintf with floats you have to do: Strange problem with "sprintf" - #19 by krupski - Programming Questions - Arduino Forum , or use dtostrf

4

Or "strcat_P" etc, preferably.

5

Ok thank you! It seems to be working! I used sprintf() and for floats I used one little algorithm. It stayed like this and it gives me exactly what I want.

char *createMessage(byte ID, float value){
  char message[200] = "Automatic Message from Cryo Lab: There seems to be a problem with";
  if (ID == 1){
    sprintf(message, "%s Electricity", message);
  }
  int d1 = value;
  float f2 = value - d1;
  int d2 = trunc(f2 * 100);
  float f3 = f2 * 100 - d2;
  sprintf(message, "%s with the value of %d.%02d\n", message, d1, d2);
  Serial.println(message);
  return message;
}
6

Have a close look at reply #1

7

I tried to understand it, but I'm not sure. In this last case is sprintf turning "message" a reference?

8

You're returning a pointer to an entity on the stack, mere nanoseconds before that entity goes out of scope.
Either make "message" static, use a global, or pass the pointer to the buffer into the function