Question

My background is not in C (it’s in Real Studio – similar to VB) and I’m really struggling to split a comma-delimited string since I’m not used to low-level string handling.

I’m sending strings to an Arduino over serial. These strings are commands in a certain format. For instance:

@20,2000,5!
@10,423,0!

‘@’ is the header indicating a new command and ‘!’ is the terminating footer marking the end of a command. The first integer after ‘@’ is the command id and the remaining integers are data (the number of integers passed as data may be anywhere from 0 – 10 integers).

I’ve written a sketch that gets the command (stripped of the ‘@’ and ‘!’) and calls a function called handleCommand() when there is a command to handle. The problem is, I really don’t know how to split this command up to handle it!

Here’s the sketch code:

String command; // a string to hold the incoming command
boolean commandReceived = false; // whether the command has been received in full

void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);
}

void loop() {
  // main loop
  handleCommand();
}

void serialEvent(){
  while (Serial.available()) {
  // all we do is construct the incoming command to be handled in the main loop

    // get the incoming byte from the serial stream
    char incomingByte = (char)Serial.read();

    if (incomingByte == '!')
    {
       // marks the end of a command
       commandReceived = true;
       return;
    }
    else if (incomingByte == '@')
    {
       // marks the start of a new command
       command = "";
       commandReceived = false;
       return;
    }
    else
    {
      command += incomingByte;
      return;
    }

  }
}

void handleCommand() {

  if (!commandReceived) return; // no command to handle

  // variables to hold the command id and the command data
  int id;
  int data[9];

  // NOT SURE WHAT TO DO HERE!!

  // flag that we've handled the command 
  commandReceived = false;
}

Say my PC sends the Arduino the string “@20,2000,5!”. My sketch ends up with a String variable (called command) that contains “20,2000,5” and the commandRecieved boolean variable is set to True so the handleCommand() function is called.

What I would like to do in the (currently useless) handleCommand() function is assign 20 to a variable called id and 2000 and 5 to an array of integers called data, i.e: data[0] = 2000, data[1] = 5, etc.

I’ve read about strtok() and atoi() but frankly I just can’t get my head around them and the concept of pointers. I’m sure my Arduino sketch could be optimised too.

Answer 1

Since you’re using the Arduino core String type, strtok and other string.h functions aren’t appropriate. Note that you can change your code to use standard C null-terminated strings instead, but using Arduino String will let you do this without using pointers.

The String type gives you indexOf and substring.

Assuming a String with the @ and ! stripped off, finding your command and arguments would look something like this:

// given: String command
int data[MAX_ARGS];
int numArgs = 0;

int beginIdx = 0;
int idx = command.indexOf(",");

String arg;
char charBuffer[16];

while (idx != -1)
{
    arg = command.substring(beginIdx, idx);
    arg.toCharArray(charBuffer, 16);

    // add error handling for atoi:
    data[numArgs++] = atoi(charBuffer);
    beginIdx = idx + 1;
    idx = command.indexOf(",", beginIdx);
}

data[numArgs++] = command.substring(beginIdx);

This will give you your entire command in the data array, including the command number at data[0], while you’ve specified that only the args should be in data. But the necessary changes are minor.