Concatenated String Doesn't Send Correctly

[mrmuszynski]

Hi all!

I'm working with sending messages from one of my sensors back to my controller for the first time, and I'm a little stumped...

I have this message defined:

MyMessage msg(CHILD_ID_LIGHT, V_VAR1);

And this code that uses it to send a message to the controller

void open_scratchpad(int expectedScratchpadValues) {
  send(msg.set("Scratchpad is open"));
  send(msg.set("string " + String(expectedScratchpadValues)));
}

I expect to get the following messages back (in this example, expectedScratchpadValues=4):

2;1;1;0;24;Scratchpad is open
2;1;1;0;24;string 4

Instead, I get:

2;1;1;0;24;Scratchpad is open
2;1;1;0;24;0

But if I do

Serial.println("string " + String(expectedScratchpadValues)

then the arduino console correctly prints

string 4

Never mind the exact code here. I'm just playing around to learn the messaging protocol, so it really doesn't mean much of anything. I just want to be able to convert ints to strings, concatenate those strings, and then pass that concatenated result back to my controller.

Thanks for you help!

[virtualmkr]

@mrmuszynski
Hi, there are some setters for message payload types but none for type String, see: Message manipulation.

You need to convert the resulting String into const char* like this:

send(msg.set(("string " + String(expectedScratchpadValues)).c_str()));

But in general I would try to avoid usage of the String data type, especially for the use case in your code example. You could use e.g.:

  char str[80];
  sprintf(str, "string %d", expectedScratchpadValues);
  send(msg.set(str));

For Arduino String read more e.g. here: The Evils of Arduino Strings

HTH