function returning String-Array

Question

i'm still new to arduino and also to C++ and have a short question. What i want to do is to write a function (in my case to parse the query-string of an URL) to get the parameter names and values (varying number of parameters).

My function looks like the following:

String function readParams()
{
   String params[x]; // x: number of params

   // fill params[0]... params[x] with parameter names

   return params;
}


void loop()
{
   // do some stuff

   String params[x] = readParams();

   for(int i=0; i < x; i++)
   {
      debug(params[i]);
   }
}

What is the correct return-type for the readParams() function in this case? And how to access the params in the main (loop) -function?

I hope the code makes it clear what i want to do.

Thanks, Marius

Answer 1

Some tips:

1. Don't use String.
2. Use char * and slice the string in-place.
3. Pass a pointer to an array of pointers and fill that with pointers to the portions of the string you have sliced.
4. Return the number of entries in the array you used.

I do something similar in my CLI library. I use a custom "getWord" function, but similar can be done with strtok().

You can read more about working with C strings here.

Answer 2

I'm inclined to agree with Majenko that you shouldn't use String in the first place, but if you want to, you can pass by reference:

const int numParams = 2;

void readParams(String (& params) [numParams])
  {
  params [0] = "foo";
  params [1] = "bar";
  }

void setup() 
{
  Serial.begin (115200);
  String params [numParams];
  readParams (params);
  for (int i = 0; i < numParams; i++)
    Serial.println (params [i]);
}

void loop() { }

An easier thing is to make the parameters a global variable.