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.

• Ciasto piekarz: what if number of parameters is not known in the beginning ? how would you initialize numPaams then ?
• Nick Gammon: Typically you would use new and make an array of whatever it is you want to make multiple copies of. Or you could consider the STL (Standard Template Library) which supports things like vectors.