Declaring and using array of structures in Arduino

Question

Printing structure variables, I get default values. What is wrong with my code?

struct SCENARIO
{
  int Lamp_Pin = -1;
  int PB_Pin = -1;
} ;

SCENARIO  _red;
SCENARIO  _yellow;
SCENARIO  _white;
SCENARIO  _stop;
SCENARIO btns[4] = { _red,  _yellow, _white, _stop};


void setup()
{ 
  _stop.Lamp_Pin = 8;
  _stop.PB_Pin = 4;

 ///// initializing other 3 objects i.e. red & white & yellow
 //// 
for (int i = 0; i < 4; i++)
  {
    Serial.print(i);Serial.print(":");
    Serial.print(btns[i].Lamp_Pin);Serial.print(":");
    Serial.println( btns[i].PB_Pin );
    //pinMode(  btns[i].Lamp_Pin, OUTPUT);
    //pinMode(  btns[i].PB_Pin, INPUT);
  }
}

The image below shows what I get. It seems I have not initialized the variables!

Answer 1

In this line:

SCENARIO btns[4] = { _red,  _yellow, _white, _stop};

you are making 4 copies of the structures and placing them in an array.

Instead you should either create the array directly with the values:

SCENARIO btns[4] = {
    {-1, -1},
    {-1, -1},
    {-1, -1},
    {8, 4}
};

And only use the array, or use pointers in the array:

SCENARIO *btns[4] = { &_red,  &_yellow, &_white, &_stop };

and then access the entries as pointers:

Serial.print(btns[i]->Lamp_Pin);
Serial.print(":");
Serial.println( btns[i]->PB_Pin );

Answer 2

The problem is that a struct is handled as a value type, not as a reference type.

This means that a copy of _stop is made and put in btns (btns[3] actually).

So what you need to do is putting pointers into the btns array:

SCENARIO* btns[4] = { &_red, &_yellow, &_white, &_stop};

The symbol * denotes a pointer, and & means the address of a variable (so it points to the variable and is not a copy).

Now when you change _stop, also btns[3] which points to _stop will show the correct initialized value.

For printing you should use:

Serial.println( (*btns[i]).LampPin);Serial.print(":");
Serial.println( (*btns[i]).PB_Pin );

Because *btns[i] is a pointer, but because this is a very often used feature, a special notation can be used:

Serial.print(btns[i]->Lamp_Pin);Serial.print(":");
Serial.println( btns[i]->PB_Pin);