Check array position for null/empty

Question

I have an array which might contain empty/null positions (e.g: array[2]=3, array[4]=empty/unassigned). I want to check in a loop whether the array position is null.

array[4]==NULL //this doesn't work

I'm pretty new to C++.
Thanks.

Edit: Here's more code; A header file contains the following declaration

int y[50];

The population of the array is done in another class,

geoGraph.y[x] = nums[x];

The array should be checked for null in the following code;

    int x=0;
    for(int i=0; i<sizeof(y);i++){
        //check for null
        p[i].SetPoint(Recto.Height()-x,y[i]);
        if(i>0){
            dc.MoveTo(p[i-1]);
            dc.LineTo(p[i]);

        }
        x+=50;
    }

Answer 1

If your array is not initialized then it contains randoms values and cannot be checked !

To initialize your array with 0 values:

int array[5] = {0};

Then you can check if the value is 0:

array[4] == 0;

When you compare to NULL, it compares to 0 as the NULL is defined as integer value 0 or 0L.

If you have an array of pointers, better use the nullptr value to check:

char* array[5] = {nullptr}; // we defined an array of char*, initialized to nullptr

if (array[4] == nullptr)
    // do something

Answer 2

You can use boost::optional (or std::optional since C++17), which was developed in particular for decision of your problem:

boost::optional<int> y[50];
....
geoGraph.y[x] = nums[x];
....
const size_t size_y = sizeof(y)/sizeof(y[0]); //!!!! correct size of y!!!!
for(int i=0; i<size_y;i++){
   if(y[i]) { //check for null
      p[i].SetPoint(Recto.Height()-x,*y[i]);
      ....
   }
}

P.S. Do not use C-type array -> use std::array or std::vector.

std::array<int, 50> y;   //not int y[50] !!!