C++ error when using struct variable

you are reffering in the function that array which is not in its scope, you need to do it as

int clearMatrix(int var,noduri *all)
 {
   cout<<all[1].nod[30]; // as here you have the base address of the array of type noduri you can refer it.
 }
 int main()
 {
noduri all[100];
clearMatrix(5, all);
return 0;
}

You are using raw arrays. That's not a good idea. Use std::vector if the size if not known at compile time, consider std::array if it is known at compile time and dynamic resizing would cause measurable performance problems.

One of the problems with raw arrays in C++ is that it's not at all(!) as easy to pass them to a function like, say, an int or a double. std::vector and std::array, in contrast, are as easy to pass to a function like any other normal type.

Here's a complete example:

#include <array>
#include <iostream>

struct noduri {
    std::array<int, 100> nod;
};

void clearMatrix(std::array<noduri, 100> const &array) {
    std::cout << array[1].nod[30];
}

int main() {
    std::array<noduri, 100> all;
    std::cout << all[1].nod[30];
}

Note that std::array is only available if your compiler supports C++11. For an older compiler, use boost::array or just do it with a std::vector.

The code you showed will not be compiled and has no any sense. If I have understood correctly you want to assign each element of the array by some value in function clearMatrix. If so then the code will look the following way

#include <iostream>

struct noduri
{
    int nod[100];
};

int clearMatrix( noduri *matrix, int size, int var )
{
   for ( int i = 0; i < size; i++ )
   {
      for ( int &n : matrix[i].nod ) n = var;
   }
}

int main()
{
    const int N = 100;   
    noduri all[N] = {};

    std::cout << all[1].nod[30] << std::endl;

    clearMatrix( all, N, 10 );

    std::cout << all[1].nod[30] << std::endl;

    return 0;
}