How to initialize static member array with a result of a function? - c++

Question

I'm translating such fragment of this Python file to C++:
SIDE = 3
LINES = []
for y in range(SIDE):
row = tuple((x, y) for x in range(SIDE))
LINES.append(row)
for x in range(SIDE):
col = tuple((x, y) for y in range(SIDE))
LINES.append(col)
LINES.append(tuple((x, x) for x in range(SIDE)))
LINES.append(tuple((SIDE - x - 1, x) for x in range(SIDE)))
LINES holds (x, y) coordinates of possible lines in Tic Tac Toe game. So for SIDE = 3 it holds:
[((0, 0), (1, 0), (2, 0)),
((0, 1), (1, 1), (2, 1)),
((0, 2), (1, 2), (2, 2)),
((0, 0), (0, 1), (0, 2)),
((1, 0), (1, 1), (1, 2)),
((2, 0), (2, 1), (2, 2)),
((0, 0), (1, 1), (2, 2)),
((2, 0), (1, 1), (0, 2))]
SIDE value can change.
What I've tried
Performance is crucial (that's why I reached for C++), so I would like to calculate LINES only once. Thus, I've chosen to implement LINES as a static member of the class TicTacToeState.
I started with such code:
static char init_lines() {
return 'a';
}
class TicTacToeState {
static char LINES;
};
char TicTacToeState::LINES = init_lines();
It works. How to change LINES to an array? Maybe vector will be better? With pairs?
Maybe static member is not the best choice, maybe there is an easier way?
How would you translate it to C++?
We know the size of LINES, it's always 2 * SIDE + 2.
Special requirement
All C++ code must be in one .cpp file, no headers. Why? Because this is fragment of a library for bot competitions and it's typical that you can submit only one file.

Answer 1

In C++ you can initialize static array members using group initialization
static int a[10] = {5}; //this will initialize first position item with 5 and rest with 0s
static char b[2] = {'b', 'b'};
static int c[2][2] = { {1,1}, {1,2} };
int main()
{
cout<< a[0] << endl; //output: 5
cout<< a[1] << endl; //output: 0
cout<< b[0] << endl; //output: b
cout<< c[0][1] << endl; //output: 1
}
Although the fact is you need to know size of the array not like in Python's list that are dynamically
If you need to insert to the table values calculated dynamically the best way to do this is to create factory method
static int** fact(int width, int height)
{
int** a;
a = new int*[width]; //we can do it when it is DYNAMIC array!
a[0] = new int[height];
a[1] = new int[height];
for(int i = 0; i < width; i++)
for(int k = 0; k < height; k++)
a[i][k] = i*k;
return a;
}
static int** c = fact(2, 2); //you can call it with your SIDE var
int main()
{
cout<< c[1][1] << endl; //output: 1
}
Of course you can process it in loops
The same approach will be proper when you will decide to use std Vector class which is equvalent of Python's dynamic list

Answer 2

I suppose you could do this using a lambda function like this:
#include <vector>
#include <iostream>
const auto SIDE = 3U;
struct coord
{
unsigned x;
unsigned y;
coord(unsigned x, unsigned y): x(x), y(y) {}
};
static const auto lines = [] // lambda function
{
// returned data structure
std::vector<std::vector<coord>> lines;
for(auto y = 0U; y < SIDE; ++y)
{
lines.emplace_back(); // add a new line to back()
for(auto x = 0U; x < SIDE; ++x)
lines.back().emplace_back(x, y); // add a new coord to that line
}
for(auto x = 0U; x < SIDE; ++x)
{
lines.emplace_back();
for(auto y = 0U; y < SIDE; ++y)
lines.back().emplace_back(x, y);
}
lines.emplace_back();
for(auto i = 0U; i < SIDE; ++i)
lines.back().emplace_back(i, i);
lines.emplace_back();
for(auto i = 0U; i < SIDE; ++i)
lines.back().emplace_back(SIDE - i - 1, i);
return lines;
}(); // NOTE: () is important to run the lambda function
int main()
{
for(auto const& line: lines)
{
std::cout << "(";
for(auto const& coord: line)
std::cout << "(" << coord.x << ", " << coord.y << ")";
std::cout << ")\n";
}
}
Output:
((0, 0)(1, 0)(2, 0))
((0, 1)(1, 1)(2, 1))
((0, 2)(1, 2)(2, 2))
((0, 0)(0, 1)(0, 2))
((1, 0)(1, 1)(1, 2))
((2, 0)(2, 1)(2, 2))
((0, 0)(1, 1)(2, 2))
((2, 0)(1, 1)(0, 2))