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odomobo wrote: ↑Tue Apr 30, 2019 9:01 pm Actually, something I just realized is that c++ doesn't like the notation for pointers to normal arrays, so std::array will help us out here. Maybe there's a way to do it, but if so it's beyond me.

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// These are vectors for the bishop. Ex: mvsNW[A1][0] is the number of squares. [A1][1-7] are the to squares.
// Note that we have to declare the dimensions in reverse order, since we're explicitly nesting the types
std::array<std::array<u08, 8>, 64> mvsNW;
std::array<std::array<u08, 8>, 64> mvsNE;
std::array<std::array<u08, 8>, 64> mvsSE;
std::array<std::array<u08, 8>, 64> mvsSW;

// This is an array of pointers to the above arrays.
// Note that this is storing the exact same pointer as befores, but now it's preserving type information.
// We could have mvsB itself be a std::array, but it's not necessary.
std::array<std::array<u08, 8>, 64> * mvsB[] = { &mvsNW, &mvsNE, &mvsSW, &mvsSE };

// In this code I declare unsigned char *ptr and in line 4 it is assigned each array address by a for loop
// Then It is used with pointer arithmetic. But, I thought it would be possible to use it as an array name as in ptr[fs][j]
void WBf(threadS *t, u08 id, u08 depth, moveS *m) {
  unsigned char *ptr = &t->inv[id][t->top[id]];  m->fs = t->sqr[id];
  if (*ptr) WSBBG(t, id, m->fs, ptr, &t->stk[id][t->top[id]]);
  for (u08 i = 0; i < 4; i++) { 
    // Why are we dereferencing, and then holding a reference to the underlying array object? Why not leave it as a pointer?
    // Because otherwise we'd need to explicitly deference like this:
    // (*mvs)[m->fs][j]
    // BTW, if we just dereferenced, but didn't hold a reference, semantically we're asking the compiler to copy the contents of the array onto the stack.
    auto & mvs = *mvsB[i];
    for (u08 j = 1; j <= mvs[m->fs][0]; j++) {
      m->ts = mvs[m->fs][j];
      m->id = t->brd[m->ts];
      m->typ = (m->id) ? WBC : WBM;
      MakeMove(t, m);
      if (depth > 0) Perft(t, depth - 1);
      TakeBack(t, m);
    }
  }
}

Let me know if you have any questions

odomobo wrote: ↑Tue Apr 30, 2019 11:11 pm I played around with it, and figured out how to do it with a natural array. However, I personally find the notation more confusing.

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u08 mvsNW[64][8];
//...

u08 (*mvsB[])[64][8] = { &mvsNW, &mvsNE, &mvsSW, &mvsSE };

// ...

auto & mvs = *mvsB[i];
m->ts = mvs[m->fs][j];

I also found another way to do this, since pointers in c/c++ can be indexed like arrays, and since the size of the [64] dimension of mvsNW isn't necessary to perform a lookup into the table. I also find this to be somewhat confusing.

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u08 mvsNW[64][8];
//...

u08 (*mvsB[])[8] = { &mvsNW[0], &mvsNE[0], &mvsSW[0], &mvsSE[0] };

// ...

u08 (*mvs)[8] = mvsB[i]; // can instead use auto; I am just showing the type here, which is a pointer to u08[8]
m->ts = mvs[m->fs][j];

Finally, you could do the last one using std::array instead of native arrays. I trust you can figure out how to do that.

Ultimately, it shouldn't matter which way you do it, as long as you don't accidentally copy a bunch of data to the stack.

Michael Sherwin wrote: ↑Tue Apr 30, 2019 11:22 pm

odomobo wrote: ↑Tue Apr 30, 2019 11:11 pm I played around with it, and figured out how to do it with a natural array. However, I personally find the notation more confusing.

Code: Select all

u08 mvsNW[64][8];
//...

u08 (*mvsB[])[64][8] = { &mvsNW, &mvsNE, &mvsSW, &mvsSE };

// ...

auto & mvs = *mvsB[i];
m->ts = mvs[m->fs][j];

I also found another way to do this, since pointers in c/c++ can be indexed like arrays, and since the size of the [64] dimension of mvsNW isn't necessary to perform a lookup into the table. I also find this to be somewhat confusing.

Code: Select all

u08 mvsNW[64][8];
//...

u08 (*mvsB[])[8] = { &mvsNW[0], &mvsNE[0], &mvsSW[0], &mvsSE[0] };

// ...

u08 (*mvs)[8] = mvsB[i]; // can instead use auto; I am just showing the type here, which is a pointer to u08[8]
m->ts = mvs[m->fs][j];

Finally, you could do the last one using std::array instead of native arrays. I trust you can figure out how to do that.

Ultimately, it shouldn't matter which way you do it, as long as you don't accidentally copy a bunch of data to the stack.

Thanks!

lucasart wrote: ↑Wed May 01, 2019 7:10 am

Michael Sherwin wrote: ↑Tue Apr 30, 2019 11:22 pm

odomobo wrote: ↑Tue Apr 30, 2019 11:11 pm I played around with it, and figured out how to do it with a natural array. However, I personally find the notation more confusing.

Code: Select all

u08 mvsNW[64][8];
//...

u08 (*mvsB[])[64][8] = { &mvsNW, &mvsNE, &mvsSW, &mvsSE };

// ...

auto & mvs = *mvsB[i];
m->ts = mvs[m->fs][j];

I also found another way to do this, since pointers in c/c++ can be indexed like arrays, and since the size of the [64] dimension of mvsNW isn't necessary to perform a lookup into the table. I also find this to be somewhat confusing.

Code: Select all

u08 mvsNW[64][8];
//...

u08 (*mvsB[])[8] = { &mvsNW[0], &mvsNE[0], &mvsSW[0], &mvsSE[0] };

// ...

u08 (*mvs)[8] = mvsB[i]; // can instead use auto; I am just showing the type here, which is a pointer to u08[8]
m->ts = mvs[m->fs][j];

Finally, you could do the last one using std::array instead of native arrays. I trust you can figure out how to do that.

Ultimately, it shouldn't matter which way you do it, as long as you don't accidentally copy a bunch of data to the stack.

Thanks!

Instead of looking for complicated solutions to an artificial problem, you should rethink your code, in such a way that a normal C array can be used (C arrays can be multidimensional).

odomobo wrote: ↑Wed May 01, 2019 3:03 pm lucasart is right. Instead of an array of a pointer of 2d arrays, you're better off having a single 3d array. There are techniques you can use such that this will strictly be more efficient, never less efficient.

Code: Select all

u08 mvsB[][64][8];

// references to sub-arrays
u08 (&mvsNW)[64][8] = mvsB[0];
u08 (&mvsNE)[64][8] = mvsB[1];
// ...

void WBf(threadS *t, u08 id, u08 depth, moveS *m) {
  unsigned char *ptr = &t->inv[id][t->top[id]];  m->fs = t->sqr[id];
  if (*ptr) WSBBG(t, id, m->fs, ptr, &t->stk[id][t->top[id]]);
  for (u08 i = 0; i < 4; i++) { 
    auto & mvs = mvsB[i];
    for (u08 j = 1; j <= mvs[m->fs][0]; j++) {
      m->ts = mvs[m->fs][j];
      m->id = t->brd[m->ts];
      m->typ = (m->id) ? WBC : WBM;
      MakeMove(t, m);
      if (depth > 0) Perft(t, depth - 1);
      TakeBack(t, m);
    }
  }
}

When you take a reference to a sub-array like this, the machine code is really storing a pointer to the first index in that sub-array.

Michael Sherwin wrote: ↑Sat Apr 20, 2019 8:58 pm I want to use a pointer to a two dimensional array like it was an array name. This code works and I am okay with that but it uses pure pointer arithmetic, however, for readability I want to use the pointer like array notation instead.

class MyData {
public:
  int a, b, c;
  int64_t d, e;
  char *array[20][40];
};

void MyClass::myfunction(MyData& data) 
{
   auto ch = data.array[2][7];
   ...
}