Feature request: matrix slicing

[xuhao1]

Hi there, my program needs to create an array to store some data, it looks like

#Marching...
...
    vertlist = [[0, 0, 0] for i in range(12)]
    for _i in ti.static(range(12)):
        if edges & 2**_i:
            _p0 = ti.static(edges_grid_xyz)[_i][0]
            _p1 = ti.static(edges_grid_xyz)[_i][1]
            p0 = [i + _p0[0]*step, j + _p0[1]*step, k + _p0[2]*step]
            p1 = [i + _p1[0]*step, j + _p1[1]*step, k + _p1[2]*step]
            p = self.vertexInterp(p0, p1, tsdf[p0], tsdf[p1], isolevel)
            vertlist[_i][0] = p[0]
            vertlist[_i][1] = p[1]
            vertlist[_i][2] = p[2]
    
    _i = 0
    while triTable[cubeindex, _i] != -1:
        self.add_triangle(vertlist[triTable[cubeindex, _i]], 
            vertlist[triTable[cubeindex, _i + 1]], vertlist[triTable[cubeindex, _i +2]], triangle_idx)
        triangle_idx += 1
        _i += 3

If a use a normal list here for vertlist, it will raise:

TypeError: list indices must be integers or slices, not Expr

If I change vertlist to

                    vertlist = ti.Matrix([[0, 0, 0] for i in range(12)])

It will raise

                        self.add_triangle(vertlist[triTable[cubeindex, _i], 0],
                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The 0-th index of a Matrix/Vector must be a compile-time constant integer, got <class 'taichi.lang.expr.Expr'>.
This is because matrix operations will be **unrolled** at compile-time for performance reason.
If you want to *iterate through matrix elements*, use a static range:
  for i in ti.static(range(3)):
    print(i, "-th component is", vec[i])
See https://docs.taichi.graphics/lang/articles/advanced/meta#when-to-use-for-loops-with-tistatic for more details.

I hope to parallelize this piece of code so the vertlist should not be a global accessible field, so how should I make this code works?

[strongoier]

Could you try ti.init(dynamic_index=True) and then use vertlist = ti.Matrix([[0, 0, 0] for i in range(12)])? Allowing variable indices of vectors/matrices is still an experimental feature and not officially released yet. You may observe performance regression for now, but it should work properly. We are actively working on it, so feedback is highly welcome!

[xuhao1]

@strongoier Thanks for your help! By enabling the dynamics index, the error disappears. But another problem arises.
vertlist[i] does give an element of the matrix, but not a column of the matrix, how can I pass a row of the matrix to some function like

vertlist[i,:]

[strongoier]

Oops, matrix slicing is not supported yet. Could you use a helper function or expression like ti.Vector([vertlist[i, 0], vertlist[i, 1], vertlist[i, 2]]) for now? We will add this feature soon.

[xuhao1]

@strongoier Thanks! I am writing in ti.Vector([vertlist[i, 0], vertlist[i, 1], vertlist[i, 2]]) style now. Looking forward for this feature.