# Interacting with external arrays

Although Taichi fields are mainly used in Taichi-scope, in some cases efficiently manipulating Taichi field data in Python-scope could also be helpful.

We provide various interfaces to copy the data between Taichi fields and external arrays. The most typical case maybe copying between Tachi fields and Numpy arrays. Let's take a look at two examples below.

Export data in Taichi fields to a NumPy array via `to_numpy()`. This allows us to export computation results to other Python packages that support NumPy, e.g. `matplotlib`.

``````@ti.kerneldef my_kernel():   for i in x:      x[i] = i * 2
x = ti.field(ti.f32, 4)my_kernel()x_np = x.to_numpy()print(x_np)  # np.array([0, 2, 4, 6])``````

Import data from NumPy array to Taichi fields via `from_numpy()`. This allows people to initialize Taichi fields via NumPy arrays. E.g.,

``x = ti.field(ti.f32, 4)x_np = np.array([1, 7, 3, 5])x.from_numpy(x_np)print(x)  # 1print(x)  # 7print(x)  # 3print(x)  # 5``

## API reference#

We provide interfaces to copy data between Taichi field and external arrays. External arrays refers to NumPy arrays or PyTorch tensors.

For details, check Field in API references

## External array shapes#

Shapes of Taichi fields (see Scalar fields) and those of corresponding NumPy arrays are closely connected via the following rules:

• For scalar fields, the shape of NumPy array is exactly the same as the Taichi field:
``````field = ti.field(ti.i32, shape=(233, 666))field.shape  # (233, 666)
array = field.to_numpy()array.shape  # (233, 666)
field.from_numpy(array)  # the input array must be of shape (233, 666)``````
• For vector fields, if the vector is `n`-D, then the shape of NumPy array should be `(*field_shape, vector_n)`:
``````field = ti.Vector.field(3, ti.i32, shape=(233, 666))field.shape  # (233, 666)field.n      # 3
array = field.to_numpy()array.shape  # (233, 666, 3)
field.from_numpy(array)  # the input array must be of shape (233, 666, 3)``````
• For matrix fields, if the matrix is `n*m`, then the shape of NumPy array should be `(*field_shape, matrix_n, matrix_m)`:
``````field = ti.Matrix.field(3, 4, ti.i32, shape=(233, 666))field.shape  # (233, 666)field.n      # 3field.m      # 4
array = field.to_numpy()array.shape  # (233, 666, 3, 4)
field.from_numpy(array)  # the input array must be of shape (233, 666, 3, 4)``````

## Using external arrays as Taichi kernel arguments#

Use the type hint `ti.ext_arr()` for passing external arrays as kernel arguments. For example:

``````import taichi as tiimport numpy as np
ti.init()
n = 4m = 7
val = ti.field(ti.i32, shape=(n, m))
@ti.kerneldef test_numpy(arr: ti.ext_arr()):  for i in range(n):    for j in range(m):      arr[i, j] += i + j
a = np.empty(shape=(n, m), dtype=np.int32)
for i in range(n):  for j in range(m):    a[i, j] = i * j
test_numpy(a)
for i in range(n):  for j in range(m):    assert a[i, j] == i * j + i + j``````
##### note

Struct-for's are not supported on external arrays.