Tensor
Tensor is a multilinear function that can be used to represent linear relationships between vectors, scalars, and other tensors. The basic examples of these linear relations are the inner product, the outer product, the linear map, and the Cartesian product. In the \(n\) dimensional space, its coordinates have \(n^{r}\) components. Each component is a function of coordinates, and these components are also linearly transformed according to certain rules when the coordinates are transformed. \(r\) is called the rank or order of this tensor (not related to the rank or order of the matrix).
A tensor is a special data structure that is similar to arrays and matrices. Tensor is the basic data structure in MindSpore network operations. This chapter describes the attributes and usage of tensors and sparse tensors.
Creating a Tensor
There are multiple methods for creating tensors. When building a tensor, you can pass the Tensor, float, int, bool, tuple, list, and numpy.ndarray types.
Generating a tensor based on data
You can create a tensor based on data. The data type can be set or automatically inferred by the framework.
import mindspore as ms
x = ms.Tensor(0.1)
Generating a tensor from the NumPy array
You can create a tensor from the NumPy array.
import numpy as np
arr = np.array([1, 0, 1, 0])
tensor_arr = ms.Tensor(arr)
print(type(arr))
print(type(tensor_arr))
<class 'numpy.ndarray'>
<class 'mindspore.common.tensor.Tensor'>
If the initial value is numpy.ndarray, the generated Tensor data type corresponds to numpy.ndarray.
Generating a tensor by using init
When init is used to initialize a tensor, the init, shape, and dtype parameters can be transferred.
init: supports the subclass of initializer.shape: supportslist,tuple, andint.dtype: supports mindspore.dtype.
import mindspore as ms
from mindspore.common.initializer import One, Normal
ms.set_seed(1)
tensor1 = ms.Tensor(shape=(2, 2), dtype=ms.float32, init=One())
# Generates an array with values sampled from Normal distribution N(0.01,0.1) in order to initialize a tensor
tensor2 = ms.Tensor(shape=(2, 2), dtype=ms.float32, init=Normal())
print("tensor1:\n", tensor1)
print("tensor2:\n", tensor2)
tensor1:
[[1. 1.]
[1. 1.]]
tensor2:
[[-0.00128023 -0.01392901]
[ 0.0130886 -0.00107818]]
The init is used for delayed initialization in parallel mode. Usually, it is not recommended to use init interface to initialize parameters.
Inheriting attributes of another tensor to form a new tensor
from mindspore import ops
oneslike = ops.OnesLike()
x = ms.Tensor(np.array([[0, 1], [2, 1]]).astype(np.int32))
output = oneslike(x)
print(output)
print("input shape:", x.shape)
print("output shape:", output.shape)
[[1 1]
[1 1]]
input shape: (2, 2)
output shape: (2, 2)
Outputting a constant tensor of a specified size
shape is the size tuple of a tensor, which determines the dimension of the output tensor.
import mindspore as ms
zeros = ops.Zeros()
output = zeros((2, 2), ms.float32)
print(output)
[[0. 0.]
[0. 0.]]
During Tensor initialization, dtype can be specified to, for example, mstype.int32, mstype.float32 or mstype.bool_.
Tensor Attributes
Tensor attributes include shape, data type, transposed tensor, item size, number of bytes occupied, dimension, size of elements, and stride per dimension.
shape: a tuple.
dtype: a data type of MindSpore.
T: a transpose of a
Tensor.itemsize: the number of bytes occupied by each element in
Tensor, which is an integer.nbytes: the total number of bytes occupied by
Tensor, which is an integer.ndim: rank of
Tensor, that is, len(tensor.shape), which is an integer.size: the number of all elements in
Tensor, which is an integer.strides: the number of bytes to traverse in each dimension of
Tensor, which is a tuple.
x = ms.Tensor(np.array([[1, 2], [3, 4]]), ms.int32)
print("x_shape:", x.shape)
print("x_dtype:", x.dtype)
print("x_transposed:\n", x.T)
print("x_itemsize:", x.itemsize)
print("x_nbytes:", x.nbytes)
print("x_ndim:", x.ndim)
print("x_size:", x.size)
print("x_strides:", x.strides)
x_shape: (2, 2)
x_dtype: Int32
x_transposed:
[[1 3]
[2 4]]
x_itemsize: 4
x_nbytes: 16
x_ndim: 2
x_size: 4
x_strides: (8, 4)
Tensor Indexing
Tensor indexing is similar to NumPy indexing Indexing starts from 0, negative indexing means indexing in reverse order, and colons : and ... are used for slicing.
tensor = ms.Tensor(np.array([[0, 1], [2, 3]]).astype(np.float32))
print("First row: {}".format(tensor[0]))
print("value of bottom right corner: {}".format(tensor[1, 1]))
print("Last column: {}".format(tensor[:, -1]))
print("First column: {}".format(tensor[..., 0]))
First row: [0. 1.]
value of bottom right corner: 3.0
Last column: [1. 3.]
First column: [0. 2.]
Tensor Operation
There are many operations between tensors, including arithmetic, linear algebra, matrix processing (transposing, indexing, and slicing), and sampling. The usage of tensor computation is similar to that of NumPy. The following describes several operations.
Common arithmetic operations include: addition (+), subtraction (-), multiplication (*), division (/), modulo (%), and exact division (//).
x = ms.Tensor(np.array([1, 2, 3]), ms.float32)
y = ms.Tensor(np.array([4, 5, 6]), ms.float32)
output_add = x + y
output_sub = x - y
output_mul = x * y
output_div = y / x
output_mod = y % x
output_floordiv = y // x
print("add:", output_add)
print("sub:", output_sub)
print("mul:", output_mul)
print("div:", output_div)
print("mod:", output_mod)
print("floordiv:", output_floordiv)
add: [5. 7. 9.]
sub: [-3. -3. -3.]
mul: [ 4. 10. 18.]
div: [4. 2.5 2. ]
mod: [0. 1. 0.]
floordiv: [4. 2. 2.]
Concat connects a series of tensors in a given dimension.
data1 = ms.Tensor(np.array([[0, 1], [2, 3]]).astype(np.float32))
data2 = ms.Tensor(np.array([[4, 5], [6, 7]]).astype(np.float32))
op = ops.Concat()
output = op((data1, data2))
print(output)
print("shape:\n", output.shape)
[[0. 1.]
[2. 3.]
[4. 5.]
[6. 7.]]
shape:
(4, 2)
Stack combines two tensors from another dimension.
data1 = ms.Tensor(np.array([[0, 1], [2, 3]]).astype(np.float32))
data2 = ms.Tensor(np.array([[4, 5], [6, 7]]).astype(np.float32))
op = ops.Stack()
output = op([data1, data2])
print(output)
print("shape:\n", output.shape)
[[[0. 1.]
[2. 3.]]
[[4. 5.]
[6. 7.]]]
shape:
(2, 2, 2)
Conversion Between Tensor and NumPy
Tensor and NumPy can be converted to each other.
Tensor to NumPy
Use asnumpy() to convert Tensor to NumPy.
zeros = ops.Zeros()
output = zeros((2, 2), ms.float32)
print("output: {}".format(type(output)))
n_output = output.asnumpy()
print("n_output: {}".format(type(n_output)))
output: <class 'mindspore.common.tensor.Tensor'>
n_output: <class 'numpy.ndarray'>
NumPy to Tensor
Use asnumpy() to convert NumPy to Tensor.
output = np.array([1, 0, 1, 0])
print("output: {}".format(type(output)))
t_output = ms.Tensor(output)
print("t_output: {}".format(type(t_output)))
output: <class 'numpy.ndarray'>
t_output: <class 'mindspore.common.tensor.Tensor'>
Sparse Tensor
A sparse tensor is a special tensor in which the value of the most significant element is zero.
In some scenario s(such as recommendation systems, molecular dynamics, graph neural networks), the data is sparse. If you use common dense tensors to represent the data, you may introduce many unnecessary calculations, storage, and communication costs. In this case, it is better to use sparse tensor to represent the data.
MindSpore now supports the two most commonly used CSR and COO sparse data formats.
The common structure of the sparse tensor is <indices:Tensor, values:Tensor, shape:Tensor>. indices means the indexes of non-zero elements, values means the values of non-zero elements, and shape means the dense shape of the sparse tensor. In this structure, we define data structure CSRTensor, COOTensor, and RowTensor.
CSRTensor
The compressed sparse row (CSR) is efficient in both storage and computation. All the non-zero values are stored in values, and their positions are stored in indptr(row) and indices (column).
indptr: 1-D integer tensor, indicating the start and end points of the non-zero elements in each row of the sparse data invalues. The index data type can only be int32.indices: 1-D integer tensor, indicating the position of the sparse tensor non-zero elements in the column and has the same length asvalues. The index data type can only be int32.values: 1-D tensor, indicating that the value of the non-zero element corresponding to theCSRTensorand has the same length asindices.shape: indicates the shape of a compressed sparse tensor. The data type isTuple. Currently, only 2-DCSRTensoris supported.
For details about
CSRTensor, see mindspore.CSRTensor.
The following are some examples of using the CSRTensor:
import mindspore as ms
indptr = ms.Tensor([0, 1, 2])
indices = ms.Tensor([0, 1])
values = ms.Tensor([1, 2], dtype=ms.float32)
shape = (2, 4)
# CSRTensor construction
csr_tensor = ms.CSRTensor(indptr, indices, values, shape)
print(csr_tensor.astype(ms.float64).dtype)
Float64
The above code generates a CSRTensor as shown in the following equation:
COOTensor
The COO (Coordinate Format) sparse tensor format is used to represent a collection of nonzero elements of a tensor on a given index. If the number of non-zero elements is N and the dimension of the compressed tensor is ndims, then:
indices: 2-D integer tensor. Each row indicates a non-zero element subscript. Shape:[N, ndims]. The index data type can only be int32.values: 1-D tensor of any type, indicating the value of the non-zero element. Shape:[N].shape: indicates the shape of a compressed sparse tensor. Currently, only 2-DCOOTensoris supported.
For details about
COOTensor, see mindspore.COOTensor.
The following are some examples of using COOTensor:
import mindspore as ms
import mindspore.nn as nn
indices = ms.Tensor([[0, 1], [1, 2]], dtype=ms.int32)
values = ms.Tensor([1, 2], dtype=ms.float32)
shape = (3, 4)
# COOTensor construction
coo_tensor = ms.COOTensor(indices, values, shape)
print(coo_tensor.values)
print(coo_tensor.indices)
print(coo_tensor.shape)
print(coo_tensor.astype(ms.float64).dtype) # COOTensor cast to another data type
[1. 2.]
[[0 1]
[1 2]]
(3, 4)
Float64
The preceding code generates COOTensor as follows:
RowTensor
RowTensor is used to compress tensors that are sparse in the dimension 0. If the dimension of RowTensor is [L0, D1, D2, ..., DN ] and the number of non-zero elements in the dimension 0 is D0, then L0 >> D0.
indices: 1-D integer tensor, indicating the position of non-zero elements in the dimension 0 of the sparse tensor. The shape is[D0].values: indicating the value of the corresponding non-zero element. The shape is[D0, D1, D2, ..., DN].dense_shape: indicates the shape of a compressed sparse tensor.
For the detailed documentation of
RowTensor, see the code example in mindspore.RowTensor. A code example is as follows:
import mindspore as ms
import mindspore.nn as nn
class Net(nn.Cell):
def __init__(self, dense_shape):
super(Net, self).__init__()
self.dense_shape = dense_shape
def construct(self, indices, values):
x = ms.RowTensor(indices, values, self.dense_shape)
return x.values, x.indices, x.dense_shape
indices = ms.Tensor([0])
values = ms.Tensor([[1, 2]], dtype=ms.float32)
out = Net((3, 2))(indices, values)
print("non-zero values:", out[0])
print("non-zero indices:", out[1])
print("shape:", out[2])
non-zero values: [[1. 2.]]
non-zero indices: [0]
shape: (3, 2)
The preceding code generates RowTensor as shown in the following equation: