# mars.tensor.matmul¶

`mars.tensor.``matmul`(a, b, sparse=None, out=None, **kw)[source]

Matrix product of two tensors.

The behavior depends on the arguments in the following way.

• If both arguments are 2-D they are multiplied like conventional matrices.

• If either argument is N-D, N > 2, it is treated as a stack of matrices residing in the last two indexes and broadcast accordingly.

• If the first argument is 1-D, it is promoted to a matrix by prepending a 1 to its dimensions. After matrix multiplication the prepended 1 is removed.

• If the second argument is 1-D, it is promoted to a matrix by appending a 1 to its dimensions. After matrix multiplication the appended 1 is removed.

Multiplication by a scalar is not allowed, use `*` instead. Note that multiplying a stack of matrices with a vector will result in a stack of vectors, but matmul will not recognize it as such.

`matmul` differs from `dot` in two important ways.

• Multiplication by scalars is not allowed.

• Stacks of matrices are broadcast together as if the matrices were elements.

Parameters
• a (array_like) – First argument.

• b (array_like) – Second argument.

• out (Tensor, optional) – Output argument. This must have the exact kind that would be returned if it was not used. In particular, it must have the right type, and its dtype must be the dtype that would be returned for dot(a,b). This is a performance feature. Therefore, if these conditions are not met, an exception is raised, instead of attempting to be flexible.

Returns

output – Returns the dot product of a and b. If a and b are both 1-D arrays then a scalar is returned; otherwise an array is returned. If out is given, then it is returned.

Return type

Tensor

Raises

ValueError – If the last dimension of a is not the same size as the second-to-last dimension of b. If scalar value is passed.

`vdot`

Complex-conjugating dot product.

`tensordot`

Sum products over arbitrary axes.

`dot`

alternative matrix product with different broadcasting rules.

Notes

The matmul function implements the semantics of the @ operator introduced in Python 3.5 following PEP465.

Examples

For 2-D arrays it is the matrix product:

```>>> import mars.tensor as mt
```
```>>> a = [[1, 0], [0, 1]]
>>> b = [[4, 1], [2, 2]]
>>> mt.matmul(a, b).execute()
array([[4, 1],
[2, 2]])
```

For 2-D mixed with 1-D, the result is the usual.

```>>> a = [[1, 0], [0, 1]]
>>> b = [1, 2]
>>> mt.matmul(a, b).execute()
array([1, 2])
>>> mt.matmul(b, a).execute()
array([1, 2])
```

Broadcasting is conventional for stacks of arrays

```>>> a = mt.arange(2*2*4).reshape((2,2,4))
>>> b = mt.arange(2*2*4).reshape((2,4,2))
>>> mt.matmul(a,b).shape
(2, 2, 2)
>>> mt.matmul(a,b)[0,1,1].execute()
98
>>> mt.sum(a[0,1,:] * b[0,:,1]).execute()
98
```

Vector, vector returns the scalar inner product, but neither argument is complex-conjugated:

```>>> mt.matmul([2j, 3j], [2j, 3j]).execute()
(-13+0j)
```

Scalar multiplication raises an error.

```>>> mt.matmul([1,2], 3)
Traceback (most recent call last):
...
ValueError: Scalar operands are not allowed, use '*' instead
```