How to multiply a matrix by a vector in PyTorch - pytorch

Question

I'm playing around with PyTorch with the aim of learning it, and I have a very dumb question: how can I multiply a matrix by a single vector?
Here's what I've tried:
>>> import torch
>>> a = torch.rand(4,4)
>>> a
0.3162 0.4434 0.9318 0.8752
0.0129 0.8609 0.6402 0.2396
0.5720 0.7262 0.7443 0.0425
0.4561 0.1725 0.4390 0.8770
[torch.FloatTensor of size 4x4]
>>> b = torch.rand(4)
>>> b
0.1813
0.7090
0.0329
0.7591
[torch.FloatTensor of size 4]
>>> a.mm(b)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: invalid argument 2: dimension 1 out of range of 1D tensor at /Users/soumith/code/builder/wheel/pytorch-src/torch/lib/TH/generic/THTensor.c:24
>>> a.mm(b.t())
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: t() expects a 2D tensor, but self is 1D
>>> b.mm(a)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: matrices expected, got 1D, 2D tensors at /Users/soumith/code/builder/wheel/pytorch-src/torch/lib/TH/generic/THTensorMath.c:1288
>>> b.t().mm(a)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: t() expects a 2D tensor, but self is 1D
On the other hand, if I do
>>> b = torch.rand(4,2)
then my first attempt, a.mm(b), works fine. So the problem is just that I'm multiplying a vector rather than a matrix --- but how can I do this?

Answer 1

You're looking for
torch.mv(a,b)
Note that for the future, you may also find torch.matmul() useful. torch.matmul() infers the dimensionality of your arguments and accordingly performs either dot products between vectors, matrix-vector or vector-matrix multiplication, matrix multiplication or batch matrix multiplication for higher order tensors.

Answer 2

This is a self-answer to supplement #mexmex's correct and useful answer.
In PyTorch, unlike numpy, 1D Tensors are not interchangeable with 1xN or Nx1 tensors. If I replace
>>> b = torch.rand(4)
with
>>> b = torch.rand((4,1))
then I will have a column vector, and matrix multiplication with mm will work as expected.
But this is not necessary, because as #mexmex points out there is an mv function for matrix-vector multiplication, as well as a matmul function that dispatches the appropriate function depending on the dimensions of its input.