# Convert python numpy array to double

122 visualizaciones (últimos 30 días)
Peter el 4 de Oct. de 2014
Comentada: winkmal el 17 de Mayo de 2022
The plot call below will throw an error because x is not a matlab type. How do you convert a python numpy array to a regular matlab matrix?
x = py.numpy.random.random([4,4]);
plot(x)
Sincerely, Peter
##### 3 comentariosMostrar 1 comentario más antiguoOcultar 1 comentario más antiguo
Peter el 4 de Oct. de 2014
Geoff, yes I try to.
Peter el 4 de Oct. de 2014
>> x
x =
Python ndarray with properties:
T: [1x1 py.numpy.ndarray]
base: [1x1 py.NoneType]
ctypes: [1x1 py.numpy.core._internal._ctypes]
data: [1x1 py.buffer]
dtype: [1x1 py.numpy.dtype]
flags: [1x1 py.numpy.flagsobj]
flat: [1x1 py.numpy.flatiter]
imag: [1x1 py.numpy.ndarray]
itemsize: 8
nbytes: 128
ndim: 2
real: [1x1 py.numpy.ndarray]
shape: [1x1 py.tuple]
size: 16
strides: [1x1 py.tuple]
[[ 0.84501423 0.72129285 0.53197632 0.56672641]
[ 0.353229 0.40170347 0.99848745 0.74374568]
[ 0.15023563 0.72432396 0.05446134 0.00455388]
[ 0.85040029 0.65360657 0.35074251 0.08965406]]
>>

Iniciar sesión para comentar.

David Garrison el 27 de Ag. de 2020
Beginning in MATLAB R2018b, Python functions that accept numpy arrays may also accept MATLAB arrays without explicit conversion. When necessary, a numpy array can be created explicitly from a MATLAB array. For example, if you have a supported version of Python that is installed with the numpy library, you can do the following:
>> x = rand(2,2); % MATLAB array
>> y = py.numpy.array(x); % numpy array
y =
Python ndarray:
0.5943 0.8064
0.6133 0.1372
Use details function to view the properties of the Python object.
Use double function to convert to a MATLAB array.
Also beginning in MATLAB R2018b, it is possible to convert numeric numpy arrays returned from Python into MATLAB arrays. For example:
>> y = py.numpy.random.random([int32(2), int32(2)]) % numpy array
y =
Python ndarray:
0.5943 0.8064
0.6133 0.1372
Use details function to view the properties of the Python object.
Use double function to convert to a MATLAB array.
>> x = 2*double(y) % MATLAB array
x =
1.1885 1.6129
1.2266 0.2744
See the MATLAB documentation on Passing Matrices and Multidimensional Arrays for additional Information.
##### 3 comentariosMostrar 1 comentario más antiguoOcultar 1 comentario más antiguo
abraham rodriguez el 18 de Sept. de 2021
@Badr Hisham In latest R2021a, you can pass directly a numpy ndarray to double() and it will convert to a native matlab matrix.
winkmal el 17 de Mayo de 2022
y = py.numpy.random.random([int32(2), int32(2)])
gives
Python Error: AttributeError: 'array.array' object has no attribute 'fromstring'
probably due to this. Python 3.9 here.
Maybe you can update your post, since Python 3.7 support will be removed on R2022a.

Iniciar sesión para comentar.

### Más respuestas (3)

Jim Hokanson el 4 de Nov. de 2014
data = double(py.array.array('d',py.numpy.nditer(x))); %d is for double, see link below on types
data = reshape(data,[4 4])'; %Could incorporate x.shape here ...
py.array.array is apparently the way Matlab suggests getting data from Python into Matlab (see link above) https://docs.python.org/2/library/array.html
This however requires an iterable over which the array can be constructed, hence the call to nditer()
Once the value is an array, then Matlab has written functionality for casting to a Matlab type, in this case via double.
##### 6 comentariosMostrar 4 comentarios más antiguosOcultar 4 comentarios más antiguos
Dev-iL el 16 de Mayo de 2017
Editada: Dev-iL el 16 de Mayo de 2017
Hi Christoph,
Thanks for the code! However, it doesn't support the conversion of scalar arrays:
Error using reshape
Size vector must have at least two elements.
Error in matpy.nparray2mat (line 55)
result=reshape(result,fliplr(data_size));
This can be solved by modifying the first `if` statement in nparray2mat to:
if any(numel(data_size) == [0,1])
Cheers!
Eric Cousineau el 30 de Mayo de 2017
To build further upon y'all's work, I've made a really rough prototype, using `matpy`, to get `numpy`-friendly proxy that allows more natural referencing, indexing / slicing, etc., via `subsref` and `subsassgn`:
Example code:
pyA = py.numpy.eye(3);
mlA = NumPyProxy(pyA);
mlA(:)
double(mlA(:))
mlA(3, 2)
sub = mlA([2, 1], 3)
double(sub) % 2017-05-30T01:09-04:00 - Presently a bug, wrong order
sub2 = mlA([1, 2], 3)
double(sub2)
mlA([2, 1], 3) = 100 * [1, 2]
mlA(:) = 5
Note that PyProxy also permits casting of >1-D arrays, using `subsref` tricks, whereas MATLAB R2016b presently does not permit passing 2-D matrices (at least, as far as I've tried using other people's examples).
This is an ongoing effort to test some Drake functionality: GitHub Issue
This has only been tested / tinkered with in R2016b, and still needs refinement / robustification.
I will try to see if hacks like this aren't needed in future versions of MATLAB.
Thanks a ton for taking the time to post y'all's stuff!

Iniciar sesión para comentar.

Shaowu Pan el 27 de Sept. de 2017
Editada: per isakson el 27 de Sept. de 2017
Weird discussion...
def npArray2Matlab(x):
return matlab.double(x.tolist())
##### 2 comentariosMostrar NingunoOcultar Ninguno
Laszlo Kormoczi el 14 de Nov. de 2018
Then how to use this in MATLAB?
timo kvamme el 2 de En. de 2019
Thank you, this worked

Iniciar sesión para comentar.

Jim Hokanson el 4 de Oct. de 2014
Editada: Jim Hokanson el 4 de Oct. de 2014
Here's a very ugly solution.
temp = cellfun(@cell,cell(x.tolist),'un',0);
data = cell2mat(vertcat(temp {:}));
A slightly cleaner solution:
data = typecast(uint8(char(x.flat.base.data)),'double');
data = reshape(data,[4 4])'; %Could incorporate x.shape here ...
Then of course:
plot(data)
Sorry I don't have time to explain how you would know that you need to do these things. I have to run!
Jim
##### 4 comentariosMostrar 2 comentarios más antiguosOcultar 2 comentarios más antiguos
Peter el 6 de Oct. de 2014
Editada: Peter el 6 de Oct. de 2014
Jim, it's a bit complicated. For example, if I run:
for jj = 1:10
x = py.numpy.random.random([10,10]);
try
data = typecast(uint8(char(x.flat.base.data)),'double');
disp(numel(data))
catch me
warning(me.message)
end
end
I should get 100 doubles, but I end up with something very different:
Warning: The first input must contain a multiple of 8 elements to convert from uint8 (8 bits) to double (64 bits).
29.00
8.00
7.00
10.00
Warning: The first input must contain a multiple of 8 elements to convert from uint8 (8 bits) to double (64 bits).
24.00
20.00
Warning: The first input must contain a multiple of 8 elements to convert from uint8 (8 bits) to double (64 bits).
56.00
Do you have any idea why the typecasting is broken? I would guess that it's something to do with the conversion from the flat.base to char.
Peter.
Jim Hokanson el 8 de Oct. de 2014
Editada: Jim Hokanson el 8 de Oct. de 2014
Interesting. The problem occurs when there is a zero in the underlying raw buffer (not a zero value but a 0 byte). I'm not sure if this desired or if it is a bug.

Iniciar sesión para comentar.

### Categorías

Más información sobre Call Python from MATLAB en Help Center y File Exchange.

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by