Tim - 6 months ago 39

Python Question

I am currently working on a project where I need do some steps of processing with legacy Matlab code (using the Matlab engine) and the rest in Python (numpy).

I noticed that converting the results from Matlab's

`matlab.mlarray.double`

`numpy.ndarray`

Here is some example code for creating an ndarray with 1000 elements from another ndarray, a list and an mlarray:

`import timeit`

setup_range = ("import numpy as np\n"

"x = range(1000)")

setup_arange = ("import numpy as np\n"

"x = np.arange(1000)")

setup_matlab = ("import numpy as np\n"

"import matlab.engine\n"

"eng = matlab.engine.start_matlab()\n"

"x = eng.linspace(0., 1000.-1., 1000.)")

print 'From other array'

print timeit.timeit('np.array(x)', setup=setup_arange, number=1000)

print 'From list'

print timeit.timeit('np.array(x)', setup=setup_range, number=1000)

print 'From matlab'

print timeit.timeit('np.array(x)', setup=setup_matlab, number=1000)

Which takes the following times:

`From other array`

0.00150722111994

From list

0.0705359556928

From matlab

7.0873282467

The conversion takes about 100 times as long as a conversion from list.

Is there any way to speed up the conversion?

Answer

Moments after posting the question I found the solution.

For one-dimensional arrays, access only the `_data`

property of the Matlab array.

```
import timeit
print 'From list'
print timeit.timeit('np.array(x)', setup=setup_range, number=1000)
print 'From matlab'
print timeit.timeit('np.array(x)', setup=setup_matlab, number=1000)
print 'From matlab_data'
print timeit.timeit('np.array(x._data)', setup=setup_matlab, number=1000)
```

prints

```
From list
0.0719847538787
From matlab
7.12802865169
From matlab_data
0.118476275533
```

For multi-dimensional arrays you need to reshape the array afterwards. In the case of two-dimensional arrays this means calling

```
np.array(x._data).reshape(x.size[::-1]).T
```