numpy array equivalent for += operator

Question

I often do the following:

import numpy as np

def my_generator_fun():
    yield x # some magically generated x

A = []
for x in my_generator_fun():
    A += [x]
A = np.array(A)

Is there a better solution to this which operates on a numpy array from the start and avoids the creation of a standard python list?

Note that the += operator allows to extend an empty and dimensionless array with an arbitrarily dimensioned array whereas np.append and np.concatenate demand for equally dimensioned arrays.

possible duplicate of stackoverflow.com/questions/4535374/initialize-a-numpy-array — John Ruddell
– John Ruddell, Commented Jul 15, 2014 at 15:06
no, my point is slightly different: image the case where you successively built up the array or you want to initialize each element differently. However, I really need the exactly same functionality, not only the same result. — morph
– morph, Commented Jul 15, 2014 at 15:13
why are you even doing a loop with this? why not just make A = [[0,1],[1,2],[3,4]] ? — John Ruddell
– John Ruddell, Commented Jul 15, 2014 at 15:19
another possible duplicate.. you really need to research what you are looking for first... stackoverflow.com/questions/10346336/… — John Ruddell
– John Ruddell, Commented Jul 15, 2014 at 15:20

Jaime · Accepted Answer · 2014-07-15 17:27:10Z

4

Use np.fromiter:

def f(n):
    for j in range(n):
        yield j

>>> np.fromiter(f(5), dtype=np.intp)
array([0, 1, 2, 3, 4])

If you know beforehand the number of items the iterator is going to return, you can speed things up using the count keyword argument:

>>> np.fromiter(f(5), dtype=np.intp, count=5)
array([0, 1, 2, 3, 4])

answered Jul 15, 2014 at 17:27

Jaime

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Comments

mdurant · Accepted Answer · 2014-07-15 15:10:02Z

0

To get the same array A, do:

A = numpy.arange(5)

Arrays are not in general meant to be dynamically sized, but you could use numpy.concatenate.

answered Jul 15, 2014 at 15:10

mdurant

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3 Comments

mdurant Over a year ago

I see your requirement has changed - what in general are you trying to achieve?

morph Over a year ago

basically said, i will achieve the same functionality like I know it from the += operator in basic python lists for numpy arrays

Paul H Over a year ago

@morph look into np.hstack and np.vstack

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