Python Iterating 2D Array, Return Array Value

Question

I have created a 2D 10x10 Array. using Numpy I want to iterate over the array as efficiently as possible.

However I would like to return the array values. essentially iterating over the 10x10 array 10 times and return a 1x10 array each time.

 import datetime
 import numpy as np
 import random

 start = datetime.datetime.now()
 a = np.random.uniform(low=-1, high=1, size=(10,10))
 print("Time :",datetime.datetime.now() - start)

 for x in np.nditer(a):
    print(x)

the result is as follows:

0.5738994777717537
0.24988408410910767
0.8391827831682657
0.0015975845830569213
0.54477459840569
0.14091622639476165
-0.36517132895234106
-0.06311125453484467
-0.6572544506539948

...

100 times

However I would expect the result to be:

[0.5738994777717537,
0.24988408410910767,
0.8391827831682657,
0.0015975845830569213,
0.54477459840569,
0.14091622639476165,
-0.36517132895234106,
-0.06311125453484467,
-0.6572544506539948],[...]

...

10 times

Any help would be appreciated!

First, why are you using nditer? What documentation are you working from? Why not just x.tolist()? Or for x in a: print(x)? nditer effectively flattens the array, providing each value without regard to the row/column structure. That should be obvious from the nditer documentation. Also nditer, at least when called from Python, is not a speed tool. It is not faster than simple iteration. — hpaulj
– hpaulj, Commented Aug 19, 2019 at 18:18
Efficient and iterate don't belong together when talking about numpy arrays. Focus on using the fast compiled whole-array methods, not iteration. — hpaulj
– hpaulj, Commented Aug 19, 2019 at 18:26
You are just interested in slicing. Don't try to iterate, just have a look at numpy slicing. You are just looking for a[0], a[1] etc. — pythonic833
– pythonic833, Commented Aug 19, 2019 at 18:46

Baptiste Vauthey · Accepted Answer · 2019-08-19 18:14:05Z

1

To directly answer your question, this does exactly what you want:

import numpy as np
a = np.random.uniform(low=-1, high=1, size=(10,10))
print(','.join([str(list(x)) for x in a]))

This will print

[-0.2403881196886386, ... , 0.8518165986395723],[-0.2403881196886386, ... , 0.8518165986395723], ..., [-0.2403881196886386, ... , 0.8518165986395723]

The reason you're printing just the elements of the array is due to the way nditer works. nditer iterates over single elements, even at a multidimensional level, whereas you want to iterate over just the first dimension of the array. For that, for x in a: works as intended.

Edit

Here is a good link if you want to read up on how nditer works: https://docs.scipy.org/doc/numpy/reference/arrays.nditer.html#arrays-nditer

answered Aug 19, 2019 at 18:14

Baptiste Vauthey

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1 Comment

hpaulj Over a year ago

That indexing reference is not very useful unless you read all the way to end, and start working in cython.

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