numpy structured arrays: help understanding output

Question

I am trying to learn how to use numpy's structured arrays. Specifically, I was trying to add information to more than one field at a time. I tried:

import numpy as np

numrec = np.zeros(8, dtype=[('col0', 'int16'), ('col1', 'int16'),
                            ('col2', 'int16'), ('col3', 'int16')])

numrec[['col1','col2']][0:2] = [(3,5), (1,8)]
print numrec

The above does not work. The values are not added to the columns specified. What is surprising is that I do not get any error when I run it. Can someone please explain what is happening?

Thanks.

AFoglia · Accepted Answer · 2011-03-04 22:52:55Z

9

You are setting values on a temporary.

numrec[["col1", "col2"]]

returns a copy of the array. You can see this by the OWNDATA flag.

>>> numrec[["col1", "col2"]].flags["OWNDATA"]
True

When you index a numpy array with a list, numpy returns a copy of the data. It has to be a copy, because, in general, the list may not resolve to a regular, ordered view of the underlying data. (This holds for any numpy array, not just structured arrays.)

Compare

>>> numrec[["col1"]].flags["OWNDATA"]
True
>>> numrec["col1"].flags["OWNDATA"]
False

Also, if a numpy array is a view, the base member holds the underlying array.

>>> id(numrec["col1"].base) == id(numrec)
True

edited Mar 4, 2011 at 22:52

answered Mar 4, 2011 at 21:25

AFoglia

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4 Comments

Paul Over a year ago

spot on. It might also be worth mentioning that this is true for all "fancy indexed" arrays, not just structured arrays. And I'll put a link to this related recent post here: stackoverflow.com/questions/5127991/…

Curious2learn Over a year ago

Thanks!! I did not know this. So, if I have two lists listone (of length 3) and listwo (of length 3) can I update first three elements in 'col1' and 'col2' of numrec to equal these lists simultaneously. I know I can do numrec['col1'][0:3] = listone and numrec['col2'][0:3] = listtwo. But can I do it together in one command. Thanks.

AFoglia Over a year ago

Paul, I may not have emphasized it, but that's what I didn't say "When you index a numpy structured array..." Anyway, I could have made it clearer. So I just added a parenthetical to that effect.

AFoglia Over a year ago

Curious2learn, I don't think there is a way to do what you want in one simple command, though I would appreciate someone proving me wrong.

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