Finding value at indexes present in a 2D array in Python

Question

I have a 2D array size(2,3) and a list

Input:

a=['airplane','automobile','bird','cat','deer','dog','frog','horse','ship','truck']

b=[[4 2 8][1 7 0]] #2D numpy array shape (2,3) containing indices of list a

Output:

c = [['deer','bird','ship'],['automobile','horse','airplane']]

Is there any pythonic way or shortcut to achieve the output without iterating over each index value?

What are you going to do with the output? There might be a better way of representing your data. — gr1zzly be4r
– gr1zzly be4r, Commented Apr 20, 2016 at 15:28
why "without iterating over each index value"? Is there a good reason for that restriction? — Tadhg McDonald-Jensen
– Tadhg McDonald-Jensen, Commented Apr 20, 2016 at 15:30
You can use np.take to have a 2D array as output : np.take(a,b). — Divakar
– Divakar, Commented Apr 20, 2016 at 15:48
@TadhgMcDonald-Jensen I have got the array b after a number of operations on numpy array. Also the array b is of shape(n,3) where n is very large over 100000. So I am concerned about the time it will take to iterate using loop. But numpy operation will take less time for sure. — Kapes
– Kapes, Commented Apr 20, 2016 at 15:59

Eric · Accepted Answer · 2016-04-20 16:48:48Z

2

If you make your list a np.array as well, all you need is a[b]:

>>> import numpy as np
>>> keys = np.array(['airplane','automobile','bird','cat','deer','dog','frog','horse','ship','truck'])
>>> indices = np.array([[4,2,8],[1,7,0]])
>>> keys[indices]
array([['deer', 'bird', 'ship'],
       ['automobile', 'horse', 'airplane']], 
      dtype='<U10')

answered Apr 20, 2016 at 16:48

Eric

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Comments

Colin Dickie · Accepted Answer · 2016-04-20 16:09:20Z

0

This does the job:

a=['airplane','automobile','bird','cat','deer','dog','frog','horse','ship','truck']
b=numpy.array([[4,2,8],[1,7,0]])


c = [[a[idx] for idx in row] for row in b]

edited Apr 20, 2016 at 16:09

answered Apr 20, 2016 at 15:37

Colin Dickie

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7 Comments

gr1zzly be4r Over a year ago

This iterates over each index.

C Panda Over a year ago

@gr1zzlybe4r Well, as b is a list you have to traverse and index. That's the whole point of a list.

Kapes Over a year ago

I want to keep the array b as numpy array for further operations for my code

Colin Dickie Over a year ago

@Kapes I've edited my answer to make b a numpy array. This won't alter the rest of the code. But I'm confused over what you want, or why this wouldn't be acceptable. Keen to help if I can.

Tadhg McDonald-Jensen Over a year ago

@ColinDickie see Kapes response to why they don't want this

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