Sort python dictionary based on a given column in an array

Question

I have the following kind of dictionary

import operator
my_dict = {'a':np.array((1,2)),'b':np.array((3,4))}

and I need to sorted based on the 1st column of the arrays. I can sort this other dictionary

my_dict2 = {'a':np.array((1)),'b':np.array((3))}

using

sorted(my_dict2.items(), key=operator.itemgetter(1),reverse=True)

but trying the same approach with my_dict, i.e.

sorted(my_dict.items(), key=operator.itemgetter(1),reverse=True)

throws the error message

The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

I have tried to play with the argument in itemgetter, but I cannot order my_dict.

• Florian H: Possible duplicate of How to sort a Python dictionary by value?
• arghtype: Possible duplicate of Sorting arrays in NumPy by column

Answer 1

I'm not exactly sure how to use operator to get subscripted values. So what I did was this:

sorted(my_dict.items(), key=lambda x: x[1][0],reverse=True)

Explanation:
x[1][0] means you are taking the 1 and 3 out of (1,2) and (3,4).
x[1][1] means you are taking the 2 and 4 out of (1,2) and (3,4).
x[0] is the same as itemgetter(0) which will sort by a and b, which is the keys of your dictionary.