Remove duplicates from array and elements in matching positions in another array

Question

I have two numpy array, I want to remove duplicate values from the first array (including the original value) and remove the items in the matching positions in the second array.

For example:

a = [1, 2, 2, 3]
b = ['a', 'd', 'f', 'c']

Becomes:

a = [1, 3]
b = ['a', 'c']

I need to do this efficiently and not use the naive solution which is time consuming

Answer 1

Here's one with np.unique -

unq,idx,c = np.unique(a, return_index=True, return_counts=True)
unq_idx = np.sort(idx[c==1])
a_out = a[unq_idx]
b_out = b[unq_idx]

Sample run -

In [34]: a
Out[34]: array([1, 2, 2, 3])

In [35]: b
Out[35]: array(['a', 'd', 'f', 'c'], dtype='|S1')

In [36]: unq,idx,c = np.unique(a, return_index=1, return_counts=1)
    ...: unq_idx = idx[c==1]
    ...: a_out = a[unq_idx]
    ...: b_out = b[unq_idx]

In [37]: a_out
Out[37]: array([1, 3])

In [38]: b_out
Out[38]: array(['a', 'c'], dtype='|S1')

Answer 2

Since you are open to NumPy, you may wish to consider Pandas, which uses NumPy internally:

import pandas as pd

a = pd.Series([1, 2, 2, 3])
b = pd.Series(['a', 'd', 'f', 'c'])

flags = ~a.duplicated(keep=False)
idx = flags[flags].index

a = a[idx].values
b = b[idx].values

Result:

print(a, b, sep='\n')

array([1, 3], dtype=int64)
array(['a', 'c'], dtype=object)