How can I convert a two column array to a matrix with counts of occurences?

Question

I have the following numpy array:

import numpy as np

pair_array = np.array([(205, 254), (205, 382), (254, 382), (18, 69), (205, 382), 
                       (31         


        

Answer 1

One way of doing it is appending the pair_array with pair_array reversed at axis 1 which can be done using [::-1]. And to append use np.vstack/np.r_/np.concatenate.

Now use pd.crosstab to perform cross tabulation.

all_vals = np.r_[pair_array, pair_array[:, ::-1]]
pd.crosstab(all_vals[:, 0], all_vals[:, 1])

col_0  18   31   69   183  205  254  267  382
row_0                                        
18       0    0    1    0    0    0    0    0
31       0    0    0    1    0    0    1    1
69       1    0    0    0    0    0    0    0
183      0    1    0    0    0    0    1    1
205      0    0    0    0    0    1    0    2
254      0    0    0    0    1    0    0    1
267      0    1    0    1    0    0    0    0
382      0    1    0    1    2    1    0    0

As @QuangHoang pointed when there are identical pairs occurring more than one time i.e [(18, 18), (18, 18), ...], then use

rev = pair_array[:, ::-1]
m = (pair_array == rev)
rev = rev[~np.all(m, axis=1)]
all_vals = np.r_[pair_arr, rev]