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

You could create a data frame of the appropriate size with zeros beforehand and just increment the appropriate cells by looping over the pairs:

import numpy as np
import pandas as pd

pair_array = np.array([(205, 254), (205, 382), (254, 382), (18, 69), (205, 382),
                       (31, 183), (31, 267), (31, 82), (183, 267), (183, 382)])

vals = sorted(set(pair_array.flatten()))
n = len(vals)

df = pd.DataFrame(np.zeros((n, n), dtype=np.int), columns=vals, index=vals)

for r, c in pair_array:
    df.at[r, c] += 1
    df.at[c, r] += 1

print(df)

Output:

     18   31   69   82   183  205  254  267  382
18     0    0    1    0    0    0    0    0    0
31     0    0    0    1    1    0    0    1    0
69     1    0    0    0    0    0    0    0    0
82     0    1    0    0    0    0    0    0    0
183    0    1    0    0    0    0    0    1    1
205    0    0    0    0    0    0    1    0    2
254    0    0    0    0    0    1    0    0    1
267    0    1    0    0    1    0    0    0    0
382    0    0    0    0    1    2    1    0    0