Pandas: how to convert a list into a matrix grouped by a column?

Question

I have a pandas dataframe where the first column (CUSTOMER) is the name of the customer and the customer\'s name is repeated once for every product the customer has purchase

Answer 1

Self merge with crosstab

d1 = df.merge(df, on='Customer').query('Product_x != Product_y')
pd.crosstab(d1.Product_x, d1.Product_y)

Product_y  A  B  C
Product_x         
A          0  2  1
B          2  0  1
C          1  1  0

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You can see this answer to get a better idea how to speed the crosstab up. The key insight for this problem was the self merging.