How to calculate a table of pairwise counts from long-form data frame

Question

I have a \'long-form\' data frame with columns id (the primary key) and featureCode (categorical variable). Each record has between 1 and 9 values

Answer 1

I would use SQL, in R it is available with the sqldf Package.

Extract all possible combinations something like:

sqldf("select distinct df1.featureCode, df2.featureCode
       from df df1, df df2       
       ")

Then you can extract the result elements:
(Maybe just use a for loop for all combinations)

PCLI - PPLC

sqldf("select count(df1.id)
       from df df1, df df2
       where df1.id = df2.id
       and df1.featureCode = 'PCLI' and df2.featureCode = 'PPLC'
       ")

PPLC - PPL

sqldf("select count(df1.id)
       from df df1, df df2
       where df1.id = df2.id
       and df1.featureCode = 'PPLC' and df2.featureCode = 'PPL'
       ")

PCLI - PPL

sqldf("select count(df1.id)
       from df df1, df df2
       where df1.id = df2.id
       and df1.featureCode = 'PCLI' and df2.featureCode = 'PPL'
       ")

There is for sure some easier solution out there especially if you got more combinations to consider. Maybe a search for contingency table helps you out.