How do I turn monadic data into dyadic data in R (country-year into pair-year)?

Question

I have data organized by country-year, with a ID for a dyadic relationship. I want to organize this by dyad-year.

Here is how my data is organized:

Answer 1

We can reshape from 'long' to 'wide' using different methods. Two are described below.

Using 'data.table', we convert the 'data.frame', to 'data.table' (setDT(mydf)), create a sequence column ('ind'), grouped by 'dyadic_id' and 'year'. Then, we convert the dataset from 'long' to 'wide' format using dcast.

library(data.table)
setDT(mydf)[, ind:= 1:.N, by = .(dyadic_id, year)]
dcast(mydf, dyadic_id+year~ paste('country_codes', ind, sep='_'), value.var='country_codes')
#   dyadic_id year country_codes_1 country_codes_2
#1:         1 1990             200              20
#2:         1 1991             200              20
#3:         2 1990             300              10
#4:         3 1990             100              10
#5:         4 1991             500             200

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Or using dplyr/tidyr, we do the same i.e. grouping by 'dyadic_id', 'year', create a 'ind' column (mutate(...), and use spread from tidyr to reshape to 'wide' format.

library(dplyr)
library(tidyr)
mydf %>% 
    group_by(dyadic_id, year) %>%
    mutate(ind= paste0('country_codes', row_number())) %>% 
    spread(ind, country_codes)
#    dyadic_id  year country_codes1 country_codes2
#       (dbl) (dbl)          (dbl)          (dbl)
#1         1  1990            200             20
#2         1  1991            200             20
#3         2  1990            300             10
#4         3  1990            100             10
#5         4  1991            500            200