# Computing relative values between two rows of data, returned when grouping by "MatchId", one for each team in the match.
# Script solution for Laura de Jong

## function to compute relative scores between two rows, to use next in mutate
fun = function(x){
  c(x[1] - x[2], x[2]-x[1])
}

## use function above on particular variables and rename
rel_scores = TeamStats %>%
  group_by(MatchId) %>%
  mutate_at(vars(Appearances:GkShortAccuracy), list(~ fun)) %>% # run the function above over variables Appearances to GkShortAccuracy (the scoring variables)
  rename_at(vars(Appearances:GkShortAccuracy), ~ paste0(names(TeamStats %>% select(Appearances:GkShortAccuracy)), "_rel")) # rename calculated variables starting with "rel_"

## join datasets, with and without relative scores, together
TeamStats_join_rel = TeamStats %>%
  inner_join(rel_scores, by = c("Date", "MatchId", "TeamId", "OppositionId", "Venue", "LeagueOrTournament")) %>%
  select(noquote(order(colnames(.)))) %>% ## order variables by alphabetic order
  select(c("Date", "MatchId", "TeamId", "OppositionId", "Venue", "LeagueOrTournament"), everything()) %>% ## place independent variables at left, followed by everything else
  arrange(MatchId)