A.12 Using mutate and group_by

When we add/edit a column with mutate after group_by has been used, the operation is performed on each group separately. For example, for each team and each game, let’s find how many days have passed since the team’s previous game.

Code

dd = dd %>%
  arrange(date) %>%
  group_by(team) %>%
  mutate(days.rest = c(NA, diff(date)))

## Let's look at one team
dd %>% 
  filter(team == 'PHI')

# A tibble: 82 × 8
# Groups:   team [1]
   date       team  score opp   opp.score gid      ha    days.rest
   <date>     <chr> <dbl> <chr>     <dbl> <chr>    <chr>     <dbl>
 1 2021-10-20 PHI     117 NOP          97 22100009 away         NA
 2 2021-10-22 PHI     109 BKN         114 22100021 home          2
 3 2021-10-24 PHI     115 OKC         103 22100038 away          2
 4 2021-10-26 PHI      99 NYK         112 22100050 away          2
 5 2021-10-28 PHI     110 DET         102 22100065 home          2
 6 2021-10-30 PHI     122 ATL          94 22100082 home          2
 7 2021-11-01 PHI     113 POR         103 22100096 home          2
 8 2021-11-03 PHI     103 CHI          98 22100111 home          2
 9 2021-11-04 PHI     109 DET          98 22100119 away          1
10 2021-11-06 PHI     114 CHI         105 22100135 away          2
# ℹ 72 more rows