Exclude column in `dplyr` `mutate_at` while using data in this column

Question

I want to rescale all variables (but year and gender) in a df by one specific year, grouped by gender:

Answer 1

Use position in mutate_at

library(dplyr)

df %>%
  group_by(gender) %>%
  mutate_at(-c(1, 2), ~ifelse(year == 3, 0, . - .[year == 3]))

#  gender  year var_a var_b
#       
# 1 m          1    -2    -2
# 2 m          2    -1    -1
# 3 m          3     0     0
# 4 m          4     1     1
# 5 m          5     2     2
# 6 f          1    -2    -2
# 7 f          2    -1    -1
# 8 f          3     0     0
# 9 f          4     1     1
#10 f          5     2     2

In case, if you do not know the position of columns beforehand you can first find it

cols <- which(names(df) %in% c("gender", "year"))

df %>%
  group_by(gender) %>%
  mutate_at(-cols, ~ifelse(year == 3, 0, . - .[year == 3]))

---

Or select columns which starts_with

df %>%
  group_by(gender) %>%
  mutate_at(vars(starts_with("var")), ~ifelse(year == 3, 0, . - .[year == 3]))