I am using mutate to create a column depending on the first value of a group
library(tidyverse)
test = data.frame(grp = c(1,1,1,2,2,2), x = c(1,2,3,1,2,3), y = c(1,2,3,1,2,3))
test
grp x y
1 1 1 1
2 1 2 2
3 1 3 3
4 2 1 1
5 2 2 2
6 2 3 3
test %>% group_by(grp) %>%
mutate(y = ifelse(grp[[1]] == x[[1]], y-1, y))
grp x y
<dbl> <dbl> <dbl>
1 1 1 0
2 1 2 0
3 1 3 0
4 2 1 1
5 2 2 1
6 2 3 1
However output is not as I expected.
Expected output is
grp x y
<dbl> <dbl> <dbl>
1 1 1 0
2 1 2 1
3 1 3 2
4 2 1 1
5 2 2 2
6 2 3 3
Can you please explain what is happening and how best to get my expected solution?
CodePudding user response:
Because of the x[[1]], you are always comparing the group values of each row with the the x value of the first row. I think you want grp==x within ifelse()
CodePudding user response:
You need to remove the index [[1]]
from grp
since it will only change the first value of that group and use that to replace y
. Since grp
is the group you should avoid indexing it. Just use it as is, i.e.
library(dplyr)
test %>%
group_by(grp) %>%
mutate(new_y = ifelse(grp == first(x), y-1, y))
# A tibble: 6 × 4
# Groups: grp [2]
grp x y new_y
<dbl> <dbl> <dbl> <dbl>
1 1 1 1 0
2 1 2 2 1
3 1 3 3 2
4 2 1 1 1
5 2 2 2 2
6 2 3 3 3