Mutate dynamically created variable dplyr - r

Question

I am trying to create a function in which I summarise several columns in a dataframe using several functions and then mutate the output of these functions later.
A simpler example is given below:
group_mean_plus_one <- function(df, groups, var){
df %>%
group_by(across({{groups}})) %>%
summarise(across({{ var }},
.fns = list(mean = ~mean(.x, na.rm=TRUE),
sd = ~sd(.x, na.rm=TRUE)),
.names = "{.col}_{.fn}")) %>%
mutate("mean_plus_one_{{var}}" := !!rlang::expr("{{var}}_mean + 1"))
}
tibble(mtcars) %>%
group_mean_plus_one(groups = cyl, var = hp)
Here the idea is that we group by each of the variables in group and summarise each of the variables in var using the given functions.
Further on we wish to refer to the the variables created in the summarise block and mutate new variables from these. However, I am struggling with referring to these dynamically created variable names from the summarise block.
Running the above returns:
# A tibble: 3 x 4
cyl hp_mean hp_sd mean_plus_one_hp
<dbl> <dbl> <dbl> <chr>
1 4 82.6 20.9 {{var}}_mean + 1
2 6 122. 24.3 {{var}}_mean + 1
3 8 209. 51.0 {{var}}_mean + 1
when instead I want it to return:
# A tibble: 3 x 4
cyl hp_mean hp_sd mean_plus_one_hp
<dbl> <dbl> <dbl> <dbl>
1 4 82.6 20.9 83.6
2 6 122. 24.3 123.
3 8 209. 51.0 210.
Any help is much appreciated, thanks in advance :)

Answer 1

We could convert to string, and use .data
group_mean_plus_one <- function(df, groups, var){
var1 <- rlang::as_string(rlang::ensym(var))
df %>%
group_by(across({{groups}})) %>%
summarise(across({{ var }},
.fns = list(mean = ~mean(.x, na.rm=TRUE),
sd = ~sd(.x, na.rm=TRUE)),
.names = "{.col}_{.fn}")) %>%
mutate("mean_plus_one_{{var}}" := .data[[str_c(var1, "_mean")]] + 1)
}
-testing
tibble(mtcars) %>%
group_mean_plus_one(groups = cyl, var = hp)
# A tibble: 3 x 4
cyl hp_mean hp_sd mean_plus_one_hp
<dbl> <dbl> <dbl> <dbl>
1 4 82.6 20.9 83.6
2 6 122. 24.3 123.
3 8 209. 51.0 210.