If you ever tried to pass a string as an argument to a dplyr function you probably got frustrated. This just doesn't work. But don't give up, instead have a look at the solution below.

dplyr-strings.R

suppressMessages(library(dplyr))
data("mtcars")

mtcars %>% group_by(cyl) %>% summarise(n = n())
#> # A tibble: 3 x 2
#>     cyl     n
#>   <dbl> <int>
#> 1     4    11
#> 2     6     7
#> 3     8    14

var <- "cyl"
# this doesn't work
mtcars %>% group_by(var) %>% summarise(n = n())
#> Error: Column `var` is unknown

# that's the solution
mtcars %>% group_by(!!sym(var)) %>% summarise(n = n())
#> # A tibble: 3 x 2
#>     cyl     n
#>   <dbl> <int>
#> 1     4    11
#> 2     6     7
#> 3     8    14

# this works but has an unintended side effect
mtcars %>% group_by(get(var)) %>% summarise(n = n())
#> # A tibble: 3 x 2
#>   `get(var)`     n
#>        <dbl> <int>
#> 1          4    11
#> 2          6     7
#> 3          8    14