如何在R中动态地改变数据帧中的列

我想改变a的值 dataframe tibble 列定义为变量( col_name ).我试过了 !!col_name 没有成功。

library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
col_name <- "b" # the column to select

df <- tibble(a = c(1,2,3), b = c(2,4,6))

df %>%
  mutate(b = if_else((b == 6 & a == 3), 8, b))  # this works 
#> # A tibble: 3 x 2
#>       a     b
#>   <dbl> <dbl>
#> 1     1     2
#> 2     2     4
#> 3     3     8

# but this doesn't
df %>%
  mutate(!!col_name := if_else((!!col_name == 6 & a == 3), 8, !!col_name))
#> Error: Problem with `mutate()` input `b`.
#> x `false` must be a double vector, not a character vector.
#> i Input `b` is `if_else(("b" == 6 & a == 3), 8, "b")`.
Created on 2020-10-13 by the reprex package (v0.3.0)