Using mutate from dplyr inside a function: getting around non-standard evaluation

男娘i 2022-08-22 03:58 19阅读 0赞

To edit or add columns to a `data.frame`, you can use `mutate` from the `dplyr` package:

library(dplyr)
    mtcars %>% mutate(new_column = mpg + wt)

Here, `dplyr` uses non-standard evaluation in finding the contents for `mpg` and `wt`, knowing that it needs to look in the context of`mtcars`. This is nice for interactive use, but not so nice for using`mutate` inside a function where `mpg` and `wt` are inputs to the function.

The goal is to write a function `f` that takes the columns in `mtcars`you want to add up as strings, and executes `mutate`. Note that we also want to be able to set the new column name. A first naive approach might be:

f = function(col1, col2, new_col_name) {
        mtcars %>% mutate(new_col_name = col1 + col2)
    }

The problem is that `col1` and `col2` are not interpreted, in stead`dplyr` tries looking for `col1` and `col2` in mtcars. In addition, the name of the new column will be `new_col_name`, and not the content of `new_col_name`. To get around non-standard evaluation, you can use the `lazyeval` package. The following function does what we expect:

library(lazyeval)
    f = function(col1, col2, new_col_name) {
        mutate_call = lazyeval::interp(~ a + b, a = as.name(col1), b = as.name(col2))
        mtcars %>% mutate_(.dots = setNames(list(mutate_call), new_col_name))
    }
    head(f('wt', 'mpg', 'hahaaa'))
       mpg cyl disp  hp drat    wt  qsec vs am gear carb hahaaa
    1 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4 23.620
    2 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4 23.875
    3 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1 25.120
    4 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1 24.615
    5 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2 22.140
    6 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1 21.560

The important parts here are, given the call to `f` above:

*  `lazyeval::interp(~ a + b, a = as.name(col1), b = as.name(col2))` this creates the expression `wt + mpg`.
 *  `mutate_(mutate_call)` where `mutate_` is the version of mutate that uses standard evaluation (SE).
 *  `setNames(list(mutate_call), new_col_name))` sets the output name to the content of `new_col_name`, i.e. `hahaaa`.