在dplyr动词中将字符串作为参数传递 [英] Passing strings as arguments in dplyr verbs

问题描述

我希望能够为 dplyr 动词定义自变量

I would like to be able to define arguments for dplyr verbs

condition <- "dist > 50"

，然后在<$ c中使用这些字符串$ c> dplyr 函数：

require(ggplot2)
ds <- cars
ds1 <- ds %>%
   filter (eval(condition))
ds1

但是它会引发错误

Error: filter condition does not evaluate to a logical vector. 

代码应评估为：

  ds1<- ds %>%
     filter(dist > 50)
  ds1

结果是：

ds1

ds1

   speed dist
1     14   60
2     14   80
3     15   54
4     18   56
5     18   76
6     18   84
7     19   68
8     20   52
9     20   56
10    20   64
11    22   66
12    23   54
13    24   70
14    24   92
15    24   93
16    24  120
17    25   85

问题：

如何在<$ c中将字符串作为参数传递$ c> dplyr 动词？

推荐答案

自2014年以来，使用< a href = https://rlang.r-lib.org/reference/quasiquotation.html rel = noreferrer> rlang的准报价。

Since these 2014 answers, two new ways are possible using rlang's quasiquotation.

方便为了便于比较，该语句 dist> 50 直接包含在 dplyr :: filter（）中。

library(magrittr)

# The filter statement is hard-coded inside the function.
cars_subset_0 <- function( ) {
  cars %>%
    dplyr::filter(dist > 50)
}
cars_subset_0()

结果：

   speed dist
1     14   60
2     14   80
3     15   54
4     18   56
...
17    25   85

使用NSE的郎朗方法（非标准评估）。如使用dplyr 小插图进行编程，语句 dist> 50 由 rlang :: enquo处理（） ，它使用某种黑魔法来查看参数，查看用户键入的内容，然后将该值返回给定值。然后rlang的 !! 取消对输入的引用，以便在周围环境中立即对其进行评估。

rlang approach with NSE (nonstandard evaluation). As described in the Programming with dplyr vignette, the statement dist > 50 is processed by rlang::enquo(), which "uses some dark magic to look at the argument, see what the user typed, and return that value as a quosure". Then rlang's !! unquotes the input "so that it’s evaluated immediately in the surrounding context".

# The filter statement is evaluated with NSE.
cars_subset_1 <- function( filter_statement ) {
  filter_statement_en <- rlang::enquo(filter_statement)
  message("filter statement: `", filter_statement_en, "`.")

  cars %>%
    dplyr::filter(!!filter_statement_en)
}
cars_subset_1(dist > 50)

结果：

filter statement: `~dist > 50`.
<quosure>
expr: ^dist > 50
env:  global
   speed dist
1     14   60
2     14   80
3     15   54
4     18   56
17    25   85

rlang方法传递字符串。语句 dist> 50 作为显式字符串传递给函数，并由 rlang :: parse_expr（） ，然后用 !! 取消引用。

rlang approach passing a string. The statement "dist > 50" is passed to the function as an explicit string, and parsed as an expression by rlang::parse_expr(), then unquoted by !!.

# The filter statement is passed a string.
cars_subset_2 <- function( filter_statement ) {
  filter_statement_expr <- rlang::parse_expr(filter_statement)
  message("filter statement: `", filter_statement_expr, "`.")

  cars %>%
    dplyr::filter(!!filter_statement_expr)
}
cars_subset_2("dist > 50")

结果：

filter statement: `>dist50`.
   speed dist
1     14   60
2     14   80
3     15   54
4     18   56
...
17    25   85

使用 dplyr :: select（），事情变得更简单。显式字符串只需要 !! 。

Things are simpler with dplyr::select(). Explicit strings need only !!.

# The select statement is passed a string.
cars_subset_2b <- function( select_statement ) {
  cars %>%
    dplyr::select(!!select_statement)
}
cars_subset_2b("dist")

这篇关于在dplyr动词中将字符串作为参数传递的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！