使用select_和starts_with R [英] Using select_ and starts_with R

问题描述

为什么此代码不起作用?

Why doesn't this code work?

mtcars %>% select_("starts_with('d')")

Error in eval(expr, envir, enclos) : could not find function "starts_with"

这是简化的示例.我正在尝试将select_命令传递给函数.

This is simplified example. I am trying to pass the select_ command to a function.

推荐答案

select()和select_()之间的区别是它们对参数的非标准/标准评估.如果将starts_with()之类的函数用作select_()的参数，则应使用代字号引起来:

The difference between select() and select_() is their non-stadard / standard evaluation of the argument. If a function like starts_with() is used as an argument of select_() it should be quoted with a tilde:

library(dplyr)
mtcars %>% select_(~starts_with('d'))

这将产生与通常使用select相同的输出:

This yields the same output as the normal use of select:

identical(mtcars %>% select_(~starts_with('d')), mtcars %>% select(starts_with('d')))
#[1] TRUE

有关更多信息，请参见关于非标准评估的小插曲:vignette("nse").

For more information see the vignette on non-standard evaluation: vignette("nse").

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