问题描述
我想在 dplyr 的函数中使用变量名作为字符串。请参见下面的示例：
I want to use variable names as strings in functions of dplyr. See the example below:
df <- data.frame(
color = c("blue", "black", "blue", "blue", "black"),
value = 1:5)
filter(df, color == "blue")
它工作得很好，但我想通过字符串引用 color 这个：
It works perfectly, but I would like to refer to color by string, something like this:
var <- "color"
filter(df, this_probably_should_be_a_function(var) == "blue").
我会很高兴，以任何方式做到这一点，超级快乐，要阅读 dplyr 语法。
I would be happy, to do this by any means and super-happy to make use of easy-to-read dplyr syntax.
推荐答案
自 dplyr 0.3 每个 dplyr 函数使用非标准评估（NSE，请参阅和）有一个标准评估（SE）双胞胎以下划线结尾。这些可以用于传递变量。对于过滤器，它将是 filter _ 。使用 filter _ 可以将逻辑条件作为字符串传递。
As of dplyr 0.3 every dplyr function using non standard evaluation (NSE, see release post and vignette) has a standard evaluation (SE) twin ending in an underscore. These can be used for passing variables. For filter it will be filter_. Using filter_ you may pass the logical condition as a string.
filter_(df, "color=='blue'")
# color value
# 1 blue 1
# 2 blue 3
# 3 blue 4
使用逻辑条件来处理字符串当然是直接的
Construing the string with the logical condition is of course straighforward
l <- paste(var, "==", "'blue'")
filter_(df, l)
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