如何在dplyr 0.7.0中将列名称指定为字符元素？ [英] How can you specify a column name as character element in dplyr 0.7.0?

问题描述

从>另一个问题的答案中借用：

library(dplyr)
df <- data.frame( 
  color = c("blue", "black", "blue", "blue", "black"), 
  value = 1:5)

通常呈现的示例采用以下形式...

The typically presented example takes the form of ...

# As of dplyr 0.7, new functions were introduced to simplify the situation
col_name <- quo(color)
df %>% filter((!!col_name) == val)

# Remember to use enquo within a function
filter_col <- function(df, col_name, val){
  col_name <- enquo(col_name)
  df %>% filter((!!col_name) == val)
}
filter_col(df, color, 'blue')

...但是如果您想使用字符串指定颜色列的名称怎么办？

... but what if you wanted to have the name of the color column specified by a string?

例如

column_name <- "color"
col_name <- quo(column_name) # <- something else goes here
df %>% filter((!!col_name) == "blue")

或调用签名为 filter_col（df， color， blue）的函数？

推荐答案

按照aosmith的链接，我们可以找到lukeA的答案...对此用例进行了修改：

Following aosmith's link takes us to lukeA's answer... which modified for this use case is:

library(dplyr)
library(rlang)
df <- data.frame( 
  color = c("blue", "black", "blue", "blue", "black"), 
  value = 1:5)

# In interactive mode
column_name <- rlang::sym("color")
df %>% filter((!!column_name) == "blue")

# In a function
filter_col <- function(df, col_name_as_string, val){
  col_name <- rlang::sym(col_name_as_string)
  df %>% filter((!!col_name) == val)
}
filter_col(df, 'color', 'blue')

关键部分是 rlang :: sym（）产生的对象可以被expa由非定元运算符 !! 找到。尽管此问题的答案是另一个问题的重复a>我将暂时搁置它，因为我认为这对问题的指定方式更为重要/对这个问题不能有太多正确的答案。 ：）

The key part is that rlang::sym() produces an object that can be expanded by the unquo operator !!. Although the answer to this question is a duplicate of another question I'm going to leave it for now because I believe this is a bit more on point in how the question is specified / there can't be too many right answers to this problem. :)

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