使用 $ 和字符值动态选择数据框列

本文介绍了使用 $ 和字符值动态选择数据框列的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我有一个包含不同列名的向量，我希望能够遍历每个列名以从 data.frame 中提取该列.例如，考虑数据集 mtcars 和一些存储在字符向量 cols 中的变量名称.当我尝试使用 cols 的动态子集从 mtcars 中选择一个变量时，这些都不起作用

I have a vector of different column names and I want to be able to loop over each of them to extract that column from a data.frame. For example, consider the data set mtcars and some variable names stored in a character vector cols. When I try to select a variable from mtcars using a dynamic subset of cols, nether of these work

cols <- c("mpg", "cyl", "am") col <- cols[1] col # [1] "mpg" mtcars$col # NULL mtcars$cols[1] # NULL

我怎样才能让这些返回与

how can I get these to return the same values as

mtcars$mpg

此外，我如何遍历 cols 中的所有列以获取某种循环中的值.

Furthermore how can I loop over all the columns in cols to get the values in some sort of loop.

for(x in seq_along(cols)) { value <- mtcars[ order(mtcars$cols[x]), ] }

推荐答案

你不能用 $ 做那种子集.在源代码 (R/src/main/subset.c) 中，它指出:

You can't do that kind of subsetting with $. In the source code (R/src/main/subset.c) it states:

/*$子集运算符.我们需要确保只评估第一个参数.第二个将是需要匹配而不是评估的符号.*/

/*The $ subset operator. We need to be sure to only evaluate the first argument. The second will be a symbol that needs to be matched, not evaluated. */

第二个论点?什么?！你必须意识到 $ 和 R 中的其他所有东西一样，(包括例如 ( , + , ^etc) 是一个函数，它接受参数并被评估.df$V1 可以重写为

Second argument? What?! You have to realise that $, like everything else in R, (including for instance ( , + , ^ etc) is a function, that takes arguments and is evaluated. df$V1 could be rewritten as

`$`(df , V1)

或者确实

`$`(df , "V1")

可是……

`$`(df , paste0("V1") )

...例如永远不会工作，其他任何必须首先在第二个参数中评估的东西也不会工作.您只能传递一个从不求值的字符串.

...for instance will never work, nor will anything else that must first be evaluated in the second argument. You may only pass a string which is never evaluated.

改为使用 [ (或 [[ 如果您只想提取单列作为向量).

Instead use [ (or [[ if you want to extract only a single column as a vector).

例如

var <- "mpg" #Doesn't work mtcars$var #These both work, but note that what they return is different # the first is a vector, the second is a data.frame mtcars[[var]] mtcars[var]

您可以不使用循环执行排序，使用do.call 来构造对order 的调用.下面是一个可重现的示例:

You can perform the ordering without loops, using do.call to construct the call to order. Here is a reproducible example below:

# set seed for reproducibility set.seed(123) df <- data.frame( col1 = sample(5,10,repl=T) , col2 = sample(5,10,repl=T) , col3 = sample(5,10,repl=T) ) # We want to sort by 'col3' then by 'col1' sort_list <- c("col3","col1") # Use 'do.call' to call order. Seccond argument in do.call is a list of arguments # to pass to the first argument, in this case 'order'. # Since a data.frame is really a list, we just subset the data.frame # according to the columns we want to sort in, in that order df[ do.call( order , df[ , match( sort_list , names(df) ) ] ) , ] col1 col2 col3 10 3 5 1 9 3 2 2 7 3 2 3 8 5 1 3 6 1 5 4 3 3 4 4 2 4 3 4 5 5 1 4 1 2 5 5 4 5 3 5