我有一个具有不同列名的向量,并且我希望能够遍历每个列名,以便从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.当我尝试使用cols的动态子集从mtcars中选择一个变量时,这些工作还是会
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如何获取这些值以返回与
how can I get these to return the same values as
mtcars$mpg此外,我如何遍历cols中的所有列以某种形式获取值.
Furthermore how can I loop over all the columns in cols to get the values in some sort of loop.
for(x in seq_along(cols)) { value <- mtcars[ order(mtcars$cols[x]), ] }推荐答案
您不能使用$进行这种子设置.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. 我们需要确保仅评估第一个参数. 第二个将是需要匹配的符号,无需评估. */
/*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?!您必须意识到$像R中的其他所有内容(包括例如(,+,^等)都是一个函数,它接受参数并被求值. df$V1可以改写为
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
`$`(df , V1)或者实际上
`$`(df , "V1")但是...
`$`(df , paste0("V1") )...例如将永远不起作用,也必须首先在第二个参数中求值的其他任何东西.您只能传递经过 从不 评估的字符串.
...for instance will never work, nor will anything else that must first be evaluated in the second argument. You may only pass a string which is never evaluated.
请改为使用[(如果要仅将单个列提取为向量,则请使用[[).
Instead use [ (or [[ if you want to extract only a single column as a vector).
例如,
var <- "mpg" #Doesn't work mtcars$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]使用do.call构造对order的调用,您可以执行不带循环的排序.这是下面的可重现示例:
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更多推荐
使用$和字符值动态选择数据框列
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