遍历R中列名的特定子集 [英] Looping over a specific subset of column names in R

问题描述

我有一个数据框AData，我已经提取了它的列名称的一个子集，说SpecialName。我想知道如何在for循环中引用这些列。

我现在的代码如下所示：

< pre $ SpecialNames< - setdiff（colnames（AData），colnames（BData））

for（i in SpecialNames）{

AData $ i< - NULL＃对AData $ i做某些事情，例如删除它或其他东西

}


唉，AData $我似乎并没有引用名称为i的数据框AData的列。有没有一种不同的语法可以给我这个？

我在这篇文章中读到指出：$是用于交互式的使用，而是在编程时，也就是当列名需要使用[或[[，因此我用示例[[paste0（i，'.impt'）]]替换了示例$ i.imp。
$ b $基于这个评论，我猜想，我一直在寻找的语法可能是AData $ [i]或AData $ [[i]]或AData $ [[paste0（i）]]，但是没有一个看起来像这样任何想法？

解决方案

不知道是什么你在做什么，很难说是否是循环的路径。然而，希望这将有助于你的方式：

  ##示例数据总是很好
 set.seed （1）
 mydf < -  data.frame（A = c（1,1,1,2,2,3,3,3,4,4,4），
 B =字母[c（1,1,1,1,2,2,2,2,2,2,2）]，
矩阵（样本（100,36，替换= TRUE），nrow = 12 ））
 
 ##这是您的特殊向量名称
特殊名称<  -  setdiff（名称（mydf），c（A，B））
 
 ##这是一个`for`循环，它将打印在specialnames中命名的每列的前两行
 ## 
 ##这不是我通常会在R中执行的
 ## -------------------------------------------- ----------- 
 for（i in seq_along（specialnames））{
 print（head（mydf [specialnames [i]]，2））
} 
  

---

注意事项（也许）：

$ b $ ul

for（i in seq_along（specialnames））： seq_along （或 i in 1：length（sp eicamenames）或类似的东西）是很重要的。

你似乎误解了 [和 [[。尝试以下方法来了解它们的作用：

• mydf [A]


• mydf [[A]]


• mydf [ 1，c（A，B）]



• 在这里。

I have a dataframe AData that of which I have extracted a certain subset of its column names say SpecialNames. I would like to know how to reference these columns in a for loop.

My current code looks like this:

SpecialNames <- setdiff(colnames(AData), colnames(BData))

for ( i in SpecialNames ) {

    AData$i <- NULL # Do something to AData$i such as delete it or something else

}

Alas, AData$i does not seem to reference the column of dataframe AData with name i. Is there a different syntax that would give me that?

I read in this post here that: "the $ is for interactive usage. Instead, when programming, i.e. when the column name is to be interpreted, you need to use [ or [[, hence I replaced sample$i.imp with sample[[paste0(i, '.impt')]]".

Based on this comment, I guessed that perhaps the syntax I have been looking for is AData$[i] or AData$[[i]] or AData$[[paste0(i)]] but none of these seem to work either.

Any ideas?

解决方案

Not knowing what you are doing, it's hard to say whether a for loop is the way to go or not; however, hopefully this will help get you on your way:

## Sample data is always nice
set.seed(1)
mydf <- data.frame(A = c(1, 1, 1, 2, 2, 3, 3, 3, 3, 4, 4, 4),
                   B = LETTERS[c(1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2)],
                   matrix(sample(100, 36, replace = TRUE), nrow = 12))

## Here is your vector of special names
specialnames <- setdiff(names(mydf), c("A", "B"))

## Here is a `for` loop that will print the first two rows
##   of each column named in "specialnames"
## THIS IS NOT HOW I WOULD NORMALLY DO THIS IN R
## -------------------------------------------------------
for (i in seq_along(specialnames)) {
  print(head(mydf[specialnames[i]], 2))
}

---

Matters of note (perhaps):

• for (i in seq_along(specialnames)): That seq_along (or i in 1:length(specialnames) or something like that) is important.
• You seem to have misunderstood the use of [ and [[. Try the following to get a sense of what they do:
  • mydf["A"]
  • mydf[["A"]]
  • mydf[1, c("A", "B")]
• Two questions to look at here and here.

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