从括号内反映data.table colulmn名称 [英] Refencing data.table colulmn names from within brackets

问题描述

我想创建一个函数startswith在数据表中的括号内使用，它应该返回一个包含以所提供的字符开头的列名的字符向量，例如

  DT < -  data.table（x = 1，y = 2，z1 = 1，z2 = 2）
＃语法DT [ ，startswith（z）]相当于
 DT [，。（z1，z2）] 
＃返回
 z1 z2 
 1：1 2 
  

我熟悉grep搜索文本表达式，但是我找不到一种方法来引用列名DT从一个方括号内。我尝试的一个解决方案是使用ls（）和与DT相关的环境来列出DT中的所有列，但我还没有找到一种方法来引用括号内的这个环境。 / p>

目标是为grep创建一个用于方便的函数的包装，我不想在括号内指定DT。

解决方案

当然有一个更惯用的方法，但这是我想出的：

  startswith<  -  function（pattern =z）{
 
 re<  -  paste0（^，pattern）
 
 call_info<  -  deparse（sys.calls（）[[1]]）
 
 if（grepl（（^。+ \\（） \\\）$），call_info））{
 this_name < -  sub（（^。+ \\（）（。+）（\\）$），\ \2，call_info）
} else {
 this_name < -  strsplit（call_info，\\ [）[[1]] [1] 
} 
 
 this<  -  copy（get（this_name））
 this_names<  -  names（this）
 
 eval.parent（grep（re，this_names））
 
} 
  

---

  library（data.table）
 DT<  -  data.table（x = 1，y = 2，z1 = 1，z2 = 2）
 ## 
 R& ; DT [，。（z1，z2）] 
 z1 z2 
 1：1 2 
 ## 
 R& DT [，startswith（），with = F] 
 z1 z2 
 1：1 2 
  

$ b b

---

我不得不添加 if（）{} else {} 的功能，例如

  Foo < -  function（gt）{
f<  -  gt [，startswith（），with = F ] 
＃{do something interesting with f} 
f 
} 
 ## 
 R> Foo（DT）
 z1 z2 
 1：1 2 
  

---

我认为这是一个有趣的问题，据我所知，R没有一个概念 C ++中的这个指针，但它在这种情况下肯定是有用的。基本上，我的所有我的黑客与 sys.call ， get ，等...只是这样我可以检索调用对象的名称。

I'd like to create a function "startswith' to be used within brackets in data.table. It should return a character vector containing the column names that begin with the character provided. For example

DT <- data.table(x=1, y=2, z1=1, z2=2)
# the syntax DT[, startswith("z")] is equivalent to  
DT[, .(z1, z2)]
# returns
   z1 z2
1:  1  2

I'm familiar with grep to search for text expressions, but am having trouble finding a way to refer to the column names of DT from within the brackets. One solution I attempted was to use ls() and the environment associated with DT to list all of the columns in DT, but I haven't found a way to refer to this environment from within the brackets.

The goal is to create a wrapper for grep to be used as a convenience function. I don't want to have to specify the DT from within the brackets.

解决方案

Surely there is a more idiomatic approach, but this is what I came up with:

startswith <- function(pattern = "z") {

  re <- paste0("^", pattern)

  call_info <- deparse(sys.calls()[[1]])

  if (grepl("(^.+\\()(.+)(\\)$)",call_info)) {
    this_name <- sub("(^.+\\()(.+)(\\)$)","\\2",call_info)
  } else {
    this_name <- strsplit(call_info,"\\[")[[1]][1]
  }

  this <- copy(get(this_name))
  this_names <- names(this)

  eval.parent(grep(re,this_names))

}

---

library(data.table)
DT <- data.table(x=1, y=2, z1=1, z2=2)
##
R> DT[,.(z1, z2)]
   z1 z2
1:  1  2
##
R> DT[,startswith(), with=F]
   z1 z2
1:  1  2

---

I had to add in that if () {} else {} block so that this could be used inside of functions, e.g.

Foo <- function(gt) {
  f <- gt[,startswith(),with=F]
  # {do something interesting with f}
  f
}
##
R> Foo(DT)
   z1 z2
1:  1  2

---

I think this is an interesting question though - to my knowledge, R doesn't have a concept of something like the this pointer in C++, but it would certainly be useful in situations like this. Essentially, all of my hackery with sys.call, get, etc... was just so I could retrieve the names of the calling object.

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