从R data.table中除去多个列，并使用要除去的列的参数 [英] Removing multiple columns from R data.table with parameter for columns to remove

问题描述

我试图以类似的方式操纵一些data.tables，并且想写一个函数来完成这个。我想传递一个参数，该参数包含将执行操作的列的列表。当列的向量声明为：=运算符的左侧时，这种方式工作正常，但如果先前声明（或传递到函数中），则不行。以下代码显示了问题。

  dt = data.table（a = letters，b = 1：2，c = 1 ：13）
 colsToDelete = c（'b'，'c'）
 dt [，colsToDelete：= NULL]＃不工作，但我不明白为什么不。 
 dt [，c（'b'，'c'）：= NULL]＃工作正常，但不允许传递列
  

错误是添加新列colsToDelete'，然后赋值NULL（删除它）。

当沿着这些行做事时会出现同样的问题

  dt [，colNames：= lapply（.SD，adjustValue，y = factor），.SDcols = colNames] 
  

我是R的新手，但更喜欢使用其他语言，所以这可能是一个愚蠢的问题。

：= 的LHS上的符号来添加新的列，为了方便起见：例如： DT [，col：= val] 。因此，为了区分 col 本身是来自存储在 col 中作为列名称的名称，我们检查如果LHS是名称或表达式。

如果是 name ，它会在LHS中添加具有这样名称的列，如果 expression 它得到计算。

  DT [，col：= val]＃col是列名。 
 
 DT [，（col）：= val]＃col获得计算并替换为其值
 DT [，c（col）：= val]＃与上面相同
  

首选习语是： dt [，（colsToDelete）：= NULL]

HTH

I'm trying to manipulate a number of data.tables in similar ways, and would like to write a function to accomplish this. I would like to pass in a parameter containing a list of columns that would have the operations performed. This works fine when the vector declaration of columns is the left hand side of the := operator, but not if it is declared earlier (or passed into the function). The follow code shows the issue.

dt = data.table(a = letters, b = 1:2, c=1:13)
colsToDelete = c('b', 'c')
dt[,colsToDelete := NULL] # doesn't work but I don't understand why not.
dt[,c('b', 'c') := NULL] # works fine, but doesn't allow passing in of columns

The error is "Adding new column 'colsToDelete' then assigning NULL (deleting it)." So clearly, it's interpreting 'colsToDelete' as a new column name.

The same issue occurs when doing something along these lines

dt[, colNames := lapply(.SD, adjustValue, y=factor), .SDcols = colNames]

I new to R, but rather more experienced with some other languages, so this may be a silly question.

解决方案

It's basically because we allow symbols on LHS of := to add new columns, for convenience: ex: DT[, col := val]. So, in order to distinguish col itself being the name from whatever is stored in col being the column names, we check if the LHS is a name or an expression.

If it's a name, it adds the column with the name as such on the LHS, and if expression, then it gets evaluated.

DT[, col := val] # col is the column name.

DT[, (col) := val]  # col gets evaluated and replaced with its value
DT[, c(col) := val] # same as above

The preferred idiom is: dt[, (colsToDelete) := NULL]

HTH

这篇关于从R data.table中除去多个列，并使用要除去的列的参数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！