data.table中的动态列名称 [英] Dynamic column names in data.table

问题描述

我试图将列添加到我的 data.table 中，其中的名称是动态的。另外，添加这些列时，我需要使用 by 参数。例如：

I am trying to add columns to my data.table, where the names are dynamic. I addition I need to use the by argument when adding these columns. For example:

test_dtb <- data.table(a = sample(1:100, 100), b = sample(1:100, 100), id = rep(1:10,10))
cn <- parse(text = "blah")
test_dtb[ , eval(cn) := mean(a), by = id]

# Error in `[.data.table`(test_dtb, , `:=`(eval(cn), mean(a)), by = id) : 
#  LHS of := must be a single column name when with=TRUE. When with=FALSE the LHS may be a vector of column names or positions.

另一种尝试：

cn <- "blah"
test_dtb[ , cn := mean(a), by = id, with = FALSE]
# Error in `[.data.table`(test_dtb, , `:=`(cn, mean(a)), by = id, with = FALSE) : 'with' must be TRUE when 'by' or 'keyby' is provided

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Matthew的更新：

现在可以在R-Forge的v1.8.3中使用。感谢突出显示！

请参见以下类似示例的新示例：

This now works in v1.8.3 on R-Forge. Thanks for highlighting!
See this similar question for new examples:

使用data.table，按组分配多列

推荐答案

从 data.table 1.9.4 ，您可以执行以下操作：

From data.table 1.9.4, you can just do this:

## A parenthesized symbol, `(cn)`, gets evaluated to "blah" before `:=` is carried out
test_dtb[, (cn) := mean(a), by = id]
head(test_dtb, 4)
#     a  b id blah
# 1: 41 19  1 54.2
# 2:  4 99  2 50.0
# 3: 49 85  3 46.7
# 4: 61  4  4 57.1

请参见？：= ：

DT [i，（colvector）：= val]

[...现在使用首选语法。括号足以阻止LHS成为符号。与 c（colvector）

[...] NOW PREFERRED [...] syntax. The parens are enough to stop the LHS being a symbol; same as c(colvector)

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原始答案：

您在正确的轨道上：构造一个表达式，以在对 [。data.table 是执行这种操作的 data.table 方法。再说一点，为什么不构造一个以 entire j 为参数的表达式（而不只是其左手）？

You were on exactly the right track: constructing an expression to be evaluated within the call to [.data.table is the data.table way to do this sort of thing. Going just a bit further, why not construct an expression that evaluates to the entire j argument (rather than just its left hand side)?

类似这样的方法应该可以解决问题：

Something like this should do the trick:

## Your code so far
library(data.table)
test_dtb <- data.table(a=sample(1:100, 100),b=sample(1:100, 100),id=rep(1:10,10))
cn <- "blah"

## One solution
expr <- parse(text = paste0(cn, ":=mean(a)"))
test_dtb[,eval(expr), by=id]

## Checking the result
head(test_dtb, 4)
#     a  b id blah
# 1: 30 26  1 38.4
# 2: 83 82  2 47.4
# 3: 47 66  3 39.5
# 4: 87 23  4 65.2

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