data.table中的动态列名称

Question

我正在尝试向我添加列data.table,其中名称是动态的.另外,我需要by在添加这些列时使用参数.例如:

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

---

马修更新:

这现在适用于R-Forge的v1.8.3.谢谢你的突出!
有关新示例,请参阅此类似问题:

按组使用data.table分配多个列

Answer 1

从data.table 1.9.4,你可以这样做:

## 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

见细节中?:=:

DT[i, (colvector) := val]

[...]现在首选[...]语法.parens足以阻止LHS成为一个象征; 与...一样c(colvector)

原始答案:

你是在正确的轨道上:构建一个在调用中被评估的表达式[.data.table是data.table方式来做这种事情.更进一步,为什么不构造一个表达式来计算整个 j参数(而不仅仅是它的左侧)？

像这样的东西应该做的伎俩:

## 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

Answer 2

可以用bquote构建表达式.

cn <- "blah"
expr <- bquote(.(as.name(cn)):=mean(a))
test_dtb[,eval(expr), by=id]