data.table中的动态列名

Question

我正在尝试向我的data.table添加列，其中名称是动态的。另外，我需要在添加这些列时使用by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. 

的另一种尝试：从马修

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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更新：

这现在在v1.8.3的R-伪造。感谢您的突出！
见新的实例中，这类似的问题：

Assign multiple columns using data.table, by group

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 

见详细在?:=：

DT[i, (colvector) := val]

[...] NOW PREFERRED语法。这些包具足以阻止LHS成为一种象征;同c(colvector)

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原来的答复：

你是在完全正确的轨道：构建表达呼叫内进行评估，以[.data.table是做data.table方式这种事情。进一步说，为什么不构建一个表达式，其计算结果为整个参数（而不仅仅是其左手侧）？

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

表达可以用bquote构造。

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