data.table如何从j获取列名？ [英] How does data.table get the column name from j?
问题描述
例如：
dt <- data.table()
x=1:5
> dt[,list(2,3,x)]
V1 V2 x
1: 2 3 1
2: 2 3 2
3: 2 3 3
4: 2 3 4
5: 2 3 5
x
由于某种原因，我想创建一个函数来简化data.table的构造。
For some reason, I would like to create a function to simplify data.table construction.
tt <- function(a, b, ...){
list(a=sum(a), b=sum(b), ...)
}
> dt[,tt(1:2,1:3,x)]
a b V3
1: 3 6 1
2: 3 6 2
3: 3 6 3
4: 3 6 4
5: 3 6 5
code> list ，我改用 tt ，所以它自动为我插入预定义的列。
然而，现在它不能识别 x 的快捷方式命名。
So whenever I call list, I use tt instead, so it auto inserts predefined columns for me.
However, now it doesn't recognize the shortcut naming for x.
如何提高 tt 到数据表中的列表，如果不太难？
How to improve tt to auto name column like list in data.table if it is not too hard?
dt[,tt(1:2,1:3,x)]
返回
a b x
1: 3 6 1
2: 3 6 2
3: 3 6 3
4: 3 6 4
5: 3 6 5
解决方案
Solution
tt <- function(a, b, ...){
dots <- list(...)
inferred <- sapply(substitute(list(...)), function(x) deparse(x)[1])[-1]
if(is.null(names(inferred))){
names(dots) <- inferred
} else {
names(dots)[names(inferred) == ""] <- inferred[names(inferred) == ""]
}
c(a=sum(a), b=sum(b), dots)
}
dt <- data.table(c=1:5)
x=1:5
> dt[,tt(1:2,1:3,x,c+1)]
a b x c + 1
1: 3 6 1 2
2: 3 6 2 3
3: 3 6 3 4
4: 3 6 4 5
5: 3 6 5 6
> dt[,tt(1:2,1:3,x, z=c+1)]
a b x z
1: 3 6 1 2
2: 3 6 2 3
3: 3 6 3 4
4: 3 6 4 5
5: 3 6 5 6
更新
最近我发现在Venables& S的S编程的第46页有一些错误。 Ripley。我做了一些修改，并把它在这里。希望这对一些人是有用的。
Update
Recently I found that there was some bug in page 46 of S Programming from Venables & Ripley. I made some modifications and put it here. Hopefully it would be useful to some people.
# Get the best names vector for arguments like what data.frame does.
# Modified from page 46 of S Programming from Venables & Ripley.
# http://stackoverflow.com/questions/20545476/how-does-data-table-get-the-column-name-from-j
name.args <- function(...){
# Get a list of arguments.
dots <- as.list(substitute(list(...)))[-1]
# Get names of the members if they have, otherwise "".
# If a list have no named members, it returns NULL.
nm <- names(dots)
# If all arguments are named, return the names directly.
# Otherwise it would cause a problem when do nm[logic(0)] <- list().
if (!is.null(nm) && all(nm != ""))
return(nm)
# Handle empty argument list case.
if (length(dots) == 0)
return(character(0))
# Get positions of arguments without names.
fixup <-
if (is.null(nm))
seq(along=dots)
else
nm == ""
dep <- sapply(dots[fixup], function(x) deparse(x)[1])
if (is.null(nm))
dep
else {
nm[fixup] <- dep
nm
}
}
# Example
# x <- 1:2
# name.args(x, y=3, 5:6)
# name.args(x=x, y=3)
# name.args()
推荐答案
一个简单的解决方案是传递 参数：
A simple solution would be to pass in additional arguments as named rather than unnamed arguments:
dt[,tt(1:2,1:3,x=x)] ## Note that this uses `x=x` rather than just `x`
# a b x
# 1: 3 6 1
# 2: 3 6 2
# 3: 3 6 3
# 4: 3 6 4
# 5: 3 6 5
或者对于真正的懒惰，像这样的东西）
Or for the truly lazy, something like this ;)
tt <- function(a, b, ...){
dots <- list(...)
names(dots) <- as.character(substitute(list(...))[-1])
c(a=sum(a), b=sum(b), dots)
}
dt[,tt(1:2,1:3,x)]
# a b x
# 1: 3 6 1
# 2: 3 6 2
# 3: 3 6 3
# 4: 3 6 4
# 5: 3 6 5
这篇关于data.table如何从j获取列名？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！
