r - data.table 在 R 中复制表

Question

我正在这样做:

myfun <- function(inputvar_vec){
# inputvar_vec is input vector
# do something
# result = output vector
return(result)
}

DT[, result := lapply(.SD, myfun), by = byvar, .SDcols = inputvar]

我收到以下警告:

Warning message:
`In `[.data.table`(df1, , `:=`(prop, lapply(.SD, propEventInLastK)),  :
Invalid .internal.selfref detected and fixed by taking a copy of the whole table, 
so that     := can add this new column by reference. At an earlier point, this 
data.table has been copied by R (or been created manually using structure() 
or similar). (and then some more stuff) .... `

我的猜测是因为我正在堆叠 result 向量(在 by 操作之后)，正在制作副本？

谁能建议一种方法来消除此警告？我已经使用 apply 函数完成了此操作，并且认为它在这里也应该是可扩展的。

我的另一个问题是:您可以从数据框中传递一大块行(通过使用 by 语句进行子集化)，然后调用函数 myfun 对其进行操作吗？

根据要求添加示例

# generate data
N = 10000
default=NA
value = 1
df = data.table(id = sample(1:5000, N, replace=TRUE),
                trial = sample(c(0,1,2), N, replace=TRUE),
                ts = sample(1:200, N, replace=TRUE))

#set keys
setkeyv(df, c("id", "ts"))

df[["trial"]] = as.numeric(df[["trial"]]==value)

testfun <- function(x){
  L=length(x)
  x = x[L:1]
  x = fts(data=x)
  y = rep(default, L)
  if(L>=K){
    y1 = as.numeric(moving.sum(x,K))
    y = c(y1, rep(default,L-length(y1)))
  } 
  return(y[L:1]/K)
}

df[, prop:= lapply(.SD, testfun), by = id, .SDcols = "trial"]

仍然收到相同的警告消息:

Warning message:
In `[.data.table`(df, , `:=`(prop, lapply(.SD, testfun)), by = id,  :
  Invalid .internal.selfref detected and fixed by taking a copy of the whole table, so that := can add this new column by reference. At an earlier point, this data.table has been copied by R (or been created manually using structure() or similar). Avoid key<-, names<- and attr<- which in R currently (and oddly) may copy the whole data.table. Use set* syntax instead to avoid copying: setkey(), setnames() and setattr(). Also, list(DT1,DT2) will copy the entire DT1 and DT2 (R's list() copies named objects), use reflist() instead if needed (to be implemented). If this message doesn't help, please report to datatable-help so the root cause can be fixed.

Answer 1

问题出现在

df[["trial"]] = as.numeric(df[["trial"]]==value)

这不是data.table方法

data.table 方法是使用 :=

 df[, trial := as.numeric(trial == value)]

应该避免这个问题。

了解为什么要制作副本(因此内部自引用可能会无效)，请参阅 Understanding exactly when a data.table is a reference to (vs a copy of) another data.table

重要的是要认识到不存在 [[<-方法data.table因此[[<-.data.frame被调用，它将复制整个对象，而且不会做 data.table 那样的任何小心的事情。方法(例如 [<-.data.table )确实(返回有效的 data.table 。