data.table一致性分配参考n = 1行vs n> 1

Question

我有以下设置我想要剪切文本字符串，产生一个列表，然后重新指定为data.table作为嵌套列。它适用于n> 1行，但在n = 1行时不适用。我能做些什么来确保一致性：

library(data.table) 

cutdash <- function(x) {strsplit(x, '-')} 

## n > 1 row 
x <- data.frame(
    text = c("I paid for books-He was not interesting in his teaching.", 'good'), 
    id = 1:2, stringsAsFactors = FALSE) 

## n = 1 row 
y <- data.frame(text = "I paid for books-He was not interesting in his teaching.", 
    id = 3, stringsAsFactors = FALSE) 


## good 
x2 <- data.table::data.table(x) 
x2[, text := cutdash(text)][] 

##              text id 
## 1: I paid for books,He was not interesting in his teaching. 1 
## 2:              good 2 

## bad 
y2 <- data.table::data.table(y) 
y2[, text := cutdash(text)][] 

##    text id 
## 1: I paid for books 3 

## Warning message: 
## In `[.data.table`(y2, , `:=`(text, cutdash(text))) : 
## Supplied 2 items to be assigned to 1 items of column 'text' (1 unused) 

不是说第二个数据帧有1行只允许第一个元素被重新分配。

Answer 1

您需要list或.来包装你到str_split调用（如data.table [内部列表的别名，像这样：

y2[, text := .(cutdash(text))][] 

或

y2[, text := list(cutdash(text))][]