将数据添加到空r data.frame

Question

In R, If I make a data.frame with one column, I can added others

> data <- data.frame(n=c(1:4))
> data
  n
1 1
2 2
3 3
4 4
> data$n2 <- 2
> data
  n n2
1 1  2
2 2  2
3 3  2
4 4  2

but, If I make a empty data.frame, I can't add new columns

> data <- data.frame()
> data
data frame with 0 columns and 0 rows
> data$n2 <- 2
Error in `$<-.data.frame`(`*tmp*`, "n2", value = 2) : 
  replacement has 1 row, data has 0

why ? how I can add new columns to empty data.frame ?

Answer 1

You can add a column to an empty data.frame, but to match the existing data.frame's dimensions, the assigned vector needs to have length zero:

data <- data.frame()
data$n2 <- numeric()
data
# [1] n2
# <0 rows> (or 0-length row.names)

(In your first example, although the value on the RHS of the assignment didn't have the same length as the existing columns, it was "recycled" to form a column of the necessary length. When the existing data.frame has no rows, though, recycling can't be used to make the column lengths match.)