一些 J 表达式会保留列名,而有些则不会:
library(data.table)
d = data.table(hello = 1)
d[, .(hello)]
# hello
# 1: 1
d[, c(.(hello))]
# V1
# 1: 1
d[, {.(hello)}]
# hello
# 1: 1
d[, {1; .(hello)}]
# V1
# 1: 1
d[, .(get("hello"))]
# V1
# 1: 1
d[, mget("hello")]
# hello
# 1: 1
d[, c(mget("hello"))]
# Error: value for ‘hello’ not found # WTF?
d[, {1; mget("hello")}]
# hello
# 1: 1
一般规则是什么?
d[, c(mget("hello"))]
不起作用,但是这个d[, list(mget("hello"))]
可以。 - Taher A. Ghaleb