索引对于tibble和data.frame的工作方式不同。请参见this。

library(dplyr)
test <- tibble(
  id = c("John","Jacob","Jingleheimer","Schmidt"),
  score = c(2,4,6,8)
)
 
class(test[, 2])
## [1] "tbl_df"     "tbl"        "data.frame"

class(as.data.frame(test)[, 2])
## [1] "numeric"

基本区别在于 Dataframe 默认为drop = TRUE，而tibbles默认为drop = FALSE。

class(test[, 2])
## [1] "tbl_df"     "tbl"        "data.frame"

class(test[, 2, drop = FALSE]) # same
## [1] "tbl_df"     "tbl"        "data.frame"

class(test[, 2, drop = TRUE])
## [1] "numeric"

class(as.data.frame(test)[, 2])
## [1] "numeric"

class(as.data.frame(test)[, 2, drop = TRUE]) # same
## [1] "numeric"

class(as.data.frame(test)[, 2, drop = FALSE])
## [1] "data.frame"

另请注意

class(pull(test, 2))
## [1] "numeric"

class(test[[2]])
## [1] "numeric"

class(unlist(test[, 2]))
## [1] "numeric"

sapply(test, class)
##          id       score 
## "character"   "numeric"