提取 dplyr tbl 列作为向量 [英] Extract a dplyr tbl column as a vector

问题描述

有没有更简洁的方法可以从具有数据库后端的 tbl 中获取 dplyr tbl 的一列作为向量(即数据框/表不能直接作为子集)?

Is there a more succinct way to get one column of a dplyr tbl as a vector, from a tbl with database back-end (i.e. the data frame/table can't be subset directly)?

require(dplyr)
db <- src_sqlite(tempfile(), create = TRUE)
iris2 <- copy_to(db, iris)
iris2$Species
# NULL

那太简单了，所以

collect(select(iris2, Species))[, 1]
# [1] "setosa"     "setosa"     "setosa"     "setosa"  etc.

不过好像有点笨拙.

推荐答案

使用 dplyr >= 0.7.0，您可以使用 pull 从 tbl 获取向量>.

With dplyr >= 0.7.0, you can use pull to get a vector from a tbl.

library("dplyr")
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
db <- src_sqlite(tempfile(), create = TRUE)
iris2 <- copy_to(db, iris)
vec <- pull(iris2, Species)
head(vec)
#> [1] "setosa" "setosa" "setosa" "setosa" "setosa" "setosa"

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