Extract a dplyr tbl column as a vector

Question

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)?

Answer 1

As per the comment from @nacnudus, it looks like a pull function was implemented in dplyr 0.6:

iris2 %>% pull(Species)

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For older versions of dplyr, here's a neat function to make pulling out a column a bit nicer (easier to type, and easier to read):

pull <- function(x,y) {x[,if(is.name(substitute(y))) deparse(substitute(y)) else y, drop = FALSE][[1]]}

This lets you do either of these:

iris2 %>% pull('Species')
iris2 %>% pull(Species)
iris2 %>% pull(5)

Resulting in...

 [1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4 10.4 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3 26.0 30.4 15.8 19.7 15.0 21.4

And it also works fine with data frames:

> mtcars %>% pull(5)
 [1] 3.90 3.90 3.85 3.08 3.15 2.76 3.21 3.69 3.92 3.92 3.92 3.07 3.07 3.07 2.93 3.00 3.23 4.08 4.93 4.22 3.70 2.76 3.15 3.73 3.08 4.08 4.43
[28] 3.77 4.22 3.62 3.54 4.11

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A nice way to do this in v0.2 of dplyr:

iris2 %>% select(Species) %>% collect %>% .[[5]]

Or if you prefer:

iris2 %>% select(Species) %>% collect %>% .[["Species"]]

Or if your table isn't too big, simply...

iris2 %>% collect %>% .[["Species"]]