What's the difference between using select + unlist from dplyr package and using the dollar sign?

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I've been taking an online course in which the instructor always does the following to obtain, say, the column Col1 from a data.frame object Dat:

library(dplyr)
unlist(select(Dat, Col1))

Why not simply run Dat$Col1? I notice a difference in the "presentation" of both results, but is there any other significant divergence between the two forms? Any operation will result in the same product for both?

dplyr r
1 Answers

(Posting comments as community wiki.)

These are not quite equivalent - unlist(select(.)) keeps (probably unwanted) names.

dd <- data.frame(Col1=c("abc","def"))
str(unlist(select(dd,Col1)))
##  Factor w/ 2 levels "abc","def": 1 2
##  - attr(*, "names")= chr [1:2] "Col11" "Col12"
str(dd$Col1)
##  Factor w/ 2 levels "abc","def": 1 2

Your instructor is probably just a fan of the tidyverse (@RichScriven); pull(Dat, Col1) or (for extreme "tidiness") Dat %>% pull(Col1) would be more idiomatic (@Henrik). Dat$Col1 or Dat[["Col1"]] would be the base-R equivalents (the former is more convenient for interactive use, the latter is marginally safer for programming purposes since it won't do name-completion).

It hardly matters, but the tidyverse approaches are much slower.

microbenchmark(dd$Col1,dd[["Col1"]],pull(dd,Col1),unlist(select(dd,Col1)))
Unit: microseconds
                     expr     min        lq       mean    median       uq
                  dd$Col1   5.296   10.9630   14.86871   13.4040   17.160
             dd[["Col1"]]   7.870    9.6535   15.18874   11.8270   16.635
           pull(dd, Col1)  44.160  108.7625  128.89342  117.8415  136.890
 unlist(select(dd, Col1)) 601.480 1132.8240 1436.44178 1214.4420 1378.141
      max neval cld
   31.036   100  a 
   88.842   100  a 
  422.462   100  a 
 8796.964   100   b
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