Accessing the ith element in the column bar in the dataframe foo in R can be done in two different ways:
foo[i,"bar"]
and
foo$bar[i].
Is there any difference between them? If so, which one should be used in terms of efficiency, readability, etc.?
Apologies if this has already been asked, but [] and $ characters are very elusive.
I tend to think this is an opinion based question, and therefore inappropriate for SO. But since you ask for speed considerations, I won't flag it as such. Note: There are more than the two methods you describe for indexing...
data(mtcars)
library(microbenchmark)
microbenchmark(opt_a= mtcars$disp[12],
opt_b= mtcars[12, "disp"],
opt_c= mtcars[["disp"]][12])
Unit: microseconds
expr min lq mean median uq max neval cld
opt_a 5.322 6.4620 8.34029 6.8425 7.603 56.640 100 a
opt_b 9.503 10.0735 15.41463 10.6435 11.024 354.285 100 b
opt_c 4.181 4.942 7.77386 5.322 6.082 84.009 100 a
using foo$bar[i] appears to be considerably faster than foo[i, "bar"] but not the fastest alternative
