R Tip: Introduce Indices to Avoid for() Class Loss Issues

March 8, 2018

[This article was first published on R – Win-Vector Blog, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

Here is an R tip. Use loop indices to avoid for()-loops damaging classes.

Below is an R annoyance that occurs again and again: vectors lose class attributes when you iterate over them in a for()-loop.

d <- c(Sys.time(), Sys.time())
#> [1] "2018-02-18 10:16:16 PST" "2018-02-18 10:16:16 PST"

for(di in d) {
#> [1] 1518977777
#> [1] 1518977777

Notice we printed numbers, not dates/times. To avoid this problem introduce an index, and loop over that, not over the vector contents.

for(ii in seq_along(d)) {
  di <- d[[ii]]
#> [1] "2018-02-18 10:16:16 PST"
#> [1] "2018-02-18 10:16:16 PST"

seq_along() is a handy function similar to what we discussed in R Tip: Use seq_len() to Avoid The Backwards List Trap.

The introduction of indices is ugly, as index-free iteration is generally superior. Also, as we have mentioned before, for-loops should not be considered anathema in R– they are a useful tool when used correctly.

Note base::ifelse() also loses class attributes, though dplyr::if_else() avoids the problem. Also base::lapply() and base::vapply() do not have the problem (for example try: vapply(d, as.character, character(1)) and lapply(d, class)).

In both cases R is treating a vector of numbers as a complex class by adding a class attr to the vector. This means the vector is a single object holding multiple times, not a list of individual time objects. Any subsetting that strips attr values loses the class information and the derived vector reverts to its underlying type (in this case double).

For pre-allocation ideas (an important compliment to for-loops) please see R Tip: Use vector() to Pre-Allocate Lists (also includes some discussion of for-loops).

To leave a comment for the author, please follow the link and comment on their blog: R – Win-Vector Blog.

R-bloggers.com offers daily e-mail updates about R news and tutorials about learning R and many other topics. Click here if you're looking to post or find an R/data-science job.
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

If you got this far, why not subscribe for updates from the site? Choose your flavor: e-mail, twitter, RSS, or facebook...

Comments are closed.

Search R-bloggers


Never miss an update!
Subscribe to R-bloggers to receive
e-mails with the latest R posts.
(You will not see this message again.)

Click here to close (This popup will not appear again)