Hi Avi, Definitely a learning moment. I may consider writing an ifElse() for my own use and sharing it if anyone wants it.
Jim On Sun, Oct 10, 2021 at 6:36 AM Avi Gross via R-help <r-help@r-project.org> wrote: > > This is supposed to be a forum for help so general and philosophical > discussions belong elsewhere, or nowhere. > > > > Having said that, I want to make a brief point. Both new and experienced > people make implicit assumptions about the code they use. Often nobody looks > at how the sausage is made. The recent discussion of ifelse() made me take a > look and I was not thrilled. > > > > My NAÏVE view was that ifelse() was implemented as a sort of loop construct. > I mean if I have a vector of length N and perhaps a few other vectors of the > same length, I might say: > > > > result <- ifelse(condition-on-vector-A, result-if-true-using-vectors, > result-if-false-using-vectors) > > > > So say I want to take a vector of integers from 1 to N and make an output a > second vector where you have either a prime number or NA. If I have a > function called is.prime() that checks a single number and returns > TRUE/FALSE, it might look like this: > > > > primed <- ifelse(is.prime(A, A, NA) > > > > So A[1] will be mapped to 1 and A[2} to 2 and A[3] to 3, but A[4] being > composite becomes NA and so on. > > > > If you wrote the above using loops, it would be to range from index 1 to N > and apply the above. There are many complications as R allows vectors to be > longer or to be repeated as needed. > > > > What I found ifelse() as implemented to do, is sort of like this: > > > > Make a vector of the right length for the results, initially empty. > > > > Make a vector evaluating the condition so it is effectively a Boolean > result. > > Calculate which indices are TRUE. Secondarily, calculate another set of > indices that are false. > > > > Calculate ALL the THEN conditions and ditto all the ELSE conditions. > > > > Now copy into the result all the THEN values indexed by the TRUE above and > than all the ELSE values indicated by the FALSE above. > > > > In plain English, make a result from two other results based on picking > either one from menu A or one from menu B. > > > > That is not a bad algorithm and in a vectorized language like R, maybe even > quite effective and efficient. It does lots of extra work as by definition > it throws at least half away. > > > > I suspect the implementation could be made much faster by making some of it > done internally using a language like C. > > > > But now that I know what this implementation did, I might have some qualms > at using it in some situations. The original complaint led to other > observations and needs and perhaps blindly using a supplied function like > ifelse() may not be a decent solution for some needs. > > > > I note how I had to reorient my work elsewhere using a group of packages > called the tidyverse when they added a function to allow rowwise > manipulation of the data as compared to an ifelse-like method using all > columns at once. There is room for many approaches and if a function may not > be doing quite what you want, something else may better meet your needs OR > you may want to see if you can copy the existing function and modify it for > your own personal needs. > > > > In the case we mentioned, the goal was to avoid printing selected warnings. > Since the function is readable, it can easily be modified in a copy to find > what is causing the warnings and either rewrite a bit to avoid them or start > over with perhaps your own function that tests before doing things and > avoids tripping the condition (generating a NaN) entirely. > > > > Like may languages, R is a bit too rich. You can piggyback on the work of > others but with some caution as they did not necessarily have you in mind > with what they created. > > > > > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
