See https://en.m.wikipedia.org/wiki/Modulo_operation, Variants of the definition, esp the point that Knuth recommended the floor definition. The behavior of %/% follows from the definition of %% given the documented relation in ?Arithmetic.
R is not obligated to repeat the mistakes of C or Fortran. On December 19, 2022 7:15:01 AM PST, "Göran Broström" <g...@ehar.se> wrote: > > >Den 2022-12-19 kl. 15:41, skrev Martin Maechler: >>>>>>> Göran Broström >>>>>>> on Mon, 19 Dec 2022 14:22:00 +0100 writes: >> >> > I have a long vector x with five-digit codes where the >> > first digit of each is of special interest, so I extracted >> > them through >> >> >> y <- x %/% 10000 >> >> > but to my surprise y contained the value -1 in some >> > places. It turned out that x contains -1 as a symbol for >> > 'missing value' so in effect I found that >> >> >> -1 %/% 10000 == -1 >> >> > Had to check the help page for "%/%", and the first >> > relevant comment I found was: >> >> > "Users are sometimes surprised by the value returned". >> >> > No surprise there. Further down: >> >> > ‘%%’ indicates ‘x mod y’ (“x modulo y”) and ‘%/%’ >> > indicates integer division. It is guaranteed that >> >> > ‘ x == (x %% y) + y * (x %/% y) ’ (up to rounding >> > error) >> >> > I did expect (a %/% b) to return round(a / b), like >> > gfortran and gcc, >> >> What??? I cannot believe you. > >Well, you shouldn't, I generalized too far. >> >> No time for checking now, but I bet that >> 8 / 3 gives 2 and not 3 in C and Fortran >> (and hence gcc, etc) > >But compare -8 %/% 3 in R and -8 / 3 in C/Fortran. > >G, > >> >> >> > but instead I get floor(a / b) in >> > R. What is the reason for these different definitions? And >> > shouldn't R's definition be documented? >> >> >> >> > Thanks, Göran >> >> > ______________________________________________ >> > 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. -- Sent from my phone. Please excuse my brevity. ______________________________________________ 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.