[R] Integer division
gb @end|ng |rom eh@r@@e
Mon Dec 19 16:15:01 CET 2022
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.
> > 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
> > ______________________________________________
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