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
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