[R] odd behaviour of %%?
Jonathan Williams
jonathan.williams at pharmacology.oxford.ac.uk
Wed Nov 22 11:14:40 CET 2006
Dear R Helpers,
I am trying to extract the modulus from divisions by a sequence of
fractions.
I noticed that %% seems to behave inconsistently (to my untutored eye),
thus:
> 0.1%%0.1
[1] 0
> 0.2%%0.1
[1] 0
> 0.3%%0.1
[1] 0.1
> 0.4%%0.1
[1] 0
> 0.5%%0.1
[1] 0.1
> 0.6%%0.1
[1] 0.1
> 0.7%%0.1
[1] 0.1
> 0.8%%0.1
[1] 0
> 0.9%%0.1
The modulus for 0.1, 0.2, 0.4 and 0.8 is zero, as I'd expect. But, the
modulus
for 0.3, 0.6, 0.7 and 0.9 is 0.1 - which I did not expect. I can see no
obvious
rule that predicts whether x%%0.1 will give an answer of 0 or 0.1. I could
find
no explanation of the way that %% works in the R manuals. So, I have 3
questions:-
1) Why is the modulus of 0.3%%0.1 (and 0.5%%0.1 and 0.6%%0.1...) not zero?
2) Are there any algorithms in R that use the %% operator with fractional
divisors
in this way, and do they know about its apparently inconsistent behaviour?
3) If %% is not intended for use with fractional divisors, then would it be
a
good idea to trap attempts to use them?
Thanks, in advance, for your help
Jonathan Williams
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