modulus bug in R

Question

Possible Duplicate:
Why are these numbers not equal?

Just noticed this bug in R. I'm guessing it's the way 0.6 is represented, but anyone know exactly what's going on?

According to R:

0.3 %% 0.2 = 0.1
0.4 %% 0.2 = 0
0.5 %% 0.2 = 0.1
**0.6 %% 0.2 = 0.2**
0.7 %% 0.2 = 0.1
0.8 %% 0.2 = 0

What's going on?

Solution

In addition to @joshua Ulrich's comment

from ?'%%'

%% and x %/% y can be used for non-integer y, e.g. 1 %/% 0.2, but the results are subject to representation error and so may be platform-dependent. Because the IEC 60059 representation of 0.2 is a binary fraction slightly larger than 0.2, the answer to 1 %/% 0.2 should be 4 but most platforms give 5.

also similar to why we get this

> .1 + .1 + .1 == .3
[1] FALSE

as @Ben Boker pointed out, you may want to use something like

> 3:8 %% 2 / 10
[1] 0.1 0.0 0.1 0.0 0.1 0.0

Comments

[Ben Bolker]

and (as pointed out in a now-deleted answer) the 'solution' is to use integer arithmetic if possible: 6 %% 2 instead of 0.6 %% 0.2

[mike]

Yep, ended up doing that. Thanks.

[theEricStone]

this seems to work: Mod <- function(n,m){ if( m >= n ) return(n); n.digits <- nchar(strsplit(paste(n),".",fixed=T)[[1]][2]); div <- floor(n/m); rem <- n - (div * m); return(round(rem,n.digits)) }