On 28/11/2010 4:20 PM, Cory Rieth wrote:
Hello!
I stumbled upon something odd that took a while to track down, and I wanted to
run it by here to see if I should submit a bug report. For randomly generated
numbers (from a variety of distributions) rounding them to specifically 2
digits and then multiplying them by 100 produces strange results on about 8% of
cases. The problematic numbers display as I would have expected, but do not
logically match the as.integer counterpart (additionally they will not be used
correctly by functions such as rep()). I realize there are easy workarounds,
but I wouldn't have expected this result, and it only occurs rounding to 2
decimals, i.e. changing digits to 3 and multiplying by 1000 after rounding
gives the expected result.
This is presumably another version of FAQ 7.31. Very few numbers of the
form n/100 are exactly representable in floating point, and the rounding
error sometimes shows up when you multiply by 100. A simpler version of
this is the following:
> y <- 1:99
> y[(y/100)*100 != y]
[1] 7 14 28 29 55 56 57 58
Duncan Murdoch
x<-runif(100) #generate some random numbers
y<-round(x,digits=2)*100 #round them all to two decimals, then multiply them
all by 100. I expected the results to all be integers
sum(y!=as.integer(y)) #but on about 8% of the numbers they do not
match the integer version
x[which(y!=as.integer(y))] # a list of the problem numbers from the
original distribution. They seem to be more common but not exclusive to .54 to
.57
y[which(y!=as.integer(y))] #the numbers still display as would be expected,
i.e. they are integers
as.integer(y[which(y!=as.integer(y))]) # and sometimes display as the same
number they are not logically identical to
Thanks, and sorry if I came across something that is known, or it is meant to
behave this way, I couldn't find anything.
Cory Rieth
R.version() output:
platform x86_64-apple-darwin9.8.0
arch x86_64
os darwin9.8.0
system x86_64, darwin9.8.0
status
major 2
minor 12.0
year 2010
month 10
day 15
svn rev 53317
language R
version.string R version 2.12.0 (2010-10-15)
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and provide commented, minimal, self-contained, reproducible code.
