unexpected behavior using round to 2 digits on randomly generated numbers
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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