> -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > On Behalf Of Ying Zheng > Sent: Wednesday, November 28, 2012 6:11 PM > To: r-help@r-project.org > Subject: [R] Ceiling function gives me wrong answer > > Hi all, > > I have a very simple code, but gives a wrong answer. > > a=1000 > b=1000^(1/3) > c=ceiling(a/b) > > then c=101, > > if change the code to be > a=1000 > b=10 > c=ceiling(a/b) > > then c=100 is fine. > > Thank you for the help. >
This is another variation on FAQ 7.31. You are trying to calculate the cube root using a power of 1/3. But 1/3 cannot be represented exactly in a finite binary floating-point system. If you print the value of b with enough digits you will see that it is not equal to 10 because the representation of the value 1/3 ends up being slightly smaller that one-third. > print(1/3, digits=20) [1] 0.33333333333333331483 > > b <- 1000^(1/3) > print(b, digits=20) [1] 9.9999999999999982236 Hope this is helpful, Dan Daniel Nordlund Bothell, WA USA ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
