[R] Interesting quirk with fractions and rounding

[Nordlund, Dan (DSHS/RDA)]

This is FAQ 7.31.  It is not a bug, it is the unavoidable problem of accurately representing floating point numbers with a finite number of bits of precision.  Look at the following:

> a <- 100*(23/40)
> b <- (100*23)/40
> print(a,digits=20)
[1] 57.499999999999993
> print(b,digits=20)
[1] 57.5
>

Your example with all.equal evaluates TRUE because all.equal uses a 'fuzz factor'.  From the all.equal man page

"all.equal(x, y) is a utility to compare R objects x and y testing 'near equality'."


Hope this is helpful,

Dan

Daniel Nordlund, PhD
Research and Data Analysis Division
Services & Enterprise Support Administration
Washington State Department of Social and Health Services


> -----Original Message-----
> From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Paul
> Johnson
> Sent: Thursday, April 20, 2017 2:56 PM
> To: R-help
> Subject: [R] Interesting quirk with fractions and rounding
> 
> Hello, R friends
> 
> My student unearthed this quirk that might interest you.
> 
> I wondered if this might be a bug in the R interpreter. If not a bug,
> it certainly stands as a good example of the dangers of floating point
> numbers in computing.
> 
> What do you think?
> 
> > 100*(23/40)
> [1] 57.5
> > (100*23)/40
> [1] 57.5
> > round(100*(23/40))
> [1] 57
> > round((100*23)/40)
> [1] 58
> 
> The result in the 2 rounds should be the same, I think.  Clearly some
> digital number devil is at work. I *guess* that when you put in whole
> numbers and group them like this (100*23), the interpreter does
> integer math, but if you group (23/40), you force a fractional
> division and a floating point number. The results from the first 2
> calculations are not actually 57.5, they just appear that way.
> 
> Before you close the books, look at this:
> 
> > aa <- 100*(23/40)
> > bb <- (100*23)/40
> > all.equal(aa,bb)
> [1] TRUE
> > round(aa)
> [1] 57
> > round(bb)
> [1] 58
> 
> I'm putting this one in my collection of "difficult to understand"
> numerical calculations.
> 
> If you have seen this before, I'm sorry to waste your time.
> 
> pj
> --
> Paul E. Johnson   http://pj.freefaculty.org
> Director, Center for Research Methods and Data Analysis
> http://crmda.ku.edu
> 
> To write to me directly, please address me at pauljohn at ku.edu.
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.