[R] understanding all.equal() output: "Mean relative difference"

[S Ellison]

The help page says that large differences are returned as relative difference. A look at the code shows that all.equal.numeric, the version used for a numeric first argument, uses the 'target', that is, the first parametter, as the scaling factor. Thus

c(1,4) has a difference of 3, scaled by 1, giving 3
c(2,4) has a difference of 2, scaled by 2, giving 1

... and so on.

The other potentially useful part of the help page is the bit right at the beginning than says 
"Don't use 'all.equal' directly in 'if'
expressions-either use 'isTRUE(all.equal(....))' or 'identical' if
appropriate."

which usually avoids the need to parse the result at all. 



> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Liviu Andronic
> Sent: 24 November 2011 12:45
> To: r-help at r-project.org Help
> Subject: [R] understanding all.equal() output: "Mean relative 
> difference"
> 
> Dear all
> How should one parse all.equal() output? I'm specifically 
> referring to the 'mean relative difference' messages. For example,
> > all.equal(pi, 355/113)
> [1] "Mean relative difference: 8.491368e-08"
> 
> But I'm not sure how to understand these messages. When 
> they're close to 0 (or 1xe-16), then it's intuitive. But when 
> they're big,
> > all.equal(1, 4)
> [1] "Mean relative difference: 3"
> > all.equal(2, 4)
> [1] "Mean relative difference: 1"
> > all.equal(3, 4)
> [1] "Mean relative difference: 0.3333333"
> 
> the messages start making much less sense. I tried Wikipedia 
> [1], but the description is cryptic, as is the help page. 
> Also, Fox and Weisberg (2011) don't explain this particular message.
> 
> Regards
> Liviu
> 
> [1] 
> http://en.wikipedia.org/wiki/Mean_difference#Relative_mean_difference
> 
> 
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