[R] Odp: precision issue?

[Petr PIKAL]

Hi

r-help-bounces at r-project.org napsal dne 04.03.2010 10:36:43:

> 
> Hi R Gurus,
> 
> I am trying to figure out what is going on here.
> 
> > a <- 68.08
> > b <- a-1.55
> > a-b
> [1] 1.55
> > a-b == 1.55
> [1] FALSE
> > round(a-b,2) == 1.55
> [1] TRUE
> > round(a-b,15) == 1.55
> [1] FALSE
> 
> Why should (a - b) == 1.55 fail when in fact b has been defined to be a 
- 1.
> 55?  Is this a precision issue? How do i correct this?

In real world those definitions of b are the same but not in computer 
world. See FAQ 7.31

Use either rounding or all.equal.

> all.equal(a-b, 1.55)
[1] TRUE

To all, this is quite common question and it is documented in FAQs. 
However there is no such issue in Excel or some other spreadsheet 
programs, therefore maybe a confusion from novices. 

I wonder if there could be some type of global option which will get rid 
of these users mistakes or misunderstandings by setting some threshold 
option for equality testing by use "==".

Regards
Petr




> 
> Alex
> 
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