R help - Mar 2010 - precision issue?

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?

Alex
 		 	   		  
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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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> 
>    [[alternative HTML version deleted]]
> 
> ______________________________________________
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> PLEASE do read the posting guide 
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Alexander Nervedi wrote:
> 
> 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?
> 
Read th R FAQ.
See 7.31 Why doesn't R think these numbers are equal?

Berend
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