[R] Help with abs function

[Sarah Goslee]

Please read R FAQ 7.31.

On Fri, Jun 12, 2015 at 11:39 AM, Nelson, Gary (MISC)
<gary.nelson at state.ma.us> wrote:
> I have come across some odd behavior (to me) using the abs() function that I wonder if anyone can explain.
>
> Using  R version 3.2.0, I created a vector of absolute values using  the following code:
>
>> tran<-c(7.2)
>> tgrid<-c(7.1,7.4,7.3,7.1,7.3)
>> dgrid<-abs(tgrid-tran)
>> dgrid
> [1] 0.1 0.2 0.1 0.1 0.1
>
> When I tried to extract just the rows with values>0.1, I get
>
>> dgrid[dgrid>0.1]
> [1] 0.1 0.2 0.1
>
> There should be only 1 value extracted.
>
> However, if I enter the values by hand
>
>> bgrid<-c(0.1,0.2,0.1,0.1,0.1)
>> bgrid
> [1] 0.1 0.2 0.1 0.1 0.1
>> bgrid[bgrid>0.1]
> [1] 0.2
>
> The result is correct.  So why is this happening?
>
> I did explore a little bit and found
>
>> as.character(dgrid)
> [1] "0.100000000000001"  "0.2"
> [3] "0.0999999999999996" "0.100000000000001"
> [5] "0.0999999999999996"
>
> which shows the absolute values of the negative differences  are slightly greater than the difference of 0.1
>
> Is this normal behavior for the abs() function?
>
> Thanks,
>
> Gary Nelson
-- 
Sarah Goslee
http://www.functionaldiversity.org