[R] Why does a[which(b == c[d])] not work?

Achim Zeileis
Wed, 08 Oct 2003 11:45:50 -0000

On Wednesday 08 October 2003 11:27, Thomas Bock wrote:

> Dear list,
>
> I can not understand why the expression in
>
> the subject does not work correct:
>  > dcrn[which(fn == inve[2])]
>
> numeric(0)
>
>  > inve[2]
>
> [1] 406.7
>
>  > dcrn[which(fn == 406.7)]
>
>  [1] 1.3994e-07 1.3988e-07 1.3953e-07 1.3966e-07 1.3953e-07
> 1.3968e-07

The reason is that you shouldn't compare numeric output like that,
consider the following example:

R> x <- 406.7 + 1e-5
R> x
[1] 406.7
R> x == 406.7
[1] FALSE

whereas

R> x <- 406.7 + 1e-20
R> x
[1] 406.7
R> x == 406.7
[1] TRUE

that is
  1.) `==' comparisons have a certain tolerance
  2.) the print output is not necessarily "precisely" your number

Instead of using `==' you should use a comparison with a certain
tolerance you can specify...

hth,
Z


> Is this a kick self problem or an bug?
>
> Thaks very much
> Thomas
>
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