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

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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