[R] Problem (bug?) with vector indices

[Peter Dalgaard]

"Rodrigo Drummond" <rduarte at unicamp.br> writes:

> Hi all,
> 
> I found a problem on R that looks like a bug to me. I am working with R
> for windows verson 1.9.0, but the problem also happens on the linux
> version 1.9.0. I hope that the problem is on my reasoning, and I would be
> grateful if someone could explain me why this happens.
> There goes the example:
> 
> > x<-seq(-2,2,0.001)
> > y<-rep(0,length(x))
> > for (i in x){y[1000*i+2001]<-i^2}
...
> > x[244]
> [1] -1.757
> > i<-x[244]
> > 1000*i+2001
> [1] 244
> > y[1000*i+2001]
> [1] 3.087049
> > y[244]
> [1] 0
> > y[243]
> [1] 3.087049
> 
> 
> What means that the expression 1000*i+2001, which is equal to 244, inside
> the brackets is evaluated as 243! The same occurs with every value of x
> that results in a zero value of y, the right value is stored in y one
> position before it should be, overwriting the previous value at this
> position.
> The bug is on R or on my mind?

The latter: 

Have a look at (1000*x[244]+2001)-244

Beware floating point arithmetic!

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
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