[R] Relase positive with log and zero of negative with 0

[David Winsemius]

On Nov 15, 2009, at 10:18 AM, <rkevinburton at charter.net> wrote:

> This is a very simple question but I couldn't form a site search  
> quesry that would return a reasonable result set.
>
> Say I have a vector:
>
> x <- c(0,2,3,4,5,-1,-2)
>
> I want to replace all of the values in 'x' with the log of x.  
> Naturally this runs into problems since some of the values are  
> negative or zero. So how can I replace all of the positive elements  
> of x with the log(x) and the rest with zero?

 > x <- c(0,2,3,4,5,-1,-2)
 > x <- ifelse(x>0, log(x), 0)
Warning message:
In log(x) : NaNs produced
 > x
[1] 0.0000000 0.6931472 1.0986123 1.3862944 1.6094379 0.0000000  
0.0000000

The warning is harmless as you can see, but if you wanted to avoid it,  
then:

 > x[x<=0] <- 0; x[x>0] <-log(x[x>0])

In the second command, you need to have the logical test on both sides  
to avoid replacement " out of synchrony."


--

David Winsemius, MD
Heritage Laboratories
West Hartford, CT