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

[Peter Ehlers]

David Winsemius wrote:
> 
> 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."
> 
Here is one more way, somewhat less transparent, motivated
by the examples on the ?ifelse page:

  x <- log(ifelse(x > 0, x, 1))

  -Peter Ehlers
> 
> -- 
> 
> David Winsemius, MD
> Heritage Laboratories
> West Hartford, CT
> 
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