R help - Nov 2009 - Relase positive with log and zero of negative with 0

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?

Thank you.

Kevin

G'day Kevin,

On Sun, 15 Nov 2009 7:18:18 -0800
<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?
If you do not mind a warning message:

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

If you do mind, then:

R> x <- c(0,2,3,4,5,-1,-2)
R> ind <- x>0
R> x[!ind] <- 0
R> x[ind] <- log(x[ind])
R> x
[1] 0.0000000 0.6931472 1.0986123 1.3862944 1.6094379 0.0000000 0.0000000

HTH.

Cheers,

	Berwin

========================== Full address ===========================Berwin A
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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