R help - Jan 2010 - function to set log(0)=0 not working on tables or vectors

There must be a very basic thing I am not getting...

I'm working with some entropy functions and the convention is to use
log(0)=0.

So I wrote a function:

llog<-function(x){
if (x ==0) 0 else log(x)
}

which seems to work fine for individual numbers e.g.
>llog(0/2)
[1] 0

but if I try whole vectors or tables:

p<-c(4,3,1,0)
q<-c(2,2,2,2)
llog(p/q)

I get this:

[1]  0.6931472  0.4054651 -0.6931472       -Inf


What am I missing?

Thanks!

Maja
-- 
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get a warning message which should have twigged the
answer for you?

Try ifelse() instead of if().

  -Peter Ehlers

maiya wrote:
> There must be a very basic thing I am not getting...
> 
> I'm working with some entropy functions and the convention is to use
> log(0)=0.
> 
> So I wrote a function:
> 
> llog<-function(x){
> if (x ==0) 0 else log(x)
> }
> 
> which seems to work fine for individual numbers e.g.
> 
>> llog(0/2)
> [1] 0
> 
> but if I try whole vectors or tables:
> 
> p<-c(4,3,1,0)
> q<-c(2,2,2,2)
> llog(p/q)
> 
> I get this:
> 
> [1]  0.6931472  0.4054651 -0.6931472       -Inf
> 
> 
> What am I missing?
> 
> Thanks!
> 
> Maja
-- 
Peter Ehlers
University of Calgary
403.202.3921

On Jan 17, 2010, at 8:17 PM, maiya wrote:
>
> There must be a very basic thing I am not getting...
>
> I'm working with some entropy functions and the convention is to use
> log(0)=0.
>
I suppose the outcome of that effort may depend on whether you have  
assumed the needed godlike capacities to change the laws of  
mathematics. But I suppose that as the Earth mother that might occur  
to you. Go ahead, define a new mathematics.

-- 
David
> So I wrote a function:
>
> llog<-function(x){
> if (x ==0) 0 else log(x)
> }
>
> which seems to work fine for individual numbers e.g.
>
>> llog(0/2)
> [1] 0
>
> but if I try whole vectors or tables:
>
> p<-c(4,3,1,0)
> q<-c(2,2,2,2)
> llog(p/q)
>
> I get this:
>
> [1]  0.6931472  0.4054651 -0.6931472       -Inf
>
>
> What am I missing?
>
> Thanks!
>
> Maja

David Winsemius, MD
Heritage Laboratories
West Hartford, CT