R help - Mar 2012 - Simple question regarding domain restrictions/piecewise functions in R

I am a novice R user.

I would like to be able to graph some simple piecewise functions/functions
with domain restrictions in R, but I'm having trouble defining such
functions.  For example, I would like to define the following function:

f(x)={x^2 if -1<x<x; 1 if 2<x<3}

Notably, the function is undefined outside of domain (-1,1)U(2,3).  My best
attempt in R is something like this:

f<-function(x) ifelse(-1<x & x<1,x^2,ifelse(2<=x &
x<=3,1,as.null()))

This approach works fine for values within the domain.  But, naturally, when
I try values outside of the domain I get an error.  Consequently, when I try
to graph such a function:

plot(f)

I get the following error: 

Error in ifelse(2 <= x & x <= 3, 1, as.null()) : 
  replacement has length zero

I realize that I could create vectors of x-y points and plot using the
plot(x,y) command, but it seems to me that I should be able to define a
piecewise function symbolically and just graph it.  Am I wrong?  Any help
would be greatly appreciated.

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Does

     f <- function(x){
                 ifelse((-1 < x & x < 1) | (2 < x & x <
3),x^2,NA)
            }
     plot(f,xlim=c(-3,5))

give you what you want?

     cheers,

         Rolf Turner

On 26/03/12 11:08, chad.mills wrote:
> I am a novice R user.
>
> I would like to be able to graph some simple piecewise functions/functions
> with domain restrictions in R, but I'm having trouble defining such
> functions.  For example, I would like to define the following function:
>
> f(x)={x^2 if -1<x<x; 1 if 2<x<3}
>
> Notably, the function is undefined outside of domain (-1,1)U(2,3).  My best
> attempt in R is something like this:
>
> f<-function(x) ifelse(-1<x&  x<1,x^2,ifelse(2<=x& 
x<=3,1,as.null()))
>
> This approach works fine for values within the domain.  But, naturally,
when
> I try values outside of the domain I get an error.  Consequently, when I
try
> to graph such a function:
>
> plot(f)
>
> I get the following error:
>
> Error in ifelse(2<= x&  x<= 3, 1, as.null()) :
>    replacement has length zero
>
> I realize that I could create vectors of x-y points and plot using the
> plot(x,y) command, but it seems to me that I should be able to define a
> piecewise function symbolically and just graph it.  Am I wrong?  Any help
> would be greatly appreciated.

Yes!  Thanks.  It was just the "NA" value instead of the
"as.null" that does
the trick.  Correct code for the original piecewise I stated (for those who
might be looking later) is:

f <- function(x){ 
                 ifelse((-1 < x & x <
1),x^2,ifelse((2<x&x<3),1,NA))
            } 
 plot(f,xlim=c(-1,3)) 

-Chad Mills

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