[R] using integrate on a function defined with if

[Ole Christensen]

Hi Faheem

``integrate'' requires that the function (foo in your example) 
takes a vector as the first argument and returns a vector of the same
length. 

To illustrate this, try making a check for x being a vector in your
first example

foo <- function(x){
  if(is.vector(x)) print("x is a vector ! ")
  if(x==0) 4
  else 0
}

integrate(foo,0,1)

gives 

[1] "x is a vector ! "
Error in integrate(foo, 0, 1) : evaluation of function gave a result of
wrong length


As Prof. Ripley wrote in his reply, the problem is that your function is
not returning a vector but a number.

Cheers Ole



Faheem Mitha wrote:
> 
> Dear R People,
> 
> I getting some curious behaviour with the integrate function. Consider the
> function
> 
> foo <- function(x)
>   {
>     if(x==0)
>         4
>         else
>         0
>         }
> 
> I get the error
> 
> > integrate(foo,0,1)
> Error in integrate(foo, 0, 1) : evaluation of function gave a result of
> wrong length
> 
> But now consider
> 
> foo <- function(x)
>   {
>     ifelse(x==0,4,0)
>   }
> 
> > integrate(foo,0,1)
> 0 with absolute error < 0
> 
> I am guessing this may have something to do with types/classes, but I
> don't see exactly what. I would be happy to be enlightened.
> 
> Yhe help page says the function must be
> 
> "f An R function taking a numeric first argument and returning a numeric
> vector the same length."
> 
> But I don't see any difference in what these two return. In both cases it
> is NULL.
> 
>                                   Sincerely, Faheem Mitha.
> 
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-- 
Ole F. Christensen
Department of Mathematics and Statistics
Fylde College, Lancaster University 
Lancaster, LA1 4YF, England
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