[R] unexpected result in function valuation

[Prof Brian Ripley]

What value should your formula give when x is a multiple of 2*pi?
You seem to believe 9 is correct but in fact NaN is.

Element 701 of x is approximately but not exactly 2*pi: on my system 
it is about 7*.Machine$double.eps different.  You cannot expect sin(N*pi) 
to be exactly zero for N != 0.


On Thu, 5 Jul 2007, James Foadi wrote:

> Dear all,
> I have a very small script to plot a function. Here it is:
>
> ##########################################
> sinca <- function(N,th)
>
> {
>
> return(sin((N+0.5)*th)/sin(0.5*th))
>
> }
>
> plot_sinca <- function(N)
>
> {
>
> x <- seq(-5*pi,5*pi,by=pi/100)
>
> y <- rep(0,length=length(x))
>
> for (i in 1:length(x))y[i] <- sinca(N,x[i])
>
> plot(x,y,type="l",ylim=c(0,2*N+4))
>
> return(c(x,y))
>
> }
>
> ##########################################
>
> When I load the script and run the function like this:
>
> ###########################################
>> data <- plot_sinca(4)
>> y <- data[1002:2002]
> ###########################################
>
> I notice a spike on the plot which should not be there.
> In fact I have checked and:
> ###########################################
>> y[701]
> [1] 10.07404
>> sinca(4,2*pi)
> [1] 9
> ###########################################
>
> The second result is the correct one. Why, then do
> I get the y[701]=10.07404? This function is not supposed
> to be higher than 9...
>
> Any help is greatly appreciated.
>
> Regards,
>
> J
>
> Dr James Foadi
> Membrane Protein Laboratory
> Diamond Light Source Ltd
> Chilton, Didcot
> Oxfordshire OX11 0DE
> ---
>
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>
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-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595