True.
I've corrected my function in this way:
#######################################
sinca <- function(N,th)
{
mod_th <- th%%(2*pi)
if (mod_th == 0)
{
ff <- 2*N+1
}
else
{
ff <- sin((N+0.5)*th)/sin(0.5*th)
}
return(ff)
}
#######################################
The function is equal to 2*N+1 at 0, 2Pi, 4Pi, etc.
Still having the same problem.
J
----- Original Message -----
From: jim holtman
To: James Foadi
Cc: r-help@stat.math.ethz.ch
Sent: Thursday, July 05, 2007 8:44 PM
Subject: Re: [R] unexpected result in function valuation
Very simple; it is your function. You need to step through and see that you
are evaluating close to zero:
> x[701]
[1] 6.283185
> sin((4.5*x[701]))
[1] -1.666142e-14
> sin(.5*x[701])
[1] -1.653896e-15
> sin((4.5*x[701]))/sin(.5*x[701])
[1] 10.07404
>
With numbers that small you might be losing significance. See the FAQ on
floating point numbers.
On 7/5/07, James Foadi <[EMAIL PROTECTED]> 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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--
Jim Holtman
Cincinnati, OH
+1 513 646 9390
What is the problem you are trying to solve?
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
