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 > --- > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@stat.math.ethz.ch mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide > http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > -- Jim Holtman Cincinnati, OH +1 513 646 9390 What is the problem you are trying to solve? [[alternative HTML version deleted]] ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.