On 23-Sep-10 16:52:09, Duncan Murdoch wrote:
> On 23/09/2010 11:42 AM, wangguojie2006 wrote:
>> b<-runif(1000,0,1)
>> f<-density(b)
>
> f is a list of things, including x values where the density is
> computed,
> and y values for the density there. So you could do it by linear
> interpolation using approx or approxfun. For example
>
> > b <- runif(1000,0,1)
> > flist <- density(b)
> > f <- approxfun(flist$x, flist$y)
> > f(0.2)
> [1] 0.9717893
> > f(-1)
> [1] NA
>
> If you don't like the NA for an out-of-range argument, then choose
> something different from the default for the "rule" argument to
> approxfun.
>
> Duncan Murdoch
Or, perhaps more transparently (and more explicitly modifiable):
b<-runif(1000,0,1)
f <- density(b, from=0, to=1, n=512)
plot(f$x, f$y, type="l", col="blue",
xlim=c(0,1), ylim=c(0,1.5)) ## Plot the density estimate
x0 <- 0.5 ## Target value of x
i0 <- max(which(f$x <= x0))
i1 <- min(which(f$x > x0))
u0 <- f$x[i0] ; v0 <- f$y[i1]
u1 <- f$x[i1] ; v1 <- f$y[i1]
y0 <- v0 + (v1-v0)*(x0-u0)/(u1-u0) ## Linear interpolation
points(x0, y0, pch="+", col="red") ## Add interpolated point
Ted.
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E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk>
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Date: 23-Sep-10 Time: 18:05:41
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