[R] Question with uniroot function

li li hannah.hlx at gmail.com
Thu Apr 16 16:47:21 CEST 2015

```Hi Jeff,
Thanks for the reply. I am aware that the sign needs to be different at
the ends of the starting interval.

Another question:

Is there a way to set the right end point ( (the "upper" argument in the
uniroot function below) as the point where the function takes on its
minimun, for example my function f1 below?

Thanks very much!

u1 <- -3
u2 <- 4
pi0 <- 0.8

f1 <- function(lambda,z,p1){
lambda*(p1*exp(u1*z-u1^2/2)+(0.2-p1)*exp(u2*z-u2^2/2))-(1-lambda)*pi0}

x <- seq(-20,20, by=0.1)
y <- numeric(length(x))
for (i in 1:length(x)){y[i] <- f1(x[i],p1=0.15,lambda=0.998)}
plot(y ~ x, ylim=c(-1,1))
abline(h=0)

a <- uniroot(f1, lower =-10, upper = 0,
tol = 1e-20,p1=0.15,lambda=0.998)\$root

2015-04-15 22:57 GMT-04:00 Jeff Newmiller <jdnewmil at dcn.davis.ca.us>:

> You really need to read the help page for uniroot. The sign needs to be
> different at the ends of the starting interval. This is a typical
> limitation of numerical root finders.
> ---------------------------------------------------------------------------
> Jeff Newmiller                        The     .....       .....  Go Live...
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> Sent from my phone. Please excuse my brevity.
>
> On April 15, 2015 7:20:04 PM PDT, li li <hannah.hlx at gmail.com> wrote:
> >Hi all,
> >In the following code, I am trying to use uniroot function to solve for
> >the root (a and b in code below) for function f1.
> >I am not sure why uniroot function does not give the answer since when
> >we
> >look the graph, the function does cross 0 twice.
> >Any suggestion?
> >   Thanks.
> >       Hanna
> >
> >u1 <- -3
> >u2 <- 4
> >pi0 <- 0.8
> >
> >f1 <- function(lambda,z,p1){
> >lambda*(p1*exp(u1*z-u1^2/2)+(0.2-p1)*exp(u2*z-u2^2/2))-(1-lambda)*pi0}
> >
> >a <- uniroot(f1, lower =-10, upper = 0,
> >           tol = 1e-20,p1=0.15,lambda=0.998)\$root
> >
> >b <- uniroot(f1, lower =0, upper = 10,
> >           tol = 1e-20,p1=0.15,lambda=0.998)\$root
> >
> >x <- seq(-20,20, by=0.1)
> >y <- numeric(length(x))
> >for (i in 1:length(x)){y[i] <- f1(x[i],p1=0.15,lambda=0.998)}
> >plot(y ~ x, ylim=c(-1,1))
> >abline(h=0)
> >
> >       [[alternative HTML version deleted]]
> >
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