[R] optimize

[Muhammad Rahiz]

Josh's recommendation to use predict works. At the same time, I'll work on 
your suggestions, David.

Thanks.

Muhammad Rahiz
Researcher & DPhil Candidate (Climate Systems & Policy)
School of Geography & the Environment
University of Oxford

On Mon, 3 Jan 2011, David Winsemius wrote:

>
> On Jan 3, 2011, at 9:52 AM, Muhammad Rahiz wrote:
>
>> Hi all,
>>
>> I'm trying to get the value of y when x=203 by using the intersect
>> of three curves. The horizontal curve does not meet with the other
>> two. How can I rectify the code below?
>>
>> Thanks
>>
>> Muhammad
>>
>> ts <- 1:10
>> dd <- 10:1
>>
>> ts <- seq(200,209,1)
>> dd <- c(NA,NA,NA,NA,1.87,1.83,1.86,NA,1.95,1.96)
>>
>> plot(ts,dd,ylim=c(1.5,2))
>> abline(lm(dd~ts),col="blue",lty=2)
>> abline(v=203,col="blue",lty=2)
>>
>> xy <- lm(dd~ts)
>> fc <- function(x) coef(xy)[1] + x*coef(xy)[2]
>> val <- optimize(f=function(x) abs(fc(x)-203), c(1.5,2))
>                                          ^???^
>
> Joshua Wiley's answer made me realize that your question was different
> than I thought. At this point maybe you should have made your
> objective function match on the coordinate difference to be minimized.
> You are at this point taking the absolute difference between an x-
> value, 203. and a y-value. fc(x). That makes little sense. You can use
> predict() as Joshua showed you or you can change your objective
> functions so it is more meaningful. Furthermore, I have had better
> success with squared differences that with abs(differences). I'm not
> sure whether that is due to the availability of a more informative
> derivative.  (Again, looks like homework and I refrain from posting
> completed solutions.)
>
>> abline(h=val,col="blue",lty=2)
>
> --
> David Winsemius, MD
> West Hartford, CT
>
>