[R] Solving an equation in R (and using a loop)

joris meys jorismeys at gmail.com
Fri Oct 16 00:34:54 CEST 2009

I tried :

> test <- 0:10000
> test2 <- f(0:10000)
> plot(test,test2)
> plot(test,log(test2))


> test <- seq(0,1,by=0.00001)
>  test2 <- sapply(test,f)
>  plot(test,log(test2))
>  plot(test,test2)

with the values you gave.

And according to this result, you did nothing wrong. Zero is the only
root of the function, as it is a monotone asymptotic to 0 for x>0, and
not defined for negative x. Are you sure you're looking for the root
of that function?


On Thu, Oct 15, 2009 at 11:18 PM, leohearn <leohearn at dal.ca> wrote:
> Hi
> So, I have a dataset and I'm trying to solve for a parameter in an equation
> using the dataset. Before applying more sophisticated statistical
> techniques, I want to tell R to solve it out for each observation. I know I
> have to use a loop for it (and I have done that before, but am a bit rusty),
> but before I do that, I'm trying to get R to solve it for just one
> observation, to make sure that I have that part right before applying to the
> rest. However, when I do it, I get 0 for an answer, which is a valid answer,
> but a trivial one. What am I doing wrong? (tips for the loop would also be
> appreciated, but this is my main concern right now.)
> Here's my code:
> f<-function(r){1-exp(-r*b)-(1/2)*(1-exp(-r*x1)+1-exp(-r*x2))}
> uniroot(f,c(0,1))$root
> r is the unknown. I've plugged in a couple numbers from my data for b, x1,
> and x2 such as:
> b=5500
> x1=0.01
> x2=10000
> and
> b=198000
> x1=10
> x2=500000
> and gotten 0 both times.
> Thanks in advance for your help.
> --
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