[R] Plotting one dot in a graph

[David Winsemius]

OK, looks sensible, although I said either abs() OR squared  
differences. I don't think it makes sense to use both the L1 and the  
L2 metric at the same time.

-- 
David.
On Aug 12, 2010, at 4:58 PM, TGS wrote:

> # just to clean it up for my own understanding, the "difference"  
> approach as you had suggested would be
>
> x <- seq(.2, .3, by = .00001)
> f1 <- function(x){
> 	x*cos(x)-2*x**2+3*x-1
> }
> plot(x,f1(x), type = "l")
> abline(h = -.1)
> abline(v = x[which.min(abs(diff((f1(x) - (-.1))**2)))], lty =  
> 'dotted')
> points(x = x[which.min(abs(diff((f1(x) - (-.1))**2)))], y = -.1)
>
> # and the uniroot approach is:
>
> x <- seq(.2, .3, by = .01)
> f1 <- function(x){
> 	x*cos(x)-2*x**2+3*x-1
> }
> f2 <- function(x){
> 	-.1
> }
> f3 <- function(x){
> 	f1(x) - f2(x)
> }
> plot(x,f1(x), type = "l")
> abline(h = -.1)
> abline(v = uniroot(f = f3, interval = c(.2, .3))$root, lty = 'dotted')
> points(x = uniroot(f = f3, interval = c(.2, .3))$root, y = -.1)
>
> # Thanks David!
>
>
> On Aug 12, 2010, at 1:33 PM, David Winsemius wrote:
>
>
> On Aug 12, 2010, at 4:15 PM, TGS wrote:
>
>> David, I was expecting this to work but how would I specify the  
>> vector in "diff()" in order for the following to work?
>>
>> x <- seq(.2, .3, by = .01)
>> f <- function(x){
>> 	x*cos(x)-2*x**2+3*x-1
>> }
>> plot(x,f(x), type = "l")
>> abline(h = -.1)
>> abline(v = x[which.min(abs(diff(c(-.1, f(x)))))], lty = 'dotted')
>
> f2 <- function(x) -0.1
> f3 <- function(x) f(x) -f2(x)
> abline(v=uniroot(f3, c(0.2, 0.3) )$root)
> points(x=uniroot(f3, c(0.2, 0.3) )$root, y= -0.1)
>
> If you are going to use the differences, then you probably want to  
> minimize either the abs() or the square of the differences.
>
> -- 
> David.
>>
>> On Aug 12, 2010, at 1:00 PM, David Winsemius wrote:
>>
>>
>> On Aug 12, 2010, at 3:54 PM, TGS wrote:
>>
>>> Actually I spoke too soon David.
>>>
>>> I'm looking for a function that will either tell me which point is  
>>> the intersection so that I'd be able to plot a point there.
>>>
>>> Or, if I have to solve for the roots in the ways which were  
>>> demonstrated yesterday, then would I be able to specify what the  
>>> horizontal line is, for instance in the case where y (is-not) 0?
>>
>> Isn't the abline h=0 represented mathematically by the equation y=0  
>> and therefore you are solving just for the zeros of "f" (whaich are  
>> the same as for (f-0)? If it were something more interesting, like  
>> solving the intersection of two polynomials, you would be solving  
>> for the  zeros of the difference of the equations. Or maybe I have  
>> not understood what you were requesting?
>>
>>
>>>
>>> On Aug 12, 2010, at 12:47 PM, David Winsemius wrote:
>>>
>>>
>>> On Aug 12, 2010, at 3:43 PM, TGS wrote:
>>>
>>>> I'd like to plot a point at the intersection of these two curves.  
>>>> Thanks
>>>>
>>>> x <- seq(.2, .3, by = .01)
>>>> f <- function(x){
>>>> 	x*cos(x)-2*x**2+3*x-1
>>>> }
>>>>
>>>> plot(x,f(x), type = "l")
>>>> abline(h = 0)
>>>
>>> Would this just be the uniroot strategy applied to "f"? You then  
>>> plot the x and y values with points()
>>>
>>
>>>
>>
>> David Winsemius, MD
>> West Hartford, CT
>>
>>
>
> David Winsemius, MD
> West Hartford, CT
>
>