[R] How to find the zero (only the real solution) with the package polynom ?
Ptit_Bleu
ptit_bleu at yahoo.fr
Tue Nov 6 09:30:45 CET 2007
Hi again,
I found a solution (but maybe it is not the best one) : I converted the
poynom into a function.
pderiv<-as.function((deriv(polynomial(pfit$coefficients))))
uniroot(pderiv, c(0.8,0.2))[[1]]
[1] 0.4778413
Now I just have to find the corresponding "b" to this "a" but I think it
must be quite easy (am I right ?).
I don't delete my post as it maybe be helpful to someone else but if the
Administrator wants to delete it, it is ok to me.
See you,
Ptit Bleu.
-------------------
Ptit_Bleu wrote:
>
> Hello,
>
> I have 3 columns : a, b and a*b
> I would like to find the pair (a,b) so that a*b is the minimum but not
> from the points I measured but from the fit of the curve (I have more
> points that the ones given below but I fit only on this part because I
> know that the minimum a*b is in this interval).
>
> I thought doing it this way :
>
> - to fit a*b=f(a)
> abfit<-lm(ab ~ poly(a,8,raw=T))
>
> - to use the package polynom to make the polynom
> library(polynom)
> polynomial(abfit$coefficients)
> 0.03537506 - 0.8506516*x + 8.302545*x^2 - 44.80418*x^3 + 144.2161*x^4 -
> 283.2458*x^5 + 331.1776*x^6 - 210.5144*x^7 + 55.86646*x^8
>
> - to derive the polynom
> deriv(polynomial(pfit$coefficients))
> -0.8506516 + 16.60509*x - 134.4125*x^2 + 576.8643*x^3 - 1416.229*x^4 +
> 1987.066*x^5 - 1473.601*x^6 + 446.9317*x^7
>
> - to find the zero
> solve(deriv(polynomial(pfit$coefficients)))
> [1] 0.1749897+0.0568886i 0.1749897-0.0568886i 0.3742571-0.1235393i
> 0.3742571+0.1235393i 0.4778418+0.0000000i 0.8604070+0.1306799i
> 0.8604070-0.1306799i
>
> And here is my little problem : I don't want the complex solutions, only
> the real one (0.4778418).
>
> I also tried uniroot but I got an error message :
> uniroot(deriv(polynomial(pfit$coefficients)), c(0.8,0.2))
> Error in uniroot(deriv(polynomial(abfit$coefficients)), c(0.8, 0.2)) :
> Impossible to find the function "f"
>
> Is there a way to extract this real solution ?
>
> I thank you in advance for your help,
> Have a nice day,
> Ptit Bleu.
>
> -------------------------------------------------
> Set of points
> a b a*b
> 0.8 0.033320357 0.026656286
> 0.76 0.023930636 0.018187283
> 0.72 0.0153903 0.011081016
> 0.68 0.007844914 0.005334542
> 0.64 0.001598132 0.001022804
> 0.6 -0.00315939 -0.001895634
> 0.56 -0.006402796 -0.003585566
> 0.52 -0.008379886 -0.004357541
> 0.48 -0.009528793 -0.004573821
> 0.44 -0.010198446 -0.004487316
> 0.4 -0.010638282 -0.004255313
> 0.36 -0.010937536 -0.003937513
> 0.32 -0.011169004 -0.003574081
> 0.28 -0.011343229 -0.003176104
> 0.24 -0.0114878 -0.002757072
> 0.2 -0.011630636 -0.002326127
> 0.16 -0.011724586 -0.001875934
>
>
--
View this message in context: http://www.nabble.com/How-to-find-the-zero-%28only-the-real-solution%29-with-the-package-polynom---tf4756646.html#a13602541
Sent from the R help mailing list archive at Nabble.com.
More information about the R-help
mailing list