# picewise function in nls....

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## picewise function in nls....

 dear members,                             I have two predictors, x1 and x2, and one response variable, y. Moreover, a step function models the relationship between y and x1: fx <-   (x1 <= -2)*(x1^2) + (x1 > -2 && x1 < 2)*(x1^3) + (x1 > = 2)*(x1^4) Can I include fx in an nls call  to create something like this: NLS1 <- nls(y ~ a*(sin(x2) + fx), start = list(a = 2))   ? Will the above work? Or should I do something like this: if(x1 <= -2) { NLS2 <- nls(y[x1 <= -2] ~ a*(sin(x2[x1 <= -2]) + x1^2),start = list(a = 3))   } if(x1 > -2 && x1 < 2)  { NLS3 <- nls(y[x1 > -2 && x1 < 2] ~ a*(sin(x2[x1  > -2 && x1 < 2]) + x1^3),start = list(a = 4))   } if(x1 > = 2) { NLS4 <- nls(y[x1 >= 2] ~ a*(sin(x2[x1 >= 2]) + x1^4),start = list(a = 5))   } If the first case doesn't work, is there any other method to include the step function in the nls call without resorting to the if statements? Also, the coefficient, a , gotten from the above two cases are different. Will this affect the precision of the nonlinear fir between y ~ (f(x1),f(x2)) ? very many thanks for your time and effort yours sincerely, AKSHAY M KULKARNI         [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: picewise function in nls....

 On Thu, 18 Apr 2019 10:36:10 +0000 akshay kulkarni <[hidden email]> wrote: > fx <-   (x1 <= -2)*(x1^2) + (x1 > -2 && x1 < 2)*(x1^3) + (x1 > = > 2)*(x1^4) > > Can I include fx in an nls call  to create something like this: > > NLS1 <- nls(y ~ a*(sin(x2) + fx), start = list(a = 2))   ? For now, you can, since fx does not depend on any of the parameters you optimize in the nls() call. (Actually, the model as presented should be solveable by lm(y ~ I(sin(x2) + fx) + 0), since it is linear in its only parameter.) If you make fx a function and use ifelse() to provide different outcomes depending on a condition in a vectorized fashion, you would make it easier to add new parameters later, should the need arise: fx <- function(x1, x2)         ifelse(x1 <= -2, EXPR_IF_TRUE..., EXPR_IF_FALSE...) NLS1 <- nls(y ~ a*(sin(x2) + fx(x1, x2)), ...) -- Best regards, Ivan ______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: picewise function in nls....

 dear Ivan,                    THanks for the reply......But what do you mean by "since fx does not depend on any of the parameters you optimize in the nls() call."? Can you give an example? very many thanks for your time and effort..... Yours sincerely, AKSHAY M KULKARNI ________________________________ From: Ivan Krylov <[hidden email]> Sent: Thursday, April 18, 2019 4:50 PM To: akshay kulkarni Cc: R help Mailing list Subject: Re: [R] picewise function in nls.... On Thu, 18 Apr 2019 10:36:10 +0000 akshay kulkarni <[hidden email]> wrote: > fx <-   (x1 <= -2)*(x1^2) + (x1 > -2 && x1 < 2)*(x1^3) + (x1 > = > 2)*(x1^4) > > Can I include fx in an nls call  to create something like this: > > NLS1 <- nls(y ~ a*(sin(x2) + fx), start = list(a = 2))   ? For now, you can, since fx does not depend on any of the parameters you optimize in the nls() call. (Actually, the model as presented should be solveable by lm(y ~ I(sin(x2) + fx) + 0), since it is linear in its only parameter.) If you make fx a function and use ifelse() to provide different outcomes depending on a condition in a vectorized fashion, you would make it easier to add new parameters later, should the need arise: fx <- function(x1, x2)         ifelse(x1 <= -2, EXPR_IF_TRUE..., EXPR_IF_FALSE...) NLS1 <- nls(y ~ a*(sin(x2) + fx(x1, x2)), ...) -- Best regards, Ivan         [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.