# nlsr Background, Development, Examples and Discussion In nlsr: Functions for Nonlinear Least Squares Solutions

This rather long vignette is to explain the development and testing of nlsr, an R package to try to bring the R function nls() up to date and to add capabilities for the extension of the symbolic and automatic derivative tools in R.

A particular goal in nlsr is to attempt, wherever possible, to use analytic or automatic derivatives. The function nls() uses a rather weak forward derivative approximation. A second objective is to use a Marquardt stabilization of the Gauss-Newton equations to avoid the commonly encountered "singular gradient" failure of nls(). This refers to the loss of rank of the Jacobian at the parameters for evaluation. The particular stabilization also incorporates a very simple trick to avoid very small diagonal elements of the Jacobian inner product, though in the present implementations, this is accomplished indirectly. See the section below Implementation of method

In preparing the nlsr package there is a sub-goal to unify, or at least render compatible, various packages in R for the estimation or analysis of problems amenable to nonlinear least squares solution.

A large part of the work for this package -- particularly the parts concerning derivatives and R language structures -- was initially carried out by Duncan Murdoch. Without his input, much of the capability of the package would not exist.

The package and this vignette are a work in progress, and assistance and examples are welcome. Note that there is similar work in the package Deriv (Andrew Clausen and Serguei Sokol (2015) Symbolic Differentiation, version 2.0, https://cran.r-project.org/package=Deriv), and making the current work "play nicely" with that package is desirable.

## TODOS

• how to insert numerical derivatives when Deriv unable to get result
• approximations for jacfn beyond fwd approximation. How to specify??

## Summary of capabilities in `nlsr`

### Functions in nlsr

#### coef.nlsr

extracts and displays the coefficients for a model estimated by nlxb() or nlfb() in the nlsr structured object.

```library(nlsr)
ydat<-c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443, 38.558, 50.156, 62.948,
75.995, 91.972)
tdat<-1:12
# try setting t and y here
t <- tdat
y <- ydat
sol1 <- nlxb(y~100*b1/(1+10*b2*exp(-0.1*b3*t)), start=c(b1=2, b2=5, b3=3))
coef(sol1)
print(coef(sol1))
```

#### nlsDeriv, codeDeriv, fnDeriv, newDeriv

Functions Deriv and fnDeriv are designed as replacements for the stats package functions D and deriv respectively, though the argument lists do not match exactly. newDeriv allows additional analytic derivative expressions to be added. The following is an expanded and commented version of the examples from the manual for these functions.

```require(nlsr)
newDeriv() # a call with no arguments returns a list of available functions
# for which derivatives are currently defined
newDeriv(sin(x)) # a call with a function that is in the list of available derivatives
# returns the derivative expression for that function
nlsDeriv(~ sin(x+y), "x") # partial derivative of this function w.r.t. "x"

## CAUTION !! ##
newDeriv(joe(x)) # but an undefined function returns NULL
newDeriv(joe(x), deriv=log(x^2)) # We can define derivatives, even if joe(x) is meanin
nlsDeriv(~ joe(x+z), "x")

# Some examples of usage
f <- function(x) x^2
newDeriv(f(x), 2*x*D(x))
nlsDeriv(~ f(abs(x)), "x")
nlsDeriv(~ x^2, "x")
nlsDeriv(~ (abs(x)^2), "x")

# derivatives of distribution functions
nlsDeriv(~ pnorm(x, sd=2, log = TRUE), "x")
# get evaluation code from a formula
codeDeriv(~ pnorm(x, sd = sd, log = TRUE), "x")
# wrap it in a function call
fnDeriv(~ pnorm(x, sd = sd, log = TRUE), "x")
f <- fnDeriv(~ pnorm(x, sd = sd, log = TRUE), "x", args = alist(x =, sd = 2))
f
f(1)
100*(f(1.01) - f(1))  # Should be close to the gradient
# The attached gradient attribute (from f(1.01)) is
# meaningless after the subtraction.
```

#### model2rjfun, model2ssgrfun, modelexpr, rjfundoc

These functions create functions to evaluate residuals or sums of squares at particular parameter locations.

```weed <- c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443,
38.558, 50.156, 62.948, 75.995, 91.972)
ii <- 1:12
wdf <- data.frame(weed, ii)
wmod <- weed ~ b1/(1+b2*exp(-b3*ii))
start1  <-  c(b1=1, b2=1, b3=1)
wrj <- model2rjfun(wmod, start1, data=wdf)
print(wrj)
weedux <- nlxb(wmod, start=c(b1=200, b2=50, b3=0.3))
print(weedux)

wss <- model2ssgrfun(wmod, start1, data=wdf)
print(wss)

# We can get expressions used to calculate these as follows:
wexpr.rj <- modelexpr(wrj)
print(wexpr.rj)

wexpr.ss <- modelexpr(wss)
print(wexpr.ss)

wrjdoc <- rjfundoc(wrj)
print(wrjdoc)
# We do not have similar function for ss functions
```

#### nlfb

Given a nonlinear model expressed as an expression of the form of a function that computes the residuals from the model and a start vector `par` , tries to minimize the nonlinear sum of squares of these residuals w.r.t. `par`. If `model(par, mydata)` computes an a vector of numbers that are presumed to be able to fit data `lhs`, then the residual vector is `(model(par,mydata) - lhs)` , though traditionally we write the negative of this vector. (Writing it this way allows the derivatives of the residuals w.r.t. the parameters `par` to be the same as those for `model(par,mydata).)`nlfb` tries to minimize the sum of squares of the residuals with respect to the parameters.

The method takes a parameter `jacfn` which returns the Jacobian matrix of derivatives of the residuals w.r.t. the parameters in an attribute `gradient`. If this is NULL, then a numerical approximation to derivatives is used. (??which -- and can we use the character method to define these?? What about for nlxb??) Putting the Jacobian in the attribute `gradient` allows us to combine the computation of the residual and Jacobian in the same code block if we wish, since there are generally common computations, though it does make the setup slightly more complicated. See the example below.

The start vector preferably uses named parameters (especially if there is an underlying formula). The attempted minimization of the sum of squares uses the Nash variant (Nash, 1979) of the Marquardt algorithm, where the linear sub-problem is solved by a qr method.

```shobbs.res  <-  function(x){ # scaled Hobbs weeds problem -- residual
# This variant uses looping
if(length(x) != 3) stop("hobbs.res -- parameter vector n!=3")
y  <-  c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443,
38.558, 50.156, 62.948, 75.995, 91.972)
tt  <-  1:12
res  <-  100.0*x[1]/(1+x[2]*10.*exp(-0.1*x[3]*tt)) - y
}

shobbs.jac  <-  function(x) { # scaled Hobbs weeds problem -- Jacobian
jj  <-  matrix(0.0, 12, 3)
tt  <-  1:12
yy  <-  exp(-0.1*x[3]*tt)
zz  <-  100.0/(1+10.*x[2]*yy)
jj[tt,1]   <-   zz
jj[tt,2]   <-   -0.1*x[1]*zz*zz*yy
jj[tt,3]   <-   0.01*x[1]*zz*zz*yy*x[2]*tt
jj
}

cat("try nlfb\n")
st  <-  c(b1=1, b2=1, b3=1)
low  <-  -Inf
up <- Inf
ans1 <- nlfb(st, shobbs.res, shobbs.jac, trace=TRUE)
summary(ans1)

## No jacobian function -- use internal approximation
ans1n <- nlfb(st, shobbs.res, trace=TRUE, control=list(watch=FALSE)) # NO jacfn
summary(ans1n)
## difference
coef(ans1)-coef(ans1n)
```

#### nlxb

Given a nonlinear model expressed as an expression of the form lhs ~ formula_for_rhs and a start vector where parameters used in the model formula are named, attempts to find the minimum of the residual sum of squares using the Nash variant (Nash, 1979) of the Marquardt algorithm, where the linear sub-problem is solved by a qr method. The process is to develop the residual and Jacobian functions using `model2rjfun`, then call `nlfb`.

```# Data for Hobbs problem
ydat  <-  c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443,
38.558, 50.156, 62.948, 75.995, 91.972) # for testing
tdat  <-  seq_along(ydat) # for testing
start1  <-  c(b1=1, b2=1, b3=1)
eunsc  <-   y ~ b1/(1+b2*exp(-b3*tt))
weeddata1  <-  data.frame(y=ydat, tt=tdat)
anlxb1  <-  try(nlxb(eunsc, start=start1, trace=TRUE, data=weeddata1))
summary(anlxb1)
```

#### print.nlsr

Print summary output (but involving some serious computations!) of an object of class nlsr from \code{nlxb} or \code{nlfb} from package \code{nlsr}.

```### From examples above
print(weedux)
print(ans1)
```

#### resgr, resss

For a nonlinear model originally expressed as an expression of the form lhs ~ formula_for_rhs assume we have a resfn and jacfn that compute the residuals and the Jacobian at a set of parameters. This routine computes the gradient, that is, t(Jacobian) %*% residuals.

```## Use shobbs example
RG <- resgr(st, shobbs.res, shobbs.jac)
RG
SS <- resss(st, shobbs.res)
SS
```

#### nlsSimplify and related functions

Simplifications to render derivative expresssions more usable.

The related tools are: newSimplification, sysSimplifications, isFALSE, isZERO, isONE, isMINUSONE, isCALL, findSubexprs, sysDerivs.

nlsSimplify simplifies expressions according to rules specified by newSimplification.

findSubexprs finds common subexpressions in an expression vector so that duplicate computation can be avoided.

```## nlsSimplify
nlsSimplify(quote(a + 0))
nlsSimplify(quote(exp(1)), verbose = TRUE)

nlsSimplify(quote(sqrt(a + b)))  # standard rule
## sysSimplifications
# creates a new environment whose parent is emptyenv()  Why??
str(sysSimplifications)
myrules <- new.env(parent = sysSimplifications)
## newSimplification
newSimplification(sqrt(a), TRUE, a^0.5, simpEnv = myrules)
nlsSimplify(quote(sqrt(a + b)), simpEnv = myrules)

## isFALSE
print(isFALSE(1==2))
print(isFALSE(2==2))

## isZERO
print(isZERO(0))
x <- -0
print(isZERO(x))
x <- 0
print(isZERO(x))
print(isZERO(~(-1)))
print(isZERO("-1"))
print(isZERO(expression(-1)))

## isONE
print(isONE(1))
x <- 1
print(isONE(x))
print(isONE(~(1)))
print(isONE("1"))
print(isONE(expression(1)))

## isMINUSONE
print(isMINUSONE(-1))
x <- -1
print(isMINUSONE(x))
print(isMINUSONE(~(-1)))
print(isMINUSONE("-1"))
print(isMINUSONE(expression(-1)))

## isCALL  ?? don't have good understanding of this
x <- -1
print(isCALL(x,"isMINUSONE"))
print(isCALL(x, quote(isMINUSONE)))

## findSubexprs
findSubexprs(expression(x^2, x-y, y^2-x^2))

## sysDerivs
# creates a new environment whose parent is emptyenv()  Why??
str(sysDerivs)
```

#### summary.nlsr

Provide a summary output (but involving some serious computations!) of an object of class nlsr from nlxb or nlfb from package nlsr. Examples have been given above under nlxb and nlfb.

#### wrapnlsr

Given a nonlinear model expressed as an expression of the form

```lhs ~ formula_for_rhs
```

and a start vector where parameters used in the model formula are named, attempts to find the minimum of the residual sum of squares using the Nash variant (Nash, 1979) of the Marquardt algorithm, where the linear sub-problem is solved by a qr method.

```# Data for Hobbs problem
ydat  <-  c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443,
38.558, 50.156, 62.948, 75.995, 91.972) # for testing
tdat  <-  seq_along(ydat) # for testing
start1  <-  c(b1=1, b2=1, b3=1)
eunsc  <-   y ~ b1/(1+b2*exp(-b3*tt))
weeddata1  <-  data.frame(y=ydat, tt=tdat)
## The following attempt with nls() fails!
anls1  <-  try(nls(eunsc, start=start1, trace=TRUE, data=weeddata1))
## But we succeed by calling nlxb first.
anlxb1  <-  try(nlxb(eunsc, start=start1, trace=TRUE, data=weeddata1))
st2 <- coef(anlxb1)
anls2  <-  try(nls(eunsc, start=st2, trace=TRUE, data=weeddata1))
## Or we can simply call wrapnlsr
anls2a  <-  try(wrapnlsr(eunsc, start=start1, trace=TRUE, data=weeddata1))
```

### Particular features

#### weighted nonlinear regression

Weighted regression alters the sum of squares of the parameters `prm` from

``` sum_{i} (residual_i(prm)^2)
```

to

``` sum_{i} (wts_i * residual_i(prm)^2)
```

where `wts` is the vector of weights. This is equivalent to multiplying each residual or row of the Jacobian by the square root of the respective weight.

While `nls()` has provision for (fixed) weights, the example given in the documentation of `nls()` for weighted regression actually uses a functional form. Moreover, the weights are the square roots of the predicted or model values, so are not fixed. Here we replace these weights with the fixed values of the quantity we are trying to predict, namely the `rate` variable in a kinetic model. In our example below we first use the documentation example with predicted values; note that the name of the result has been changed from that in the `nls()` documentation. This result gives different estimates of the parameters from the fixed weights used afterwards.

Note that neither `nlsr::nlxb` nor `nlsr::nlfb` can use the weighted.MM function directly. This is one of the more awkward aspects of nonlinear least squares and nonlinear optimization.

```Treated <- Puromycin[Puromycin\$state == "treated", ]
weighted.MM <- function(resp, conc, Vm, K)
{
## Purpose: exactly as white book p. 451 -- RHS for nls()
##  Weighted version of Michaelis-Menten model
## ----------------------------------------------------------
## Arguments: 'y', 'x' and the two parameters (see book)
## ----------------------------------------------------------
## Author: Martin Maechler, Date: 23 Mar 2001

pred <- (Vm * conc)/(K + conc)
(resp - pred) / sqrt(pred)
}
## Here is the estimation using predicted value weights
Pur.wtnlspred <- nls( ~ weighted.MM(rate, conc, Vm, K), data = Treated,
start = list(Vm = 200, K = 0.1))
summary(Pur.wtnlspred)

## nlxb cannot use this form
Pur.wtnlxbpred <- try(nlxb( ~ weighted.MM(rate, conc, Vm, K), data = Treated,
start = list(Vm = 200, K = 0.1)))
## and the structure is wrong for nlfb. See wres.MM below.
Pur.wtnlfbpred <- try(nlfb(resfn=weighted.MM(rate, conc, Vm, K), data = Treated,
start = list(Vm = 200, K = 0.1)))

## Now use fixed weights and the weights argument in nls()
Pur.wnls <- nls( rate ~ (Vm * conc)/(K + conc), data = Treated,
start = list(Vm = 200, K = 0.1), weights=1/rate)
summary(Pur.wnls)

## nlsr::nlxb should give essentially the same result
library(nlsr)
## Note that we put in the dataframe name to explicitly specify weights
Pur.wnlxb <- nlxb( rate ~ (Vm * conc)/(K + conc), data = Treated,
start = list(Vm = 200, K = 0.1), weights=1/Treated\$rate)
summary(Pur.wnlxb)
## check difference
coef(Pur.wnls) - coef(Pur.wnlxb)
## Residuals Pur.wnls -- These are weighted
## resid(Pur.wnlxb) # ?? does not work -- why?
print(res(Pur.wnlxb))
## Those from nls() are NOT weighted
resid(Pur.wnls)

## Try using a function, that is nlsr::nlfb
stw <- c(Vm = 200, K = 0.1)
wres.MM <- function(prm, rate, conc) {
Vm <- prm[1]
K <- prm[2]
pred <- (Vm * conc)/(K + conc)
(rate - pred) / sqrt(rate) # NOTE: NOT pred here
}

# test function first to see it works
print(wres.MM(stw, rate=Treated\$rate, conc=Treated\$conc))

Pur.wnlfb <- nlfb(start=stw,  resfn=wres.MM, rate = Treated\$rate, conc=Treated\$conc,
trace=FALSE) # Note: weights are inside function already here
summary(Pur.wnlfb)
print(Pur.wnlfb)
## check estimates with nls() result
coef(Pur.wnls) - coef(Pur.wnlfb)
## and with nlxb result
coef(Pur.wnlxb) - coef(Pur.wnlfb)

wres0.MM <- function(prm, rate, conc) {
Vm <- prm[1]
K <- prm[2]
pred <- (Vm * conc)/(K + conc)
(rate - pred) # NOTE: NO weights
}
Pur.wnlfb0 <- nlfb(start=stw,  resfn=wres0.MM, rate = Treated\$rate, conc=Treated\$conc,
weights=1/Treated\$rate, trace=FALSE) # Note: explicit weights
summary(Pur.wnlfb0)
print(Pur.wnlfb0)
## check estimates with nls() result
coef(Pur.wnls) - coef(Pur.wnlfb0)
## and with nlxb result
coef(Pur.wnlxb) - coef(Pur.wnlfb0)

wss.MM <- function(prm, rate, conc) {
ss <- as.numeric(crossprod(wres.MM(prm, rate, conc)))
}
library(optimr)
osol <- optimr(stw, fn=wss.MM, gr="grnd", method="Nelder-Mead", control=list(trace=0),
rate=Treated\$rate, conc=Treated\$conc)
print(osol)
## difference from nlxb
osol\$par - coef(Pur.wnlxb)
```

### Missing capabilities

#### Multi-line function expressions

Multi-line functions present a challenge. This is in part because the chain rule may have to be applied backwards (last line first), but also because there may be structures that are not always differentiable, such as if statements.

Note that the functional approach to nonlinear least squares -- accessible via function `nlfb` -- does let us prepare residuals and Jacobians of complexity limited only by the capabilities of R.

#### Automatic differentiation

This is the natural extension of Multi-line expressions. The authors would welcome collaboration with someone who has expertise in this area.

#### Vector parameters

We would like to be able to find the Jacobian or gradient of functions that have as their parameters a vector, e.g., prm. At time of writing (January 2015) we cannot specify such a vector within nlsr. ?? is this true?

#### Examples of capabilities and weaknesses

The following (rather long) script shows things that work or fail. In particular, it shows that the calls needed to get derivatives need to be set up precisely. This is not unique to R -- the same sort of attention to (syntax specific) detail is present in other systems like Macsyma/Maxima.

First we set up the Hobbs weed problem. Initially it seemed a good idea to put the data WITHIN the residual function so that it would not have to be passed to the function. In retrospect, this is a bad idea because y and t are needed. To make the use of the data explicit, it is saved in the variables `tdat` for time and `ydat` for the weed data.

```# tryhobbsderiv.R
ydat<-c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443, 38.558, 50.156, 62.948,
75.995, 91.972)
tdat<-1:12
# try setting t and y here
t <- tdat
y <- ydat
# now define a function

hobbs.res<-function(x, t, y){ # Hobbs weeds problem -- residual
# This variant uses looping
#  if(length(x) != 3) stop("hobbs.res -- parameter vector n!=3")
#  if(abs(12*x[3])>50) {
#    res<-rep(Inf,12)
#  } else {
res<-x[1]/(1+x[2]*exp(-x[3]*t)) - y
#  }
}
# test it
start1 <- c(200, 50, .3)
## residuals at start1
r1 <- hobbs.res(start1, t=tdat, y=ydat)
print(r1)
```

Now we try some of the derivative capabilities of `nlsr`.

```## NOTE: some functions may be seemingly correct for R, but we do not
## get the result desired, despite no obvious error. Always test.
require(nlsr)
# Try directly to differentiate the residual vector. r1 is numeric, so this should
# return a vector of zeros in a mathematical sense. In fact it gives an error,
# since R does not want to differentiate a numeric vector.
Jr1a <- fnDeriv(r1, "x")
# Set up a function containing expression with subscripted parameters x[]
hobbs1 <- function(x, t, y){ res<-x[1]/(1+x[2]*exp(-x[3]*t)) - y }
# test the residuals
print(hobbs1(start1, t=tdat, y=ydat))
# Alternatively, let us set up t and y
y<-c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443, 38.558, 50.156, 62.948,
75.995, 91.972)
t<-1:12
# try different calls. Note that we need to include t and y data somehow
print(hobbs1(start1))
print(hobbs1(start1, t, y))
print(hobbs1(start1, tdat, ydat))
# We remove t and y, to ensure we don't get results from their presence
rm(t)
rm(y)
# Set up a function containing an expression with named (with numbers) parameters
# Note that we need to link these to the values in x
hobbs1m <- function(x, t, y){
x001 <- x[1]
x002 <- x[2]
x003 <- x[3]
res<-x001/(1+x002*exp(-x003*t)) - y
}
print(hobbs1m(x=start1, t=tdat, y=ydat))
# Function with explicit use of the expression()
hobbs1me <- function(x, t, y){
x001 <- x[1]
x002 <- x[2]
x003 <- x[3]
expression(x001/(1+x002*exp(-x003*t)) - y)
}
print(hobbs1me(start1, t=tdat, y=ydat))
# note failure (because the expression is not evaluated?)
#
# Now try to take derivatives
Jr11m <- fnDeriv(hobbs1m, c("x001", "x002", "x003"))
# fails because expression is INSIDE a function (i.e., closure)
#
# try directly differentiating the expression
Jr11ex <-fnDeriv(expression(x001/(1+x002*exp(-x003*t)) - y)
, c("x001", "x002", "x003"))
# this seems to "work". Let us display the result
Jr11ex
# Set the values of the parameters by name
x001 <- start1[1]
x002 <- start1[2]
x003 <- start1[3]
# and try to evaluate
print(eval(Jr11ex))
# we need t and y, so set them
t<-tdat
y<-ydat
print(eval(Jr11ex))
# But there is still a problem. WHY???
#
# Let us try it piece by piece (column by column)
resx <- expression(x001/(1+x002*exp(-x003*t)) - y)
res1 <- Deriv(resx, "x001", do_substitute=FALSE)
res1
col1 <- eval(res1)
res2 <- Deriv(resx, "x002", do_substitute=FALSE)
res2
col2 <- eval(res2)
res3 <- Deriv(resx, "x003", do_substitute=FALSE)
res3
col3 <- eval(res3)
hobJac <- cbind(col1, col2, col3)
print(hobJac)
```

Clearly there is still some work to do to make the mechanism for getting derivatives more easily, and with less chance of error. Work still to do???.

### Jacobians

Jacobians, the matrices of partial derivatives of residuals with respect to the parameters, have a vector input (the parameters). `nlsr` attempts to generate the code for this computation. This is the first key improvement of `nlsr` over `nls()`. It will likely be a continuing effort to enlarge the set of functions and expressions that can be handled and to deal with them more efficiently. Also it would be nice to automate the generation of numerical approximations for those components of the Jacobian for which analytic derivatives are not available.

Note that the Jacobian inner product (J^T J) differs from the Hessian of the sum of squares by a matrix whose elements that are an inner product of the residuals with their second derivatives with respect to the parameters. This is a summation of m matrices each involving a residual times its (n by n) partial derivatives with respect to the parameters. Generally we treat this matrix as "ignorable" on the basis that the residuals should be "small", but clearly this is not always the case.

## Implementation of nonlinear least squares methods

### Gauss-Newton variants

Nonlinear least squares methods are mostly founded on some or other variant of the Gauss-Newton algorithm. The function we wish to minimize is the sum of squares of the (nonlinear) residuals r(x) where there are m observations (elements of r) and n parameters x. Hence the function is

```f(x) = sum(r(k)^2)
```

Newton's method starts with an original set of parameters x[0]. At a given iteraion, which could be the first, we want to solve

```x[k+1]  =  x[k]  -   H^(-1) g
```

where H is the Hessian and g is the gradient at x[k]. We can rewrite this as a solution, at each iteration, of

```H delta = -g
```

with

```x[k+1]  =  x[k] + delta
```

For the particular sum of squares, the gradient is

```g(x) = 2 * r(k)
```

and

```H(x) = 2 ( J' J  +  sum(r * Z))
```

where J is the Jacobian (first derivatives of r w.r.t. x) and Z is the tensor of second derivatives of r w.r.t. x). Note that J' is the transpose of J.

The primary simplification of the Gauss-Newton method is to assume that the second term above is negligible. As there is a common factor of 2 on each side of the Newton iteration after the simplification of the Hessian, the Gauss-Newton iteration equation is

```J' J  delta = - J' r
```

This iteration frequently fails. The approximation of the Hessian by the Jacobian inner-product is one reason, but there is also the possibility that the sum of squares function is not "quadratic" enough that the unit step reduces it. @Hartley61 introduced a line search along delta, while @Marq63 suggested replacing J' J with (J' J + lambda * D) where D is a diagonal matrix intended to partially approximate the omitted portion of the Hessian.

Marquardt suggested D = I (a unit matrix) or D = (diagonal part of J' J). The former approach, when lambda is large enough that the iteration is essentially

```delta = - g / lambda
```

we get a version of the steepest descents algorithm. Using the diagonal of J' J, we have a scaled version of this (see https://en.wikipedia.org/wiki/Levenberg%E2%80%93Marquardt_algorithm)

@jn77ima found that on low precision machines, it was common for diagonal elements of J' J to underflow. A very small modification to solve

```(J' J + lambda * (D + phi * I)) * delta = - g
```

where phi is a small number (phi = 1 seems to work quite well) ?? check??.

In jncnm79, the iteration equation was solved as stated. However, this involves forming the sum of squares and cross products of J, a process that loses some numerical precision. A better way to solve the linear equations is to apply the QR decomposition to the matrix J itself. However, we still need to incorporate the lambda * I or lambda * D adjustments. This is done by adding rows to J that are the square roots of the "pieces". We add 1 row for each diagonal element of I and each diagonal element of D.

In each iteration, we reduce the lambda parameter before solution. If the resulting sum of squares is not reduced, lambda is increased, otherwise we move to the next iteration. Various authors (including the present one) have suggested different strategies for this. My current opinion is that a "quick" increase, say a factor of 10, and a "slow" decrease, say a factor of 0.2, work quite well. However, it is important to check that lambda has not got too small or underflowed before applying the increase factor. On the other hand, it is useful to be able to set lambda = 0 in the code so that a pure Gauss-Newton method can be evaluated with the program(s). The current code `nlfb()` uses the line

`if (lamda<1000*.Machine\$double.eps) lamda<-1000*.Machine\$double.eps`

to ensure we get an increase. To force a Gauss-Newton algorithm, the controls `laminc` and `lamdec` are set to 0.

The Levenberg-Marquardt adjustment to the Gauss-Newton approach is the second major improvement of `nlsr` (and also its predecessor `nlmrt` and the package `minpack-lm`) over `nls()`.

### Termination and convergence tests

The success of many optimization codes often depends on knowing when no more progress can be made. For the present codes, one simple procedure increases the damping parameter lamda whenever the sum of squares cannot be decreased, with termination when the parameters are not altered by the addition of delta. While this "works", it does incur unnecessary computational effort.

A better approach is that of @BatesWatts81. Their relative offset criterion considers the potential progress that could be made in reducing the current residuals to the initial sum of squares. This is a sensible and very effective strategy for most situations, but is dangerous where the problem has very small or zero residuals, as in the case of an interpolation problem or nonlinear equations.

To illustrate this for zero residual problems, let us set up a simple test.

```## test small resid case with roffset
tt <- 1:25
ymod <- 10 * exp(-0.01*tt) + 5
n <- length(tt)
evec0 <- rep(0, n)
evec1 <- 1e-4*runif(n, -.5, .5)
evec2 <-  1e-1*runif(n, -.5, .5)
y0 <- ymod + evec0
y1 <- ymod + evec1
y2 <- ymod + evec2
mydata <- data.frame(tt, y0, y1, y2)
st <- c(aa=1, bb=1, cc=1)
```

We provide three sizes of residuals here, but will only illustrate the consequences of zero residuals (the problem with y0). Let us run the nonlinear least squares problem to get the interpolating function, first with `nls()`, then with `nlxb()`. In the second case we have turned off the trace, as the approach "works" because we have taken care in the code to provide for problems with small residuals.

```nlsfit0 <-  try(nls(y0 ~ aa * exp(-bb*tt) + cc, start=st, data=mydata, trace=TRUE))
nlsfit0
library(nlsr)
nlsrfit0 <- try(nlxb(y0 ~ aa * exp(-bb*tt) + cc, start=st, data=mydata, trace=FALSE))
nlsrfit0
```

We can approximate what `nls()` is doing by running a simple Gauss-Newton method one step at a time. It is (or was at the time of writing) easier to do this in functional form with explicit derivatives. Note that we test these functions!

```trf <- function(par, data) {
tt <- data[,"tt"]
res <- par["aa"]*exp(-par["bb"]*tt) + par["cc"] - y0
}
print(trf(st, data=mydata))
trj <- function(par, data) {
tt <- data[,"tt"]
m <- dim(data)[1]
JJ <- matrix(NA, nrow=m, ncol=3)
JJ[,1] <- exp(-par["bb"]*tt)
JJ[,2] <- - tt * par["aa"] * exp(-par["bb"]*tt)
JJ[,3] <- 1
JJ
}
Ja <- trj(st, data=mydata)
print(Ja)

library(numDeriv)
Jn <- jacobian(trf, st, data=mydata)
print(Jn)
print(max(abs(Jn-Ja)))
ssf <- function(par, data){
rr <- trf(par, data)
ss <- crossprod(rr)
}
print(ssf(st, data=mydata))

library(numDeriv)
print(jacobian(trf, st, data=mydata))
```

The traditional way to implement a Gauss Newton method was to form the sum of squares and cross-products matrix at in traditional linear regression, using the Jacobian as the data matrix. The right hand side uses the inner product of the Jacobian with the negative residuals. This approach increases the condition number of the problem, and a more direct QR decomposition of the Jacobian is now the preferred approach. However, let us try both to ensure we are getting similar answers. First the QR approach.

```gnjn <- function(start, resfn, jacfn = NULL, trace = FALSE,
data=NULL, control=list(), ...){
# simplified Gauss Newton
offset = 1e6 # for no change in parms
par <- start
cat("starting parameters:")
print(par)
res <- resfn(par, data, ...)
ssbest <- as.numeric(crossprod(res))
cat("initial ss=",ssbest,"\n")
par0 <- par
kres <- 1
kjac <- 0
keepon <- TRUE
while (keepon) {
cat("kjac=",kjac,"  kres=",kres,"  SSbest now ", ssbest,"\n")
print(par)
JJ <- jacfn(par, data, ...)
kjac <- kjac + 1
QJ <- qr(JJ)
delta <- qr.coef(QJ, -res)
ss <- ssbest + offset*offset # force evaluation
step <- 1.0
if (as.numeric(max(par0+delta)+offset) != as.numeric(max(par0+offset)) ) {
while (ss > ssbest) {
par <- par0+delta * step
res <- resfn(par, data, ...)
ss <- as.numeric(crossprod(res))
kres <- kres + 1
##           cat("step =", step,"  ss=",ss,"\n")
if (ss > ssbest) {
step <- step * stepred
} else {
par0 <- par
ssbest <- ss
}
} # end inner loop
if (kjac >= 4)  {
keepon = FALSE
cat("artificial stop at kjac=4 -- we only want to check output")
}
} else { keepon <- FALSE # done }
} # end main iteration
} # seems to need this

} # end gnjne

fitgnjn0 <- gnjn(st, trf, trj, data=mydata)
## Another way
#- set lamda = 0 in nlxb, fix laminc, lamdec
library(nlsr)
nlx00 <- try(nlxb(y0 ~ aa * exp(-bb*tt) + cc, start=st, data=mydata, trace=TRUE,
control=list(lamda=0, laminc=0, lamdec=0, watch=TRUE)))
nlx00
```

The first iteration more or less matches the `nls()` result. The problem is quite ill-conditioned, and `nls()` is using a numerical approximation to the Jacobian, so deviations of this magnitude from the iterations are not unexpected.

Let us test with a traditional Gauss-Newton.

```gnjn2 <- function(start, resfn, jacfn = NULL, trace = FALSE,
data=NULL, control=list(), ...){
# simplified Gauss Newton
offset = 1e6 # for no change in parms
par <- start
cat("starting parameters:")
print(par)
res <- resfn(par, data, ...)
ssbest <- as.numeric(crossprod(res))
cat("initial ss=",ssbest,"\n")
kres <- 1
kjac <- 0
par0 <- par
keepon <- TRUE
while (keepon) {
cat("kres=",kres,"  kjac=",kjac,"   SSbest now ", ssbest,"\n")
print(par)
JJ <- jacfn(par, data, ...)
kjac <- kjac + 1
JTJ <- crossprod (JJ)
JTr <- crossprod (JJ, res)
delta <- - as.vector(solve(JTJ, JTr))
ss <- ssbest + offset*offset # force evaluation
step <- 1.0
if (as.numeric(max(par0+delta)+offset) != as.numeric(max(par0+offset)) ) {
while (ss > ssbest) {
par <- par0+delta * step
res <- resfn(par, data, ...)
ss <- as.numeric(crossprod(res))
kres <- kres + 1
##           cat("step =", step,"  ss=",ss,"  best is",ssbest,"\n")
if (ss > ssbest) {
step <- step * stepred
} else {
par0 <- par
ssbest <- ss
}
} # end inner loop
if (kjac >= 4)  {
keepon = FALSE
cat("artificial stop at kjac=4 -- we only want to check output")
}
} else { keepon <- FALSE # done }
} # end main iteration
} # seems to need this
} # end gnjn2

fitgnjn20 <- gnjn2(st, trf, trj, data=mydata)
```

### Nonlinear equations

Solution of sets of nonlinear equations is generally NOT a problem that is commonly required for statisticians or data analysts. My experience is that the occasions where it does arise are when workers try to solve the first order conditions for optimality of a function, rather than try to optimize the function. If this function is a sum of squares, then we have a nonlinear least squares problem, and generally such problems are best approached my methods of the type discussed in this article.

Conversely, since our problem is, using the notation above, equivalent to

r(x) = 0

the solution of a nonlinear least squares problem for which the final sum of squares is zero provides a solution to the nonlinear equations. In my experience this is a valid approach to the nonlinear equations problem, especially if there is concern that a solution may not exist. Note that there are methods for nonlinear equations, some of which (??ref) are available in R packages.

## Function versus expression approach

In `nlsr`, we provide for modelling via R functions that generate the residuals and Jacobian, and we call the nonlinear least squares minimizer through `nlfb`. Alternatively, we can do the modelling via the `nlxb` function that takes a model expression as an argument and converts it to the functional form. The actual least squares minimization is then carried out by a common routine (`nlfb`). Generally `nlxb` is a lot less programming work, but it is also more limited, as we can only handle single line expressions, and some expressions may not be differentiable within the package, even if there are mathematically feasible ways to compute them analytically.

## Providing extra data to expressions

Almost all statistical functions have exogenous data that is needed to compute residuals or likelihoods and is not dependent on the model parameters. (This section starts from Notes140806.)

model2rjfun does NOT have ... args.

Should it have? i.e., a problem where we are fitting a set of time series, 1 for each plant/animal, with some sort of start parameter for each that is NOT estimated (e.g., pH of soil, some index of health).

Difficulty in such a problem is that the residuals are then a matrix, and the nlfb rather than nlxb is a better approach. However, fitting 1 series would still need this data, and example nlsrtryextraparms.txt shows that the extra parm (ms in this case) needs to be in the user's globalenv.

```rm(list=ls())
require(nlsr)
# want to have data AND extra parameters (NOT to be estimated)
traceval  <-  TRUE  # traceval set TRUE to debug or give full history
# Data for Hobbs problem
ydat  <-  c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443,
38.558, 50.156, 62.948, 75.995, 91.972) # for testing
tdat  <-  seq_along(ydat) # for testing
# A simple starting vector -- must have named parameters for nlxb, nls, wrapnlsr.
start1  <-  c(b1=1, b2=1, b3=1)
startf1  <-  c(b1=1, b2=1, b3=.1)
eunsc  <-   y ~ b1/(1+b2*exp(-b3*tt))
cat("LOCAL DATA IN DATA FRAMES\n")
weeddata1  <-  data.frame(y=ydat, tt=tdat)
cat("weeddata contains the original data\n")
ms <- 2 # define the external parameter here
cat("wdata scales y by ms =",ms,"\n")
wdata <- data.frame(y=ydat/ms, tt=tdat)
wdata
cat("estimate the UNSCALED model with SCALED data\n")
anlxbs  <-  try(nlxb(eunsc, start=start1, trace=traceval, data=wdata))
print(anlxbs)
escal <-  y ~ ms*b1/(1+b2*exp(-b3*tt))
cat("estimate the SCALED model with scaling provided in the call (ms=0.5)\n")
anlxbh  <-  try(nlxb(escal, start=start1, trace=traceval, data=weeddata1, ms=0.5))
print(anlxbh)
cat("\n scaling is now using the globally defined value of ms=",ms,"\n")
anlxb1a  <-  try(nlxb(escal, start=start1, trace=traceval, data=weeddata1))
print(anlxb1a)
ms <- 1
cat("\n scaling is now using the globally re-defined value of ms=",ms,"\n")
anlxb1b  <-  try(nlxb(escal, start=start1, trace=traceval, data=weeddata1))
print(anlxb1b)
```

## Examples of use

### Nonlinear least squares with expressions

We use the Hobbs Weeds problem (Nash, 1979 and Nash, 2014). Note that nls() fails from start1.

```require(nlsr)
traceval <- FALSE
# Data for Hobbs problem
ydat  <-  c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443,
38.558, 50.156, 62.948, 75.995, 91.972) # for testing
tdat  <-  seq_along(ydat) # for testing

# A simple starting vector -- must have named parameters for nlxb, nls, wrapnlsr.
start1  <-  c(b1=1, b2=1, b3=1)

eunsc  <-   y ~ b1/(1+b2*exp(-b3*tt))

cat("LOCAL DATA IN DATA FRAMES\n")
weeddata1  <-  data.frame(y=ydat, tt=tdat)
weeddata2  <-  data.frame(y=1.5*ydat, tt=tdat)

anlxb1  <-  try(nlxb(eunsc, start=start1, trace=TRUE, data=weeddata1,
control=list(watch=FALSE)))
print(anlxb1)

anlsb1 <-  try(nls(eunsc, start=start1, trace=TRUE, data=weeddata1))
print(anlsb1)
```

A different start causes nlxb to return a large sum of squares. Note that nls() again fails.

```startf1  <-  c(b1=1, b2=1, b3=.1)
anlsf1 <-  try(nls(eunsc, start=startf1, trace=TRUE, data=weeddata1))
print(anlsf1)

library(nlsr)
anlxf1  <-  try(nlxb(eunsc, start=startf1, trace=TRUE, data=weeddata1,
control=list(watch=FALSE)))
print(anlxf1)

# anlxb2  <-  try(nlxb(eunsc, start=start1, trace=FALSE, data=weeddata2))
# print(anlxb2)
```

We can discover quickly the difficulty here by computing the Jacobian at this "solution" and checking its singular values.

```cf1 <- coef(anlxf1)
print(cf1)
jf1 <- anlxf1\$jacobian
svals <- svd(jf1)\$d
print(svals)
```

Here we see that the Jacobian is only rank 1, even though there are 3 coefficients. It is therefore not surprising that our nonlinear least squares program has concluded we are unable to make further progress.

### Nonlinear least squares with functions

We can run the same example as above using R functions rather than expressions, but now we need to have a gradient function as well as one to compute residuals. nlsr has tools to create these functions from expressions, as we shall see here. First we again set up the data and load the package.

```require(nlsr)
traceval <- FALSE
# Data for Hobbs problem
ydat  <-  c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443,
38.558, 50.156, 62.948, 75.995, 91.972) # for testing
tdat  <-  seq_along(ydat) # for testing

# A simple starting vector -- must have named parameters for nlxb, nls, wrapnlsr.
start1  <-  c(b1=1, b2=1, b3=1)

eunsc  <-   y ~ b1/(1+b2*exp(-b3*tt))

cat("LOCAL DATA IN DATA FRAMES\n")
weeddata1  <-  data.frame(y=ydat, tt=tdat)
```
```weedrj <- model2rjfun(modelformula=eunsc, pvec=start1, data=weeddata1)

weedrj
rjfundoc(weedrj) # Note how useful this is to report status
```

### Why is resss difficult to use in optimization?

```y <- c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443, 38.558,
50.156, 62.948, 75.995, 91.972)
tt <- seq_along(y) # for testing
mydata <- data.frame(y = y, tt = tt)
f <- y ~ 100*b1/(1 + 10*b2 * exp(-0.1 * b3 * tt))
p <- c(b1 = 1, b2 = 1, b3 = 1)
rjfn <- model2rjfun(f, p, data = mydata)
rjfn(p)

myfn <- function(p, resfn=rjfn){
val <- resss(p, resfn)
}

p <- c(b1 = 2, b2=2, b3=1)

a1 <- optim(p, myfn, control=list(trace=0))
a1
```

We can also embed the function directly.

```a2 <- optim(p, function(p,resfn=rjfn){resss(p,resfn)}, control=list(trace=0))
a2
```

## Using derivative functions to generate gradient functions

?? from a different vignette, need to integrate

One of the common needs for optimization computations is the availability of accurate gradients of the objective functions. While differentiation is relatively straightforward, it is tedious and error-prone.

At 2015-1-24, I have not determined how to automate the use the output of the derivatives generated by nlsr to create a working gradient function. However, the following write-up shows how such a function can be generated in a semi-automatic way.

We use an example that appeared on the R-help mailing list on Jan 14, 2015. Responses by Ravi Varadhan and others, along with some modification I made, gave the following negative log likelihood function to be minimized.

```require(nlsr)

y=c(5,11,21,31,46,75,98,122,145,165,195,224,245,293,321,330,350,420) # data set

Nweibull2 <- function(x,prm){
la <- prm[1]
al <- prm[2]
be<- prm[3]
val2 <- la*be*(x/al)^(be-1)* exp( (x/al)^be+la*al*(1-exp((x/al)^be) ) )
val2
}
LL2J <- function(par,y) {
R = Nweibull2(y,par)
-sum(log(R))
}
```

We want the gradient of LL3J() with respect to par, and first compute the derivatives of Nweibull2() with respect to the paramters prm

We start with the central expression in Nweibull2() and compute its partial derivatives. The expression is:

```  la*be*(x/al)^(be-1)* exp( (x/al)^be+la*al*(1-exp((x/al)^be) ) )
```
```# Put in the main expression for the Nweibull pdf.
## we generate the three gradient components
g1n <- nlsDeriv(~ la*be*(x/al)^(be-1)* exp( (x/al)^be+la*al*(1-exp((x/al)^be) ) ), "la")
g1n
g2n <- nlsDeriv(~ la*be*(x/al)^(be-1)* exp( (x/al)^be+la*al*(1-exp((x/al)^be) ) ), "al")
g2n
g3n <- nlsDeriv(~ la*be*(x/al)^(be-1)* exp( (x/al)^be+la*al*(1-exp((x/al)^be) ) ), "be")
g3n
```

By copying and pasting the output above into a function structure, we get Nwei2g() below.

```Nwei2g <- function(x, prm){
la <- prm[1]
al <- prm[2]
be<- prm[3]
g1v <- la * be * (x/al)^(be - 1) * (exp((x/al)^be + la * al * (1 - exp((x/al)^be))) *
(al * (1 - exp((x/al)^be)))) + be * (x/al)^(be - 1) * exp((x/al)^be +
la * al * (1 - exp((x/al)^be)))

g2v <- la * be * (x/al)^(be - 1) * (exp((x/al)^be + la * al * (1 - exp((x/al)^be))) *
(be * (x/al)^(be - 1) * -(x/al^2) + (la * al * -(exp((x/al)^be) *
(be * (x/al)^(be - 1) * -(x/al^2))) + la * (1 - exp((x/al)^be))))) +
la * be * ((be - 1) * (x/al)^(be - 1 - 1) * -(x/al^2)) *
exp((x/al)^be + la * al * (1 - exp((x/al)^be)))

g3v <- la * be * (x/al)^(be - 1) * (exp((x/al)^be + la * al * (1 - exp((x/al)^be))) *
((x/al)^be * log(x/al) + la * al * -(exp((x/al)^be) * ((x/al)^be *
log(x/al))))) + (la * be * ((x/al)^(be - 1) * log(x/al)) +
la * (x/al)^(be - 1)) * exp((x/al)^be + la * al * (1 - exp((x/al)^be)))
gg <- matrix(data=c(g1v, g2v, g3v), ncol=3)
}
```

We can check this gradient functionusing the grad() function from package numDeriv.

```start1 <- c(lambda=.01,alpha=340,beta=.8)
start2 <- c(lambda=.01,alpha=340,beta=.7)

require(numDeriv)
ganwei <- Nwei2g(y, prm=start1)

require(numDeriv)
Nw <- function(x, y) {
Nweibull2(y, x)
} # to allow grad() to work

gnnwei <- matrix(NA, nrow=length(y), ncol=3)
for (i in 1:length(y)){
gnnwei[i,] <- gnrow
}
gnnwei
ganwei
cat("max(abs(gnnwei - ganwei))= ",   max(abs(gnnwei - ganwei)),"\n")
```

Now we can build the gradient of the objective function. This requires an application of the chain rule to the summation of logs of the elements of the quantity R. Since the derivative of log(R) w.r.t. R is simply 1/R, this is relatively simple. However, I have not found how to automate this.

```## and now we can build the gradient of LL2J
LL2Jg <- function(prm, y) {
R = Nweibull2(y,prm)
gNN <- Nwei2g(y, prm)
#    print(str(gNN)
gL <- - as.vector( t(1/R) %*% gNN)
}
# test
gaLL2J <- LL2Jg(start1, y)
gaLL2J
gnLL2J
cat("max(abs(gaLL2J-gnLL2J))= ", max(abs(gaLL2J-gnLL2J)), "\n" )
```

## Appendix A: Providing exogenous data

These examples show dotargs do NOT work for any of nlsr, nls, or minpack.lm. Use of a dataframe or local (calling) environment objects does work in all.

```library(knitr)
# read chunk (does not run code)
# We use this approach to avoid having differences between file here and in inst/extdata/
```
```## @knitr nlsdata.R
# try different ways of supplying data to R nls stuff
ydata <- c(5.308, 7.24, 9.638, 12.866, 17.069, 23.192, 31.443, 38.558,
50.156, 62.948, 75.995, 91.972)
ttdata <- seq_along(ydata) # for testing

mydata <- data.frame(y = ydata, tt = ttdata)

hobsc <- y ~ 100*b1/(1 + 10*b2 * exp(-0.1 * b3 * tt))

ste <- c(b1 = 2, b2 = 5, b3 = 3)

# let's try finding the variables

findmainenv <- function(formula, prm) {
vn <- all.vars(formula)
pnames <- names(prm)
ppos <- match(pnames, vn)
datvar <- vn[-ppos]
cat("Data variables:")
print(datvar)
cat("Are the variables present in the current working environment?\n")
for (i in seq_along(datvar)){
cat(datvar[[i]]," : present=",exists(datvar[[i]]),"\n")
}
}

findmainenv(hobsc, ste)
y <- ydata
tt <- ttdata
findmainenv(hobsc, ste)
rm(y)
rm(tt)

# ===============================

# let's try finding the variables in dotargs

finddotargs <- function(formula, prm, ...) {
dots <- list(...)
cat("dots:")
print(dots)
cat("names in dots:")
dtn <- names(dots)
print(dtn)
vn <- all.vars(formula)
pnames <- names(prm)

ppos <- match(pnames, vn)
datvar <- vn[-ppos]
cat("Data variables:")
print(datvar)
cat("Are the variables present in the dot args?\n")
for (i in seq_along(datvar)){
dname <- datvar[[i]]
cat(dname," : present=",(dname %in% dtn),"\n")
}
}

finddotargs(hobsc, ste, y=ydata, tt=ttdata)
# ===============================

y <- ydata
tt <- ttdata
tryq <- try(nlsquiet <- nls(formula=hobsc, start=ste))
if (class(tryq) != "try-error") {print(nlsquiet)} else {cat("try-error\n")}
#- OK
rm(y)
rm(tt)

tdots<-try(nlsdots <- nls(formula=hobsc, start=ste, y=ydata, tt=ttdata))
if ( class(tdots) != "try-error") {print(nlsdots)} else {cat("try-error\n")}
#- Fails
tframe <- try(nlsframe <- nls(formula=hobsc, start=ste, data=mydata))
if (class(tframe) != "try-error") {print(nlsframe)} else {cat("try-error\n")}
#- OK

library(nlsr)
y <- ydata
tt <- ttdata
tquiet <- try(nlsrquiet <- nlxb(formula=hobsc, start=ste))
if ( class(tquiet) != "try-error") {print(nlsrquiet)}
#- OK
rm(y)
rm(tt)
test <- try(nlsrdots <- nlxb(formula=hobsc, start=ste, y=ydata, tt=ttdata))
if (class(test) != "try-error") { print(nlsrdots) } else {cat("Try error\n") }
#- Note -- does NOT work -- do we need to specify the present env. in nlfb for y, tt??
test2 <- try(nlsframe <- nls(formula=hobsc, start=ste, data=mydata))
if (class(test) != "try-error") {print(nlsframe) } else {cat("Try error\n") }
#- OK

library(minpack.lm)
y <- ydata
tt <- ttdata
nlsLMquiet <- nlsLM(formula=hobsc, start=ste)
print(nlsLMquiet)
#- OK
rm(y)
rm(tt)
## Dotargs
tdots <- try(nlsLMdots <- nlsLM(formula=hobsc, start=ste, y=ydata, tt=ttdata))
if (class(tdots) != "try-error") { print(nlsLMdots) } else {cat("try-error\n") }
#-  Note -- does NOT work
## dataframe
tframe <- try(nlsLMframe <- nlsLM(formula=hobsc, start=ste, data=mydata) )
if (class(tdots) != "try-error") {print(nlsLMframe)} else {cat("try-error\n") }
#- does not work

## Uses nlmrt here for comparison
## library(nlmrt)
## txq <- try( nlxbquiet <- nlxb(formula=hobsc, start=ste))
## if (class(txq) != "try-error") {print(nlxbquiet)} else { cat("try-error\n")}
#- Note -- does NOT work
## txdots <- try( nlxbdots <- nlxb(formula=hobsc, start=ste, y=y, tt=tt) )
## if (class(txdots) != "try-error") {print(nlxbdots)} else {cat("try-error\n")}
#- Note -- does NOT work
## dataframe
## nlxbframe <- nlxb(formula=hobsc, start=ste, data=mydata)
## print(nlxbframe)
#- OK
```

# References

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nlsr documentation built on Jan. 29, 2018, 3 a.m.