Below is an example of a problem I encounter repeatedly when I write
functions.  A call works at the command line, but it does not work inside a
function, even when I have made sure that all required variables are
available within the function. The only way I know to solve it is to make
the required variable global, which of course is dangerous. What is the
elegant or appropriate way to solve this?


# The call to ns works at the command line:

> junk<-ns(DAT$Age, knots=74)

# The call to lmer works at the command line, with the call to ns nested
within it:

> junk2<-lmer(formula=Finished ~ Sex01 + ns(DAT$Age, knots= 74 ) + MinCC +
PzC + (1 | NAME ) + (1 | Year), data=myDATonly, family="binomial")

But now I want to do this within a function such as the following:

> myfn
function(myknot=74) {
         library(lme4)
         library(splines)
         cat("myknot=", myknot, "\n")
         myMER<-lmer(formula=Finished ~ Sex01 + ns(DAT$Age, knots= myknot) +
MinCC + PzC + (1 | NAME ) + (1 | Year), data=myDATonly, family="binomial")
}

> myfn(74)
myknot= 74
Error in ns(DAT$Age, knots = myknot) : object "myknot" not found

# The bad way to do it: revise myfn to make myknot a global variable:
> myfn
function(myknot=74) {
         library(lme4)
         library(splines)
         cat("myknot=", myknot, "\n")
         myknot<<-myknot
         myMER<-lmer(formula=Finished ~ Sex01 + ns(DAT$Age, knots= myknot
) + MinCC + PzC + (1 | NAME ) + (1 | Year), data=myDATonly,
family="binomial")
}

> z<-myfn(74)
myknot= 74

# Runs fine but puts a global variable into .GlobalEnv.

> sessionInfo()
R version 2.6.2 (2008-02-08)
i386-pc-mingw32

locale:
LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
States.1252;LC_MONETARY=English_United
States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252

attached base packages:
[1] splines   stats     graphics  grDevices datasets  utils     methods
base

other attached packages:
[1] lme4_0.999375-13  Matrix_0.999375-9 lattice_0.17-6

loaded via a namespace (and not attached):
[1] boot_1.2-32 grid_2.6.2
>

Thanks for any insight.

Jacob A. Wegelin
jwegelin@vcu.edu
Assistant Professor
Department of Biostatistics
Virginia Commonwealth University
730 East Broad Street Room 3006
P. O. Box 980032
Richmond VA 23298-0032
U.S.A.
http://www.people.vcu.edu/~jwegelin

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