[R] specifying arguments in functions and calling functions, within functions

[David L Carlson]

Adding on to what Sarah has said, your function appears to limit the
functions that can be passed to it, but it does not. The character strings
"mean" and "median" will fail, but passing the function name directly will
work:

> Scale <- function(x, method=mean,...) {
>    scl <- method
>    scl(x, ...)
> }

The method=c("mean", "median") is irrelevant to the function because you
never check to guarantee that only those strings are used in the function
call. This is fortunate since those strings will fail whereas passing any
function that requires a single vector as input will work just fine:

> set.seed(42)
> ex <- runif(10)
> ex[5] <- NA
> Scale(ex)
 [1] NA
> Scale(ex, na.rm=TRUE)
[1] 0.635653
> Scale(ex, "median")
Error in Scale(ex, "median") : could not find function "scl"
> Scale(ex, median)
[1] NA
> Scale(ex, median, na.rm=TRUE)
[1] 0.7050648
> Scale(ex, fivenum)
[1] 0.1346666 0.5190959 0.7050648 0.8304476 0.9370754
> Scale(ex, hist)

----------------------------------------------
David L Carlson
Associate Professor of Anthropology
Texas A&M University
College Station, TX 77843-4352

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Sarah Goslee
> Sent: Wednesday, September 26, 2012 1:26 PM
> To: K. Brand
> Cc: r-help at r-project.org
> Subject: Re: [R] specifying arguments in functions and calling
> functions, within functions
> 
> Hi,
> 
> You have some basic confusion here, and a problem likely caused by an
> object named scl that exists in your global environment.
> 
> On Wed, Sep 26, 2012 at 11:56 AM, K. Brand <k.brand at erasmusmc.nl>
> wrote:
> > Esteemed R UseRs,
> >
> > Regarding specifying arguments in functions and calling functions
> > within functions:
> >
> > ## beginning ##
> > ## some data
> > ex <- rnorm(10)
> > ex[5] <- NA
> >
> > ## example function
> > Scale <- function(x, method=c("mean", "median")) {
> >    scl <- method
> >    scl(x)
> > }
> >
> > ## both return NA
> > Scale(ex, method=median)
> > median(ex, na.rm=FALSE)
> >
> > ## both return the median
> > Scale(ex, method="median")
> > median(ex, na.rm=TRUE)
> >
> > ## 1. Why does the use of apostrophes have this effect when calling
> > ## a fucntion within a function?
> 
> Those are double quotes, not apostrophes, and they don't have that
> effect:
> 
> > Scale(ex, method=median)
> [1] NA
> > Scale(ex, method="median")
> Error in Scale(ex, method = "median") : could not find function "scl"
> >
> 
> You probably have something named scl in your global environment: you
> can see that with ls().
> 
> Take a look at:
> 
> > class(median)
> [1] "function"
> > class("median")
> [1] "character"
> 
> In your first example, you're passing a function to Scale(), which
> gives it a new name then uses it. In the second you're passing a
> character string that happens to be the name of a function, which
> Scale() gives a new name and then tries but fails to use (because it
> isn't a function).
> 
> See also:
> > Scale(ex, "Nothing")
> Error in Scale(ex, "Nothing") : could not find function "scl"
> 
> > ## 2. What's the canonical use of apostrophes in functions like the
> above:
> > ## Scale <- function(x, method=c("mean", "median")) {....
> > ## or
> > ## Scale <- function(x, method=c(mean, median)) {....
> 
> Depends on whether you want to pass a function or the name of a
> function.
> 
> > ## 3. How does one specify the arguments of a function being called
> > ## within a function? i.e. i thought the use of '...' might work in
> > ## the following but i was wrong.
> >
> > ## '...' has no apparent effect
> > Scale <- function(x, method=c("mean", "median"),...) {
> >    scl <- method
> >    scl(x)
> > }
> 
> You can pass them explicitly as named arguments, or with ... as you
> try. In either case you have to hand those off to the function you
> want to use them:
> 
> Scale <- function(x, method=c("mean", "median"),...) {
>    scl <- method
>     scl(x, ...)
> }
> 
> Scale <- function(x, method=c("mean", "median"), na.rm=FALSE) {
>    scl <- method
>     scl(x, na.rm=na.rm)
> }
> 
> > ## both return NA
> > Scale(ex, method=median, na.rm=TRUE)
> > Scale(ex, method=median, na.rm=FALSE)
> > ## end ##
> >
> >
> > I failed to comprehend anything google returned when trying to
> > understand this myself. Greatly appreciate any thoughts &/or
> > examples on this.
> 
> Many functions that come with R are written purely in R and use these
> capabilities so you can look at them for examples.
> 
> Sarah
> 
> --
> Sarah Goslee
> http://www.functionaldiversity.org
> 
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