specifying arguments in functions and calling functions, within functions

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