R help - Feb 2012 - execute array of functions

Hi all,

I'm trying to get the min and max of a sequence of number using a loop 
like the folllowing. Can anyone point me to why it doesn't work.

Thanks.

type 	<- c("min","max")
n 	<- 1:10
for (a in 1:2) 	  {
print(type[a](n)) }


-- 
Muhammad

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Muhammad Rahiz
> Sent: Tuesday, February 14, 2012 11:03 AM
> To: x.r-help
> Subject: [R] execute array of functions
> 
> Hi all,
> 
> I'm trying to get the min and max of a sequence of number using a loop
> like the folllowing. Can anyone point me to why it doesn't work.
> 
> Thanks.
> 
> type 	<- c("min","max")
> n 	<- 1:10
> for (a in 1:2) 	  {
> print(type[a](n)) }
> 
> 
> --
> Muhammad
> 
I am not sure why you are trying to use this approach in a loop, but you need to
use the get() function if you want to do this.

type <- c("min","max")
n <- 1:10
for (a in 1:2) 	  {
  print(get(type[a])(n)) 
}

But, why not use min and max on n directly?


Hope this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

That won't work because R has special rules for evaluating things in the
function position.  Examples:

*OK*

min(1:2)
"min"(1:2)
f<-min; f(1:2)
do.call(min,list(1:2))
do.call("min",list(1:2))      # do.call converts string->function


*Not OK*

("min")(1:2)              # string in function position is not
converted
f<-"min"; f(1:2)       # ditto
f<- c("min","max");  f[1](1:2)  # ditto


What you need to do is make 'f' a list of *function values, *not a
vector
of strings:

f<- c(min,max)


and then select the element of f with [[ ]] (select one element), not [ ]
(select sublist):

f[[1]](1:2)


Thus your example becomes

type    <- c(min,max)
n       <- 1:10
for (a in 1:2)    {
print(type[[a]](n)) }

Another (uglier) approach is with do.call:

type    <- c("min","max")
n       <- 1:10
for (a in 1:2)    {
print(do.call(type[a],list(n))) }


Does that help?

             -s

On Tue, Feb 14, 2012 at 14:02, Muhammad Rahiz
<muhammad.rahiz@ouce.ox.ac.uk>wrote:
> Hi all,
>
> I'm trying to get the min and max of a sequence of number using a loop
> like the folllowing. Can anyone point me to why it doesn't work.
>
> Thanks.
>
> type    <- c("min","max")
> n       <- 1:10
> for (a in 1:2)    {
> print(type[a](n)) }
>
>
> --
> Muhammad
>
> ______________________________**________________
> R-help@r-project.org mailing list
>
https://stat.ethz.ch/mailman/**listinfo/r-help<https://stat.ethz.ch/mailman/listinfo/r-help>
> PLEASE do read the posting guide http://www.R-project.org/**
> posting-guide.html <http://www.R-project.org/posting-guide.html>
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

Not sure the replies you received make it clear, but the key point is
that functions are first class objects in R like any other objects
(vectors, lists, data frames) and can be assigned, collected into
structures, indexed, etc. in the usual way. Further, if the value of
any R expression is a function object, it can be called in the usual
way on an argument list by:

Some R_Expression(comma separated argument list)

e.g.
> {y <-function(x) sin(x)+10} (pi/2) ## pi is a known named constant in R
11

Your initial confusion below actually has nothing to do with this: you
have confused unquoted names of objects in R with quoted character
strings. So, e.g.

x <- 1:10
assigns a sequence of numeric values to the name x, which essentially
points to where those values are stored in memory.

typing
>x
at the command line would print the values (by default)

typing
>"x"
prints the character string "x"

-- Bert

On Tue, Feb 14, 2012 at 11:02 AM, Muhammad Rahiz
<muhammad.rahiz at ouce.ox.ac.uk> wrote:
> Hi all,
>
> I'm trying to get the min and max of a sequence of number using a loop
like
> the folllowing. Can anyone point me to why it doesn't work.
>
> Thanks.
>
> type ? ?<- c("min","max")
> n ? ? ? <- 1:10
> for (a in 1:2) ? ?{
> print(type[a](n)) }
>
>
> --
> Muhammad
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.


-- 

Bert Gunter
Genentech Nonclinical Biostatistics

Internal Contact Info:
Phone: 467-7374
Website:
http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm