R help - Nov 2008 - basic information defining functions

Hi all,

i am looking from some insights to define own R functions. so far i  
found most basics in documentations that are around on the web. except  
for one thing:

I?d like to define some function, say:

#assume my data matrix contains vectors like data$myColumn1,data 
$myColumn2 etc.

getMyColumn <- function (columnid){

x<-data$MyColumn?columnid?[data$indexone=1 & data$index2=5]

return(x)

}

Do I need to use assign or eval first ? I tried to use paste to  
combine something like:
paste("data$MyColumn",columnid,sep="") which
did not work.

I am happy to get any help with the problem, but also thankful for  
some useful link or guide on how to define own functions properly,  
especially the dynamic naming and return part

thx in advance

bunny

> i am looking from some insights to define own R functions. so far i 
> found most basics in documentations that are around on the web. except 
> for one thing:
> 
> I?d like to define some function, say:
> 
> #assume my data matrix contains vectors like data$myColumn1,data 
> $myColumn2 etc.
Do you really mean data matrix, or do you mean data frame?
> getMyColumn <- function (columnid){
> 
> x<-data$MyColumn?columnid?[data$indexone=1 & data$index2=5]
> 
> return(x)
> 
> }
It's not terribly clear what you want to do with this function.  Question 
marks don't mean anything in the middle of a statement, so I don't know 
what 
data$MyColumn?columnid?[data$indexone=1 & data$index2=5] 
is supposed to do.

Also, you don't need the return keyword - the last line of a function is 
the return value.
> Do I need to use assign or eval first ? I tried to use paste to 
> combine something like:
paste("data$MyColumn",columnid,sep="") which
> did not work.
paste returns a string, not a variable.  You can evaluate the contents of 
the string using 
eval(parse(text=paste("data$MyColumn",columnid,sep="")))
This is however a method of last resort; there is almost always a better 
way to do things than using eval.
 
> I am happy to get any help with the problem, but also thankful for 
> some useful link or guide on how to define own functions properly, 
> especially the dynamic naming and return part
You probably don't need to bother with dynamic naming, but take a look at 
FAW on R, q7.21.

Regards,
Richie.

Mathematical Sciences Unit
HSL

------------------------------------------------------------------------
ATTENTION:

This message contains privileged and confidential inform...{{dropped:20}}

I think your problem is more with indexing than with function writing.  The main
confusion is in how to use '$', this is a shortcut to make certain
things easier, but you are trying to use the shortcut like going from France to
Germany by way of New York City because you know a great shortcut through NYC.
>From the help page for $ (> ?'$'):
"Both '[[' and '$' select a single element of the list. 
The main
     difference is that '$' does not allow computed indices, whereas
     '[[' does."

You are trying to use a computed index, look at the line above (especially the
'not') and see if there is something else there suggests what you should
be using.

This seems to be a common misunderstanding lately, could you tell us a bit about
which documentation and examples you read that lead you to think about using
'$' with computed indices and did not help you understand the need for
'[[' instead so that maybe the documentation can be improved to help
future readers?

--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Bunny, lautloscrew.com
> Sent: Tuesday, November 25, 2008 3:18 AM
> To: r-help at r-project.org
> Subject: [R] basic information defining functions
>
> Hi all,
>
> i am looking from some insights to define own R functions. so far i
> found most basics in documentations that are around on the web. except
> for one thing:
>
> I?d like to define some function, say:
>
> #assume my data matrix contains vectors like data$myColumn1,data
> $myColumn2 etc.
>
> getMyColumn <- function (columnid){
>
> x<-data$MyColumn?columnid?[data$indexone=1 & data$index2=5]
>
> return(x)
>
> }
>
> Do I need to use assign or eval first ? I tried to use paste to
> combine something like:
paste("data$MyColumn",columnid,sep="") which
> did not work.
>
> I am happy to get any help with the problem, but also thankful for
> some useful link or guide on how to define own functions properly,
> especially the dynamic naming and return part
>
> thx in advance
>
> bunny
>
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