R help - Mar 2010 - passing columns of tables as a variable to functions

hello! ive come across the following problem:
im writing a function to ease the production of maps. as my tables have 
headers, id like to include an option in my function header to pass 
these columns to use them inside the evaluation block. the function 
looks as follows:

# maptable #the name of the table containing coordinates and columns for 
measured values
# value: which column of my table to use (e.g. Alt, slope, us, etc...
# labels(maptable)
# [1] "pos.e" "pos.n" "Alt" "slope"
"us" "uus" "lc"

make.map<-function (maptable,value)
{
pos.e<-maptable$pos.e #calling the column directly works
pos.n<-maptable$pos.n #calling the column directly works
key<-maptable$value #this doesnt work. output: NULL
... rest of function, irrelevant to my question
}

for example, i call the function by:

make.map(elevation,Alt,"Altitude","Sightings")

and my interp from akima package denies work, since key is NULL

interp(pos.e,pos.n,key,...)

have i just missed something? ive browsed the help pages and the 
archive, looked into crawleys R book but didnt find any solution, though 
im pretty sure this should be achievable!

Sacha -
    The "$" notation can only be used when you literally use
the name of the column you want.  If the name of the column
is stored in a variable, you should use this notation:

       maptable[,value]

to refer to the column whose name is stored in "value".


 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu


On Tue, 16 Mar 2010, Sacha Viquerat wrote:
> hello! ive come across the following problem:
> im writing a function to ease the production of maps. as my tables have 
> headers, id like to include an option in my function header to pass these 
> columns to use them inside the evaluation block. the function looks as 
> follows:
>
> # maptable #the name of the table containing coordinates and columns for 
> measured values
> # value: which column of my table to use (e.g. Alt, slope, us, etc...
> # labels(maptable)
> # [1] "pos.e" "pos.n" "Alt" "slope"
"us" "uus" "lc"
>
> make.map<-function (maptable,value)
> {
> pos.e<-maptable$pos.e #calling the column directly works
> pos.n<-maptable$pos.n #calling the column directly works
> key<-maptable$value #this doesnt work. output: NULL
> ... rest of function, irrelevant to my question
> }
>
> for example, i call the function by:
>
> make.map(elevation,Alt,"Altitude","Sightings")
>
> and my interp from akima package denies work, since key is NULL
>
> interp(pos.e,pos.n,key,...)
>
> have i just missed something? ive browsed the help pages and the archive, 
> looked into crawleys R book but didnt find any solution, though im pretty 
> sure this should be achievable!
>
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