[R] Need to abstract changing name of column within loop

[David Winsemius]

On Mar 16, 2011, at 7:58 PM, jctoll wrote:

> Hi,
>
> I'm struggling to figure out the way to change the name of a column
> from within a loop.  The problem is I can't refer to the object by its
> actual variable name, since that will change each time through the
> loop.  My xts object is A.
>
>> head(A)
>           A.Open A.High A.Low A.Close A.Volume A.Adjusted A.Adjusted.1
> 2007-01-03  34.99  35.48 34.05   34.30  2574600      34.30   
> 11867800000
> 2007-01-04  34.30  34.60 33.46   34.41  2073700      34.41   
> 11905860000
> 2007-01-05  34.30  34.40 34.00   34.09  2676600      34.09   
> 11795140000
> 2007-01-08  33.98  34.08 33.68   33.97  1557200      33.97   
> 11753620000
> 2007-01-09  34.08  34.32 33.63   34.01  1386200      34.01   
> 11767460000
> 2007-01-10  34.04  34.04 33.37   33.70  2157400      33.70   
> 11660200000
>
> It's column names are:
>> colnames(A)
> [1] "A.Open"       "A.High"       "A.Low"        "A.Close"
> "A.Volume"     "A.Adjusted"   "A.Adjusted.1"
>
> I want to change the 7th column name:
>> colnames(A)[7]
> [1] "A.Adjusted.1"
>
> I need to do that through a reference to i:
>> i
> [1] "A"
>

It's not pretty and there may be a more direct way:
 > a
       [,1] [,2]
[1,] 1e+20 1000
[2,] 1e+02 1000
 > b <- "a"
 > assign("tmp", eval(parse(text=b)))
 > tmp
       [,1] [,2]
[1,] 1e+20 1000
[2,] 1e+02 1000
 > colnames(tmp) <- c("one","two")
 > assign( b, tmp)
 > a
        one  two
[1,] 1e+20 1000
[2,] 1e+02 1000


Trying to short circuit the process without an intermediate temporary  
structure fails:
 > colnames(eval(parse(text=b)) )<- c("two", "three")
Error in parse(`*tmp*`) : EOF whilst reading MBCS char at line 1
-- 
David

> This works:
>> colnames(get(i))[7]
> [1] "A.Adjusted.1"
>
> And this is what I want to change the column name to:
>> paste(i, ".MarketCap", sep = "")
> [1] "A.MarketCap"
>
> But how do I make the assignment?  This clearly doesn't work:
>
>> colnames(get(i))[7] <-  paste(i, ".MarketCap", sep = "")
> Error in colnames(get(i))[7] <- paste(i, ".MarketCap", sep = "") :
>  could not find function "get<-"
>
> Nor does this (it creates a new object "A.Adjusted.1" with a value of
> "A.MarketCap") :
>
> assign(colnames(get(i))[7], paste(i, ".MarketCap", sep = ""))
>
> How can I change the name of that column within my big loop?  Any
> ideas?  Thanks!
>
> Best regards,
>
>
> James
>
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David Winsemius, MD
West Hartford, CT