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

[jctoll]

On Thu, Mar 17, 2011 at 10:32 AM, Joshua Ulrich <josh.m.ulrich at gmail.com> wrote:
> On Wed, Mar 16, 2011 at 6:58 PM, jctoll <jctoll at gmail.com> 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"
>>
>> 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!
>>
> I usually make a copy of the object, change it, then overwrite the original:
>
> tmp <- get(i)
> colnames(tmp)[7] <- "foo"
> assign(i,tmp)
>
> Hope that helps.
>
>> Best regards,
>>
> Joshua Ulrich  |  FOSS Trading: www.fosstrading.com


Thank you, that does help.  It sounds like your way is similar to the
way David suggested.  At least now I can just resign myself to doing
it that way rather than torturing my mind trying to figure out a
shorter way.

Thanks,

James