[R] Adding collumn to existing data frame

[David Winsemius]

On Aug 3, 2010, at 10:35 PM, David Winsemius wrote:

>
> On Aug 3, 2010, at 8:32 PM, Ralf B wrote:
>
>> Hi experts,
>>
>> I am trying to write a very flexible method that allows me to add a
>> new column to an existing data frame. This is what I have so far:
>>
>> add.column <- function(df, new.col, name) {
>> 	n.row <- dim(df)[1]
>> 	length(new.col) <- n.row
>> 	names(new.col) <- name
>> 	return(cbind(df, new.col))
>> }
>>
>> df <- NULL
>> df <- data.frame(a=c(1,2,3))
>> df
>> # corect: added NA to new collumn
>> df <- add.column(df,c(1,2),'myNewColumn2')
>> df
>> # problem: not added, data frame should be extended with NAs
>> add.column(df,c(1,2,3,4),'myNewColumn3')
>> df
>>
>>
>> However, there are two problems:
>>
>> 1) The column name is not renamed accurately but always set to
>> 'new.col' . Surely this could be done outside the function, but it
>> would be better if its self contained.
>
> Try this:
>
> add.col <- function(df, vec, namevec){
>                          length(vec) <- nrow(df) # pads with NA's
>                          cbind(df, namevec=vec)} # names new col  
> properly
>
Actually it doesn't name column correctky...  see below for a method  
with "[ <-" .

>> 2) It does not work for cases where new.col is longer than the length
>> of the data frame. In such cases, I would like to add NA's to the  
>> data
>> frame if it has less rows.
>
> Don't have a compact answer to this. (Tried re-dimensioning with  
> "dim() <-"  but it was not accepted by the interpreter.  Would need  
> to add a test at the beginning and then pad with rows of NA's using  
> rbind before cbinding as above.
>
> add.col <- function(df, vec, namevec){
>               if (nrow(df) < length(vec) ){ df <-  # pads rows if  
> needed
>                     rbind(df, matrix(NA, length(vec)-nrow(df),  
> ncol(df),
>                                      dimnames=list( NULL,  
> names(df) ) ) ) }
>               length(vec) <- nrow(df) # pads with NA's
>               df[, namevec] <- vec; # names new col properly
> 	return(df)}
>
>>
>> Any ideas to to solve this?
>
> Has not been tested with columns of varying types.
>

David Winsemius, MD
West Hartford, CT