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
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