Question

I’d like to combine multiple columns of a data frame in R. Given data that look something like this:

names: 123   256   192   123   256
       1     2     8     2     3
       4     3     2     9     9
       8     7     1     3     8 

How would I sum the elements of the identically-named columns to produce a table like so:

names: 123   256   192
       3     5     8
       13    12    2
       11    15    1

Thank you very much.

Answer 1

As suggested by @VincentZoonekynd it is not a good idea to have multiple columns with the same name.

Anyway, you could do in this way:

df <- data.frame(A=c(1,4,8),B=c(2,3,7),C=c(8,2,1),D=c(2,9,3),E=c(3,9,8))
names(df) <- c('123','256', '192', '123', '256')

df <- t(df)     # transpose the data.frame
aggr <- by(df, INDICES=row.names(df), FUN=colSums) # collapse the rows with the same name
aggr <- as.data.frame(do.call(cbind,aggr)) # convert by() result to a data.frame

or, in one line:

aggr <- as.data.frame(do.call(cbind, by(t(df),INDICES=names(df),FUN=colSums)))