Adding a column to a dataframe in R

Question

I have the following dataframe (df)

 start     end 1    14379   32094 2   151884  174367 3   438422  449382 4   618123  621256 5   698271  714321 6   973394  975857 7   980508  982372 8   994539  994661 9  1055151 1058824 .   .       . .   .       . .   .       . 

And a long vector with numeric values (vec).

I would like to add to each row another column, with the mean of the values in the corresponding places in vec. for example, the first row will have mean(vec[14379:32094]). I have tried playing with transform but wasn't able to accomplish this simple task.

Answer 1

That is a pretty standard use case for apply():

R> vec <- 1:10 R> DF <- data.frame(start=c(1,3,5,7), end=c(2,6,7,9)) R> DF$newcol <- apply(DF,1,function(row) mean(vec[ row[1] : row[2] ] )) R> DF   start end newcol 1     1   2    1.5 2     3   6    4.5 3     5   7    6.0 4     7   9    8.0 R>  

You can also use plyr if you prefer but here is no real need to go beyond functions from base R.

Answer 2

Even if that's a 7 years old question, people new to R should consider using the data.table, package.

A data.table is a data.frame so all you can do for/to a data.frame you can also do. But many think are ORDERS of magnitude faster with data.table.

vec <- 1:10 library(data.table) DT <- data.table(start=c(1,3,5,7), end=c(2,6,7,9)) DT[,new:=apply(DT,1,function(row) mean(vec[ row[1] : row[2] ] ))]