Insert column at beginning of a data frame
How can I add a column to an R data.frame as a new first column so that all other columns are shifted by one column?
Like:
a|b|c --> new|a|b|c
I need to do this because I want the row.names to become a discrete column. This is needed because the write.arff function takes a data.frame as input but does not preserve the names when writing files.
Answer
This has been answered in the comments, but to make it clearer that there is an answer, here's a small example:
First, some sample data:
(df <- data.frame(A = 1:2, B = 3:4, row.names = c("row1", "row2")))
# A B
# row1 1 3
# row2 2 4
The suggestion from the comments. Note that the original row.names is still part of the data.
cbind(rn = rownames(df), df)
# rn A B
# row1 row1 1 3
# row2 row2 2 4
You can get rid of that by setting row.names = NULL in the cbind step. Since you are cbinding data.frames, you could also pass other arguments to data.frame if necessary (such as stringsAsFactors = FALSE if you didn't want the "rn" column to be a factor).
cbind(rn = rownames(df), df, row.names = NULL)
# rn A B
# 1 row1 1 3
# 2 row2 2 4