How to rename the rows of a dataframe on the fly?

Question

I love to rename a vector on the fly with setNames (by on-the-fly I mean a function that returns the object):

my_vector <- c(1,2,3)
setNames(my_vector, c("a","b","c"))
# a b c
# 1 2 3

That works for dataframe column names as well

my_df <- data.frame(matrix(1:9, nrow=3))
setNames(my_df, c("a","b","c"))
  a b c
1 1 4 7
2 2 5 8
3 3 6 9

Is there a way to do the same with the row names? I thought this would work, but it doesn't:

t(setNames(t(my_df), c("a","b","c")))

Answer 1

I think what you mean by on-the-fly is you want the function to return the modified object. Since rownames<- does not do that for you, you will have to define your own function:

setRowNames <- function(df, row.names) {
   rownames(df) <- row.names
   df
}

You can have that function definition at the top of your script, or even hide it in your Rprofile or Rprofile.site (see ?Startup if you are not familiar). Then inside your code, use setRowNames(my_df, c("a", "b", "c")). It is concise, flexible, and reads nicely.

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t(setNames(t(my_df), c("a","b","c"))) does not work because transposing a data.frame gives you a matrix which does not have names but rownames and colnames.