What does the error "arguments imply differing number of rows: x, y" mean?

Though this isn't a DIRECT answer to your question, I just encountered a similar problem, and thought I'd mentioned it:

I had an instance where it was instantiating a new (no doubt very inefficent) record for data.frame (a result of recursive searching) and it was giving me the same error.

I had this:

return(
  data.frame(
    user_id = gift$email,
    sourced_from_agent_id = gift$source,
    method_used = method,
    given_to = gift$account,
    recurring_subscription_id = NULL,
    notes = notes,
    stringsAsFactors = FALSE
  )
)

turns out... it was the = NULL. When I switched to = NA, it worked fine. Just in case anyone else with a similar problem hits THIS post as I did.

Your data.frame mat is rectangular (n_rows!= n_cols).

Therefore, you cannot make a data.frame out of the column- and rownames, because each column in a data.frame must be the same length.

Maybe this suffices your needs:

require(reshape2)
mat$id <- rownames(mat) 
melt(mat)

I had the same error message so I went googling a bit I managed to fix it with the following code.

df<-data.frame(words = unlist(words))

words is a character list.

This just in case somebody else needs the output to be a data frame.