Why does mutate(row_number()) fail for data.tables?

Question

When I try to use row_number() from dplyr on a data.table, it throws an error. Here's an example:

library(dplyr)
library(data.table)
mine <- data.table(a = 1:10)
mine %>% mutate(row_number())
# Error in rank(x, ties.method = "first", na.last = "keep") : 
#   argument "x" is missing, with no default

Any ideas why this is happening?

Answer 1

The dplyr::row_number() function has a mandatory parameter, which should be the name of the column that you want to do a row number for.

In your example, you should have written it like this:

library(dplyr)
library(data.table)
mine <- data.table(a = 1:10) %>% 
    mutate(row_number(a))

Because a is the name of the column that you want to add the row_number() for. Without this, then R throws the error argument "x" is missing, with no default.

However, I'd suggest using the tibble::rowid_to_column() function. It's cleaner.

library(dplyr)
library(data.table)
library(tibble)
mine <- data.table(a = 1:10) %>% 
    rowid_to_column()

I hope that helps.