Does dplyr::row_number() calculate row number for each obs? If so, how?

[Daniel.C.Li]

On the tidyverse website reference, I saw two usage mutate(mtcars, row_number() == 1L) and mtcars %>% filter(between(row_number(), 1, 10)) . It would be straight forward to think that the row_number() function is return the row number for each observation in the dataframe.

However, it has been emphasized in the documentation that the function is a window function and is similar to sortperm in other languages. As in the example:

x <- c(5, 1, 3, 2, 2, NA) row_number(x)

which yields to: c( 5 1 4 2 3 NA )

May I ask if this function is intended to report the row number for each observations? If it is, what is the logic flow behind the function call?

Thanks!

[Leon]

You can check the code for a function like so:

> row_number
function (x) 
rank(x, ties.method = "first", na.last = "keep")
<environment: namespace:dplyr>

So yes it's a dplyr function, but it's simply a wrapper around the base function rank() and if you check the arguments, you can see, that e.g. row_number() == 1L will let you manipulate based on ranks, e.g.:

mtcars %>% group_by(cyl) %>% filter(row_number(hp) == 1)

Which is equivalent to, but more robust than:

mtcars %>% group_by(cyl) %>% arrange(hp) %>% slice(1)

If you look at the help, you can see that the row_number functions 'are provided mainly as a convenience when converting between R and SQL'

Hope it helps,
Bw. Leon