Daniel Falbel - 9 months ago 51

R Question

**UPDATE: dplyr has been updated since this question was asked and now performs as the OP wanted**

I´m trying to get the second to the seventh line in a

`data.frame`

`dplyr`

I´m doing this:

`require(dplyr)`

df <- data.frame(id = 1:10, var = runif(10))

df <- df %>% filter(row_number() <= 7, row_number() >= 2)

But this throws an error.

`Error in rank(x, ties.method = "first") :`

argument "x" is missing, with no default

I know i could easily make:

`df <- df %>% mutate(rn = row_number()) %>% filter(rn <= 7, rn >= 2)`

But I would like to understand why my first try is not working.

Answer Source

The `row_number()`

function does not simply return the row number of each element and so can't be used like you want:

• ‘row_number’: equivalent to ‘rank(ties.method = "first")’

You're not actually saying what you want the `row_number`

of. In your case:

```
df %>% filter(row_number(id) <= 7, row_number(id) >= 2)
```

works because `id`

is sorted and so `row_number(id)`

is `1:10`

. I don't know what `row_number()`

evaluates to in this context, but when called a second time `dplyr`

has run out of things to feed it and you get the equivalent of:

```
> row_number()
Error in rank(x, ties.method = "first") :
argument "x" is missing, with no default
```

That's your error right there.

Anyway, that's **not** the way to select rows.

You simply need to subscript `df[2:7,]`

, or if you insist on pipes everywhere:

```
> df %>% "["(.,2:7,)
id var
2 2 0.52352994
3 3 0.02994982
4 4 0.90074801
5 5 0.68935493
6 6 0.57012344
7 7 0.01489950
```