From: Gavin Simpson <gavin.simpson_at_ucl.ac.uk>

Date: Wed 17 Aug 2005 - 03:17:51 EST

Date: Wed 17 Aug 2005 - 03:17:51 EST

On Tue, 2005-08-16 at 18:43 +0200, Laurent Valdes wrote:

> I have seen, several times, dots (like this: "y ~." ) in formula

*> descriptions, noticeably in R help.
**>
**> I am unable to see what it does correspond to.
**>
**> Any ideas ?
*

The "." is a short cut to mean all variables specified in the data argument. E.g.:

> dat <- data.frame(y = 1:10, x = 1:10, z = 1:10)

> model.frame(y ~ ., data = dat)

y x z

1 1 1 1

2 2 2 2

3 3 3 3

4 4 4 4

5 5 5 5

6 6 6 6

7 7 7 7

8 8 8 8

9 9 9 9

10 10 10 10

If the response is also found on the rhs (right-hand-side) of the formula (i.e via using ".") then it is silently droppped from the rhs.

So this is equivalent to the above formula

> model.frame(y ~ x + y + z, data = dat)

y x z

1 1 1 1

2 2 2 2

3 3 3 3

4 4 4 4

5 5 5 5

6 6 6 6

7 7 7 7

8 8 8 8

9 9 9 9

10 10 10 10

Here y isn't the response but is included as it is in ".", i.e. dat.

> a <- 1:10

> model.frame(a ~ ., data = dat)

a y x z

1 1 1 1 1

2 2 2 2 2

3 3 3 3 3

4 4 4 4 4

5 5 5 5 5

6 6 6 6 6

7 7 7 7 7

8 8 8 8 8

9 9 9 9 9

10 10 10 10 10

If we don't specify data and we don't have an object named "." (which may be impossible - I don't know) you get an error:

> model.frame(a ~ .)

Error in eval(expr, envir, enclos) : Object "." not found

I couldn't find "." documented for use in forumla (only for update(), where it means something slightly different) but I remember seeing this somewhere.

**HTH
**
G

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