R help - May 2003 - expand.grid and the first level of a factor

I do not understand this behaviour of expand.grid:
> expand.grid(x = c("b", "a"), y = c(1, 2))$x
[1] b a b a
Levels: b a
> expand.grid(x = c("b", "a"))$x
[1] b a
Levels: a b

Why the first level of the factor x depends on the number
of arguments of expand.grid? Apparently, I can set 
the order of the levels only when the number of 
arguments in > 1. In the second example, the order 
is lexicographic.

-- Giovanni
-- 
< Giovanni M. Marchetti >
Dipartimento di Statistica, Univ. di Firenze   Phone:  +39 055 4237 204
viale Morgagni, 59                             Fax:    +39 055 4223 560
I 50134 Firenze, Italy                         email:  gmm at ds.unifi.it

Dear Giovanni,

At 12:53 PM 5/3/2003 +0000, Giovanni Marchetti wrote:
>I do not understand this behaviour of expand.grid:
>
> > expand.grid(x = c("b", "a"), y = c(1, 2))$x
>[1] b a b a
>Levels: b a
> > expand.grid(x = c("b", "a"))$x
>[1] b a
>Levels: a b
>
>Why the first level of the factor x depends on the number
>of arguments of expand.grid? Apparently, I can set
>the order of the levels only when the number of
>arguments in > 1. In the second example, the order
>is lexicographic.
As the help for expand.grid states, expand.grid generates all combinations 
of values of its arguments. Take a look at the entirety of the result:

 > expand.grid(x = c("b", "a"), y = c(1, 2))
   x y
1 b 1
2 a 1
3 b 2
4 a 2


Compare, for example, to

         expand.grid(x = c("b", "a"), y = c(1, 2), z=c(TRUE,
FALSE))

which generates 8 rows.

I hope that this helps,
  John


-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox

Giovanni Marchetti wrote:
> I do not understand this behaviour of expand.grid:
> 
> 
>>expand.grid(x = c("b", "a"), y = c(1, 2))$x
> 
> [1] b a b a
> Levels: b a
> 
>>expand.grid(x = c("b", "a"))$x
> 
> [1] b a
> Levels: a b
> 
> Why the first level of the factor x depends on the number
> of arguments of expand.grid? Apparently, I can set 
> the order of the levels only when the number of 
> arguments in > 1. In the second example, the order 
> is lexicographic.
> 
> -- Giovanni

It depends on the number of arguments, because of the implementation 
(look into the code):

In principle, expand.grid(x = c("b", "a")) does the
following:

  x <- c("b", "a")
  factor(x)

whereas for expand.grid(x = c("b", "a"), y = c(1, 2)), the
levels will
be specified as in:

  factor(x, levels = unique(x))

Hence the difference.


Uwe Ligges