[R] expand.grid() function

[Gavin Simpson]

On Mon, 2008-06-23 at 06:16 -0700, Megh Dal wrote:
> Hi,
>    
>   I have one question on expand.grid() function.
>    
>   When I write following syntax :expand.grid(c("u", "l"), c("u", "l"),
> c("u", "l")) I get following as desired :
>     Var1 Var2 Var3
> 1    u    u    u
> 2    l    u    u
> 3    u    l    u
> 4    l    l    u
> 5    u    u    l
> 6    l    u    l
> 7    u    l    l
> 8    l    l    l
> 
>   However I wanted to write that in more concise manner. Therefore I
> tried : expand.grid(rep(c("u", "l"), 3)). But I did not get answer
> that I previously got. Can people here clarify me why it is not like
> that? Then what would be the mose concise way to do that?

In the first case, you have three vectors of length 2 as arguments to
expand.grid, but in the second, you have a single vector of length 6. In
the latter case, expand.grid can't expand a single vector, hence the
single column result.

This is the closest I got to what you want:

as.matrix(expand.grid(split(rep(c("u","l"), times = 3), 
          factor(rep(1:3, each = 2)))))

Which gives:
> as.matrix(expand.grid(split(rep(c("u","l"), times = 3),
factor(rep(1:3, each = 2)))))
     1   2   3  
[1,] "u" "u" "u"
[2,] "l" "u" "u"
[3,] "u" "l" "u"
[4,] "l" "l" "u"
[5,] "u" "u" "l"
[6,] "l" "u" "l"
[7,] "u" "l" "l"
[8,] "l" "l" "l"

But that isn't particularly concise...

HTH

G
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