[R] Matrix to "database" -- best practices/efficiency?

[Gabor Grothendieck]

Try this:

> mm <- matrix(1:6, 3, dimnames = list(LETTERS[1:3], letters[1:2]))
> mm
  a b
A 1 4
B 2 5
C 3 6
> library(reshape)
> melt(mm)
  X1 X2 value
1  A  a     1
2  B  a     2
3  C  a     3
4  A  b     4
5  B  b     5
6  C  b     6


On Mon, Jun 7, 2010 at 10:33 PM, Jonathan Greenberg
<greenberg at ucdavis.edu> wrote:
> I have a matrix of, say, M and N dimensions:
>
> my_matrix=matrix(c(1:60),nrow=6,ncol=10)
>
> I have two "id" vectors corresponding to the rows and columns, e.g.:
>
> id_m=seq(10,60,by=10)
> id_n=seq(100,1000,by=100)
>
> I would like to create a "proper" database (let's say a data.frame for
> this example -- i'm going to be loading these into an SQLite database,
> but we'll leave that complication out of this discussion for now) of m
> x n rows, and 3 columns, where the 3 columns relate to the values from
> m, n, and my_matrix respectively, e.g. a single row follows the form:
>
> c(id_m[a],id_n[b],my_matrix[a,b])
>
> I can, of course, for-loop this thing with an if-then, e.g.:
>
> ***
>
> for (a in 1:length(id_m))
> {
>        for (b in 1:length(id_n))
>        {
>                if ((a==1) && (b==1))
>                {
>                        my_database=c(id_m[a],id_n[b],my_matrix[a,b])
>                } else
>                {
>                        my_database=rbind(my_database,c(id_m[a],id_n[b],my_matrix[a,b]))
>                }
>        }
> }
>
> ***
>
> But my gut is telling me this is an incredibly inefficient way of
> doing this -- is there a faster approach to doing this same process?
> Thanks!
>
> --j
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>