function to populate a matrix based on a lookup to another matrix ?

Hi,

This is a beginner R question.

I have a  4x4 matrix  named 'lookup' with the following values:

           1         2               3          4
1 0.000000 2.828427 5.656854 8.485281
2 2.828427 0.000000 2.828427 5.656854
3 5.656854 2.828427 0.000000 2.828427
4 8.485281 5.656854 2.828427 0.000000

I then create a new empty matrix named 'dd' with specfic row and col 
names :

  1   3   4   3    3   1
1 NA NA NA NA NA NA
2 NA NA NA NA NA NA
3 NA NA NA NA NA NA
4 NA NA NA NA NA NA
3 NA NA NA NA NA NA
2 NA NA NA NA NA NA
1 NA NA NA NA NA NA

I want to be able populate the cells in 'dd' using 'lookup' based on the 
specified rownames and colnames of 'dd'. For example, the cell in 'dd' 
where the rowname =2 and the colname = 1 should be assigned the value 
2.828427 .

I've tried several ways of doing this with the apply function  but 
without success. I can do it with a for loop but I want to avoid that 
for efficiency reasons.

After running the function, as an example, the first column of 'dd' 
should look like this :

            1
1 0.000000000
2 2.828427125
3 5.656854249
4 8.485281374
3 5.656854249
2 2.828427125
1 0.000000000

Can anyone please help me identify the required function or an 
alternative way of achieving the same result? Hopefully this is simple 
and I'm just not seeing it.

Thanks
Peter