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

From: Uwe Ligges <ligges at statistik.uni-dortmund.de>
To: peter leonard <pfleonard at hotmail.com>
CC: r-help at stat.math.ethz.ch
Subject: Re: [R] function to populate a matrix based on a lookup to another 
matrix ?
Date: Sat, 31 May 2003 17:58:27 +0200

peter leonard wrote:

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

 dd <- lookup[as.numeric(rownames(dd)), as.numeric(colnames(dd))]
and after that restoring dd's row- and colnames.

Anyway, I guess you don't need to create "dd". Just calculate those 
rownames (rn) and colnames (cn) as integers. Then the following works:

 dd <- lookup[rn, cn]

Uwe Ligges