[R] function to populate a matrix based on a lookup to another matrix ?

[Uwe Ligges]

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