[R] cbind in a loop...better way? | summary

[David L Carlson]

Actually Jeff Laake's can be made even shorter with

sapply(mat_list, as.vector)

David C

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Evan Cooch
Sent: Thursday, October 9, 2014 7:37 AM
To: Evan Cooch; r-help at r-project.org
Subject: Re: [R] cbind in a loop...better way? | summary

Two solutions proposed -- not entirely orthogonal, but both do the 
trick. Instead of nesting cbin in a loop (as I did originally -- OP, 
below),

1\   do.call(cbind, lapply(mat_list, as.vector))

or

2\   sapply(mat_list,function(x) as.vector(x))


Both work fine. Thanks to Jeff Laake (2) + David Carlson (1) for their 
suggestions.


On 10/8/2014 3:12 PM, Evan Cooch wrote:
> ...or some such. I'm trying to work up a function wherein the user 
> passes a list of matrices to the function, which then (1) takes each 
> matrix, (2) performs an operation to 'vectorize' the matrix (i.e., 
> given an (m x n) matrix x, this produces the vector Y of length  m*n 
> that contains the columns of the matrix x, stacked below each other), 
> and then (3) cbinds them together.
>
> Here is an example using the case where I know how many matrices I 
> need to cbind together. For this example, 2 square (3x3) matrices:
>
>  a <- matrix(c,0,20,50,0.05,0,0,0,0.1,0),3,3,byrow=T)
>  b <- matrix(c(0,15,45,0.15,0,0,0,0.2,0),3,3,byrow=T)
>
> I want to vec them, and then cbind them together. So,
>
> result  <- cbind(matrix(a,nr=9), matrix(b,nr=9))
>
> which yields the following:
>
>       [,1]  [,2]
>  [1,]  0.00  0.00
>  [2,]  0.05  0.15
>  [3,]  0.00  0.00
>  [4,] 20.00 15.00
>  [5,]  0.00  0.00
>  [6,]  0.10  0.20
>  [7,] 50.00 45.00
>  [8,]  0.00  0.00
>  [9,]  0.00  0.00
>
> Easy enough. But, I want to put it in a function, where the number and 
> dimensions  of the matrices is not specified. Something like
>
> Using matrices (a) and (b) from above, let
>
>   env <- list(a,b).
>
> Now, a function (or attempt at same) to perform the desired operations:
>
>   vec=function(matlist) {
>
>       n_mat=length(matlist);
>       size_mat=dim(matlist[[1]])[1];
>
>       result=cbind()
>
>        for (i in 1:n_mat) {
>          result=cbind(result,matrix(matlist[[i]],nr=size_mat^2))
>                           }
>
>      return(result)
>
>    }
>
>
> When I run vec(env), I get the *right answer*, but I am wondering if 
> there is a *better* way to get there from here than the approach I use 
> (above). I'm not so much interested in 'computational efficiency' as I 
> am in stability, and flexibility.
>
> Thanks...
>
> .
>

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