[R] assigning values to elements of matrixes

[Anthoni, Peter (IMK)]

Hi,

How about the following:
foo2 <- function(s,i,j,value)
{
  M = get(paste("M_",s,sep=""))
  M[i,j] = value
  assign(paste("M_",s,sep=""),M, envir = .GlobalEnv) 
}

foo2("a",1,2,15)

cheers
Peter

> On 23 Dec 2015, at 09:44, Matteo Richiardi <matteo.richiardi at gmail.com> wrote:
> 
> I am following the example I find on ?assign:
> 
> a <- 1:4
> assign("a[1]", 2)
> 
> This appears to create a new variable named "a[1]" rather than
> changing the value of the vector.
> 
> Am I missing something here? How can I assign a value to a specified
> element of a vector/matrix?
> 
> Of course, my problem is slightly more involved, but I guess the above
> is its core. For those interested, I have the two matrixes
> M_a <- matrix(0,10,10)
> M_b <- matrix(0,10,10)
> 
> I want to have a function that assigns a value to a specific element
> of one of the matrix, as in
> foo <- function(s,i,j,value){
>  assign(paste("M_",s)[i,j],value)
> }
> 
> This however does not work:
> 
> foo('a',1,1,1)
> 
> Error in paste("M_", s)[1, j] : incorrect number of dimensions
> 
> Following the ?assign help, I tried
> 
> foo2 <- function(s,i,j,value){
>  assign(paste("M_",s,"[i,j]"),value, envir = .GlobalEnv)
> }
> 
> but this produces the problem I described above (namely, a new
> variable is created, rather than replacing the specified element of
> the matrix).
> 
> I know that in this example I could simply pass the matrix as an
> argument to the function, but I am interested in understanding how
> 'assign' work.
> 
> Many thanks for your help.
> 
> Matteo
> 
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