# [R] filling the matrix row by row in the order from lower to larger elements

ilai keren at math.montana.edu
Fri Apr 6 22:18:27 CEST 2012

```I maybe missing something but this seems like an indexing problem
which doesn't require a loop at all. Something like this maybe?

(input<-matrix(c(5,1,3,7,2,6,4,8),nc=2))
output <- matrix(0,max(input),2)
output[input[,1],1] <- 100
output[input[,2],2] <- 100
output

Cheers

On Fri, Apr 6, 2012 at 1:49 PM, Dimitri Liakhovitski
<dimitri.liakhovitski at gmail.com> wrote:
> Hello, everybody!
>
> I have a matrix "input" (see example below) - with all unique entries
> that are actually unique ranks (i.e., start with 1, no ties).
> I want to assign a value of 100 to the first row of the column that
> contains the minimum (i.e., value of 1).
> Then, I want to assign a value of 100 to the second row of the column
> that contains the value of 2, etc.
> The results I am looking for are in "desired.results".
> My code (below) does what I need. But it's using a loop through all
> the rows of my matrix and searches for a matrix element every time.
> My actual matrix is very large. Is there a way to do it more efficiently?
> Thank you very much for the tips!
> Dimitri
>
> input<-as.matrix(data.frame(a=c(5,1,3,7),b=c(2,6,4,8)))
> (input)
> desired.result<-as.matrix(data.frame(a=c(100,0,100,0),b=c(0,100,0,100)))
> (desired.result)
> result<-as.matrix(data.frame(a=c(0,0,0,0),b=c(0,0,0,0)))
> for(i in 1:nrow(input)){ # i<-1
>  mymin<-i
>  mycoords<-which(input==mymin,arr.ind=TRUE)
>  result[i,mycoords[2]]<-100
>  input[mycoords]<-max(input)
> }
> (result)
>
> --
> Dimitri Liakhovitski
>
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