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

Carl Witthoft carl at witthoft.com
Fri Apr 6 23:14:37 CEST 2012

I think the OP wants to fill values in an arbitrarily large matrix. 
Now, first of all, I'd like to know what his real problem is, since this 
seems like a very tedious and unproductive matrix to produce.  But in 
the meantime,  since he also left out important information, let's 
assume the input matrix is N rows by M columns, and that he wants 
therefore to end up with N instances of "100", not counting the original 
value of 100 that is one of his ranking values (a bad BAD move IMHO).

Then either loop or lapply over an equation like (I've expanded things 
more than necessary for clarity
for (k in 1:N){
foo <- which (inmatrix == k,arr.ind=T)
result[k,foo[2]] <-100

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

output <- matrix(0,max(input),2)
output[input[,1],1] <- 100
output[input[,2],2] <- 100


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)

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