Re: [R] row-echelon form (was no subject)

[Spencer Graves]

     How about the following: 

> A <- array(1:6, dim=c(3, 2))
> A.qr <- qr(A)
> qr.R(A.qr)
         [,1]      [,2]
[1,] -3.741657 -8.552360
[2,]  0.000000  1.963961
>
   I'm no expert, either, and I don't have time now to research this 
further.  Perhaps someone else can further enlighten us both. 
spencer graves

John Fox wrote:

Dear Spencer,

I'd be surprised if the qr decomposition as computed in R weren't a lot more
stable numerically, but I'm no expert. As well, I don't know how to get the
row-echelon form from the qr decomposition -- though I suspect that you or
someone else on the list is about to enlighten me.
Regards,
John 

 

-----Original Message-----
From: Spencer Graves [mailto:[EMAIL PROTECTED] 
Sent: Wednesday, March 03, 2004 10:45 PM
To: John Fox
Cc: 'Aimin Yan'; [EMAIL PROTECTED]
Subject: Re: [R] row-echelon form (was no subject)

How does this compare with R of the qr decomposition?  spencer graves

John Fox wrote:

   

Dear Amin,

I have a function (created just for demonstration, and reproduced 
below) for finding the row-echelon form of a matrix. I'm 
     

sure that many 
   

list members could produce something that's better numerically, but 
this should be OK at least for toy problems.

John

--------- snip -------------

rowEchelonForm <- function(X, tol=.Machine$double.eps){
  if ((!is.matrix(X)) || (!is.numeric(X))) stop("argument 
     

must be a 
   

numeric matrix")
  Z <- X
  for (i in 1:min(dim(X))){
      if (i > 1) Z[i-1,] <- 0
      which <- which.max(abs(Z[,i]))  # find maximum pivot 
     

in current 
   

column at or below current row
      pivot <- X[which, i]
      if (abs(pivot) <= tol) next     # check for 0 pivot
      if (which > i) X[c(i,which),] <- X[c(which,i),]  # 
     

exchange rows
   

      X[i,] <- X[i,]/pivot            # pivot
      row <- X[i,]                    
      X <- X - outer(X[,i], row)      # sweep
      X[i,] <- row                    # restore current row
      }
  n <- nrow(X)
  for (i in 1:n) if (max(abs(X[i,])) <= tol) X[c(i,n),] <- 
     

X[c(n,i),]   #
   

0 rows to bottom
  X
  }
      



     

-----Original Message-----
From: [EMAIL PROTECTED] 
[mailto:[EMAIL PROTECTED] On Behalf Of Aimin Yan
Sent: Wednesday, March 03, 2004 2:42 PM
To: [EMAIL PROTECTED]
Subject: [R] (no subject)

how to produce a  Row Reduced Echelon Form for a matrix in R?
Aimin Yan
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