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Matrix Construction; Subdiagonal

8 messages · Sundar Dorai-Raj, Bill Venables, Stu Field +1 more

Sundar Dorai-Raj
# 
Does this help?

A <- matrix(0, 6, 6)
vec <- 1:5
A[row(A) == col(A) + 1] <- vec

--sundar
On Wed, Mar 11, 2009 at 4:42 PM, Stu Field <sgf at colostate.edu> wrote:
Sundar Dorai-Raj
# 
You can always write your own function:

myDiag <- function(x, vec, k) {
  x[row(x) == col(x) - k] <- vec
  x
}

myDiag(A, vec, -1)

Of course, you should probably do some input checking too.

--sundar
On Wed, Mar 11, 2009 at 4:57 PM, Stu Field <sgf at colostate.edu> wrote:
Bill Venables
# 
Why should there be a simple way?  R is not primarily a matrix language.

Perhaps this will help:

xyz <- function (v, k) {
    n <- length(v) + abs(k)
    x <- matrix(0, n, n)
    if (k == 0) 
        diag(x) <- v
    else if (k < 0) 
    { ## sub-diagonal
        j <- 1:(n+k)
        i <- (1 - k):n
        x[cbind(i, j)] <- v
    } else 
    { ## super-diagonal
        i <- 1:(n-k)
        k <- (1 + k):n
        x[cbind(i, j)] <- v
    }
    x
}

modify, extend, simplify, ... as you wish.

Bill Venables. 


Bill Venables
http://www.cmis.csiro.au/bill.venables/ 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Stu Field
Sent: Thursday, 12 March 2009 9:43 AM
To: r-help at r-project.org
Subject: [R] Matrix Construction; Subdiagonal

I'm trying to enter a vector into the subdiagonal of a matrix but  
cannot find a command in R which corresponds to the MatLab version of  
diag(vec, k), where vec = the vector of interest, and k = the diagonal  
(k=0 for the diagonal; k=-1 for the subdiagonal; k=1 for  
superdiagonal, etc.)
Is there an equivalent command in R?

I'm looking for something like this:
vec = seq(1, 5, 1)        # vector of interest

A = xyz(vec,-1)           # creates a 6x6 matrix with vec on the  
subdiagonal
where xyz is some function similar to diag, but with differing  
arguments.

I can't believe there is not a simple way to do this...
Thanks for your help,

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Stu Field, PhD
Postdoctoral Fellow
Department of Biology
Colorado State University
1878 Campus Delivery
Fort Collins, CO 80523-1878
Office: E208 Anatomy/Zoology
Phone: (970) 491-5744
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~






______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
Bill Venables
# 
This is horrible on memory if you are dealing with large matrices.

Here is a slightly more slick version of the function I have already posted:


xyz <- function (v, k) {
    n <- length(v) + abs(k)
    x <- matrix(0, n, n)
    i <- (1 - min(0, k)):(n - max(0,k))
    j <- (1 + max(0, k)):(n + min(0,k))
    x[cbind(i,j)] <- v
    x
}

The point of this is that it does not hold three copies of the matrix in memory at once (x, row(x) and col(x)).  It uses the matrix index idea instead.

If you were going to be doing this a lot, you would be better writing a function `Diag<-`, say, which places a vector at a specified diagonal position of a given matrix.  This is an easy exercise for the reader! 


Bill Venables
http://www.cmis.csiro.au/bill.venables/ 


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Sundar Dorai-Raj
Sent: Thursday, 12 March 2009 10:00 AM
To: Stu Field
Cc: r-help at r-project.org
Subject: Re: [R] Matrix Construction; Subdiagonal

You can always write your own function:

myDiag <- function(x, vec, k) {
  x[row(x) == col(x) - k] <- vec
  x
}

myDiag(A, vec, -1)

Of course, you should probably do some input checking too.

--sundar
On Wed, Mar 11, 2009 at 4:57 PM, Stu Field <sgf at colostate.edu> wrote:
______________________________________________
R-help at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
1 day later
Paul Smith
#
On Wed, Mar 11, 2009 at 11:49 PM, Sundar Dorai-Raj <sdorairaj at gmail.com> wrote:
Maybe, more simply:

A <- matrix(0, 6, 6)
vec <- 1:5
diag(A[-1,]) <- vec

Paul