From bug-octave-request at bevo dot che dot wisc dot edu Tue Jan  7 08:34:45 2003
Subject: Re: Operations on matrix referenced by a list of indices
From: Etienne Grossmann <etienne at isr dot isr dot ist dot utl dot pt>
To: Igor ?ukanovi? <igor dot mat at uni-mb dot si>
Cc: bug-octave at bevo dot che dot wisc dot edu,    Etienne Grossmann <etienne@isr.isr.ist.utl.pt>
Date: Tue, 7 Jan 2003 14:47:57 +0000


  Hello,

maybe the following function can help you; you may want to make it
more efficient :

## c = loop_add (a, ii, b=1)    - Add b to a at subscripts specified by ii
##
## This function is equivalent to, but should be quicker than :
##
##    c=a; for j=1:rows(ii(:)), c(ii(j)) += b(j); end
##
## a  : R x C  : Original matrix
## ii : S x D  : Indices in a. Values not in 1:(R*C) are ignored.
## b  : S x D  : Added value(s). If b is missing, b=1 is assumed.
##  or  1 x 1                    
##
## c  : R x C    
##
## Inspired by Dirk Laurie's <dirk at calvyn dot puk dot ac dot za> code sent to
## octave-help (3 Aug 2000). 
## Author : Etienne Grossmann <etienne at isr dot ist dot utl dot pt>

function c = loop_add (a, i, b)

if isempty (i), c = a; return; end

if nargin < 3,
  b = ones (size (i));
elseif prod (size (b))==1,
  b = b * ones (size (i));
end
sz = size (a);

c = a(:);
i = i(find (i>0 & i<=prod(sz)));
[i,ii] = sort (i(:));
b = b(ii)(:);			# The values in the order I'll add them

				# The indices that interest me
jj = [find (diff (i)); rows (b)];

c(i(jj)) += diff ([0; cumsum (b)(jj)]);

c = reshape (c, sz);
endfunction

  Hth,

  Etienne

On Sun, Jan 05, 2003 at 05:48:59PM -0600, Igor ?ukanovi? wrote:
# Let A be a vector or a matrix. (e.g. A=[ 1 2 3 4 5]).
# Statement
# A([ 1 2]) = [ 7 8];
# does
# A(1)=7; A(2)=8;
# and that's good.
# Statement
# A([1 1])= [ 8 9];
# does
# A(1) = 9;
# and that's good, too.
# 
# However statement
# A([ 1 1])= A([1 1]) + [ 8 9];
# does (same as in Matlab)
# A(1) = A(1) + 9;
# and that's bad.  I want it to do
# A(1) = A(1) + 8; A(1)=A(1) + 9;
# producing 
# A(1)=A(1)+8+9;
# otherwise I would not include 1 in index list twice.
# Is my wish in conflict with other functions written in octave?
# 
# Best regards,
#                       igor dukanovic
# 
# P.S.  Info:  octave-2.1.36 
# Windows 98 on Pentium

-- 
Etienne Grossmann ------ http://www.isr.ist.utl.pt/~etienne



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