From bug-request at octave dot org Tue Dec 14 07:59:33 2004
Subject: intersect patch
From: "Hall, Benjamin" <benjamin dot hall at pw dot utc dot com>
To: "'bug at octave dot org'" <bug@octave.org>
Date: Tue, 14 Dec 2004 07:31:49 -0600

(Try 2) I have made some modifications to the octave-forge intersect
function so that it now returns up to 3 results:
	[c,ia,ib] = intersect( a, b )
where the two new return vectors are the index vectors such that a(ia) ==
b(ib) == c.  This will improve compatibility with the Matlab function of the
same name.

Below are the diffs, let me know if I should submit / identify things
differently (its my first time!)

Ben




*** intersect.m,orig	2004-12-14 08:29:21.000000000 -0500
--- intersect.m	2004-12-14 08:56:51.000000000 -0500
***************
*** 21,49 ****
  ## order. If  at var{a} and @var{b} are both column vectors return a column
  ## vector, otherwise return a row vector.
  ##
  ##  at end deftypefn
  ##  at seealso{unique, union, setxor, setdiff, ismember}
  
! function c = intersect(a,b)
    if ( nargin != 2 )
      usage ("intersect (a, b)");
    endif
  
    if ( isempty (a) || isempty (b) )
!     c = [];
!   else
  
!     ## form a and b into sets
!     a = unique (a);
!     b = unique (b);
!     
!     ## keep duplicates
!     c = sort ([ a(:) ; b(:) ]);
!     n = length(c);
!     c = c (find (c(1:n-1) == c(2:n)));
!     if ( size (b, 1) == 1 || size (a, 1) == 1 )
!       c = c.';
!     endif
  
    endif
  endfunction
--- 21,71 ----
  ## order. If  at var{a} and @var{b} are both column vectors return a column
  ## vector, otherwise return a row vector.
  ##
+ ##  at deftypefnx {Function File} {[@var{c}, @var{ia}, @var{ib}] = }
intersect ( at var{a}, @var{b})
+ ##
+ ## Return index vectors  at var{ia} and @var{ib} such that @code{a(ia)==c}
and
+ ##  at code{b(ib)==c} dot 
+ ##
  ##  at end deftypefn
  ##  at seealso{unique, union, setxor, setdiff, ismember}
  
! function [c,ia,ib] = intersect(a,b)
    if ( nargin != 2 )
      usage ("intersect (a, b)");
    endif
  
    if ( isempty (a) || isempty (b) )
!     c = []; ia = []; ib = [];
!     return;
!   endif
  
!   ## form a and b into sets
!   [a,ja] = unique (a);
!   [b,jb] = unique (b);
! 
!   c = [ a(:); b(:) ];
!   [c, ic] = sort( c );              ## [a(:);b(:)](ic) == c
!   
!   ii = find( c(1:end-1) == c(2:end) );  
! 
!   c  = c(ii);                       ## The answer
!   ia = ja(ic(ii));                  ## a(ia) == c
!   ib = jb(ic(ii+1) - length(a));    ## b(ib) == c
  
+     
+   if ( size (b, 1) == 1 || size (a, 1) == 1 )
+     c = c.';
    endif
+ 
+ 
  endfunction
+ 
+ 
+ %!# Test the routine for index vectors ia and ib
+ %!test
+ %! a = [3 2 4 5 7 6 5 1 0 13 13];
+ %! b = [3 5 12 1 1 7];
+ %! [c,ia,ib] = intersect(a,b);
+ %!assert( c,[1 3 5 7]);
+ %!assert(ia,[8 1 7 5]);
+ %!assert(ib,[5 1 2 6]);



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