;+
; NAME:
; CROSSPN
;
; AUTHOR:
; Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
; craigm@lheamail.gsfc.nasa.gov
;
; PURPOSE:
; A version of CROSSP for efficient vector cross products
;
; CALLING SEQUENCE:
; C = CROSSPN(A, B)
;
; DESCRIPTION:
;
; The function CROSSPN computes the vector cross product (outer
; product). The difference between CROSSPN and the IDL library
; function CROSSP, is that CROSSPN allows more than one cross
; product to be computed at one time (i.e., it is vectorized).
;
; Thus, in the expression "C = CROSSPN(A, B)" the vector cross
; product is computed as C = A x B. Because CROSSPN is vectorized,
; any of the following combinations are valid:
;
; * A is a 3-vector, B is a 3-vector
; ==> C is the vector cross product C = A x B
;
; * A is a 3xN array, B is a 3-vector
; ==> C(*,I) = A(*,I) x B (each A is crossed with B)
;
; * A is a 3-vector, B is a 3xN array
; ==> C(*,I) = A x B(*,I) (A is crossed with each B)
;
; * A is a 3xN array, B is a 3xN array
; ==> C(*,I) = A(*,I) x B(*,I) (component-by-component)
;
; If both A and B are arrays then they must have the same
; dimensions.
;
; INPUTS:
;
; A - a 3-vector or 3xN array.
;
; B - a 3-vector or 3xN array.
;
;
; RETURNS:
;
; The vector cross product A x B, either a 3-vector or a 3xN array
; depending on A and B.
;
; SEE ALSO:
;
; CROSSP
;
; MODIFICATION HISTORY:
; Written, CM, 10 Mar 2002
; Documented, CM, 22 Mar 2002
;
; $Id: crosspn.pro,v 1.2 2002/03/22 22:02:02 craigm Exp $
;
;-
; Copyright (C) 2002, Craig Markwardt
; This software is provided as is without any warranty whatsoever.
; Permission to use, copy, modify, and distribute modified or
; unmodified copies is granted, provided this copyright and disclaimer
; are included unchanged.
;-
function crosspn, x1, x2
n1 = n_elements(x1)/3
n2 = n_elements(x2)/3
if n1 EQ 1 AND n2 EQ 1 then begin
return, crossp(x1,x2)
endif else if n1 GT 1 AND n2 EQ 1 then begin
xr = make_array(3,n1,value=0*x1(0)*x2(0))
xr(0,*) = x1(1,*)*x2(2) - x2(1)*x1(2,*)
xr(1,*) = x1(2,*)*x2(0) - x2(2)*x1(0,*)
xr(2,*) = x1(0,*)*x2(1) - x2(0)*x1(1,*)
return, xr
endif else if n1 EQ 1 AND n2 GT 1 then begin
xr = make_array(3,n2,value=0*x1(0)*x2(0))
xr(0,*) = x1(1)*x2(2,*) - x2(1,*)*x1(2)
xr(1,*) = x1(2)*x2(0,*) - x2(2,*)*x1(0)
xr(2,*) = x1(0)*x2(1,*) - x2(0,*)*x1(1)
return, xr
endif else if n1 EQ n2 then begin
xr = make_array(3,n1,value=0*x1(0)*x2(0))
xr(0,*) = x1(1,*)*x2(2,*) - x2(1,*)*x1(2,*)
xr(1,*) = x1(2,*)*x2(0,*) - x2(2,*)*x1(0,*)
xr(2,*) = x1(0,*)*x2(1,*) - x2(0,*)*x1(1,*)
return, xr
endif else begin
message, 'ERROR: number of vectors must match'
endelse
return, 0
end