;-------------------------------------------------------------
;+
; NAME:
;       RECPOL
; PURPOSE:
;       Convert 2-d rectangular coordinates to polar coordinates.
; CATEGORY:
; CALLING SEQUENCE:
;       recpol, x, y, r, a
; INPUTS:
;       x, y = vector in rectangular form.           in
; KEYWORD PARAMETERS:
;       Keywords:
;         /DEGREES means angle is in degrees, else radians.
; OUTPUTS:
;       r, a = vector in polar form: radius, angle.  out
; COMMON BLOCKS:
; NOTES:
; MODIFICATION HISTORY:
;       R. Sterner. 18 Aug, 1986.
;       Johns Hopkins University Applied Physics Laboratory.
;       RES 13 Feb, 1991 --- added /degrees.
;       R. Sterner, 30 Dec, 1991 --- simplified.
;       R. Sterner, 25 May, 1993 --- Fixed atan (0,0) problem.
;
; Copyright (C) 1986, Johns Hopkins University/Applied Physics Laboratory
; This software may be used, copied, or redistributed as long as it is not
; sold and this copyright notice is reproduced on each copy made.  This
; routine is provided as is without any express or implied warranties
; whatsoever.  Other limitations apply as described in the file disclaimer.txt.
;-
;-------------------------------------------------------------
 
 
	pro recpol, x, y, r, a, help=hlp, degrees=degrees
 
	if (n_params(0) lt 4) or keyword_set(hlp) then begin
	  print,' Convert 2-d rectangular coordinates to polar coordinates.
	  print,' recpol, x, y, r, a
	  print,'   x, y = vector in rectangular form.           in'
	  print,'   r, a = vector in polar form: radius, angle.  out'
	  print,' Keywords:'
	  print,'   /DEGREES means angle is in degrees, else radians.'
	  return
	endif
 
	;----------------------------------------------------------------
	;  Angle complicated because atan won't take (0,0) and
	;  also because want to keep angle in 0 to 360 (2 pi) range.
	;----------------------------------------------------------------
	w = where((x ne 0) or (y ne 0), count)	; Where not both X,Y eq 0.
	a = x*0.				; Output angle array.
	if count gt 0 then a(w)=atan(y(w),x(w))	; Find angles.
	w = where(a lt 0, count)		; find A < 0 and fix.
	if count gt 0 then a(w)= a(w)+2*!dpi	; add 2 pi to angles < 0.
 
	r = sqrt(x^2 + y^2)			; Find radii.
 
	if keyword_set(degrees) then a = a*!radeg
 
	return
	end