;+
; NAME:
;  rt_fintrp
; PURPOSE: (one line)
;  interpolate into an ordered list
; DESCRIPTION:
; CATEGORY:
;  RT
; CALLING SEQUENCE:
;  y = rt_fintrp(xtab, ytab, xx)
; INPUTS:
;  xtab - array of x's
;  ytab - matching array of y's
;  x - value at which we want the interpolated y   
; OPTIONAL INPUT PARAMETERS:
;  none
; KEYWORD INPUT PARAMETERS:
;  none
; KEYWORD OUTPUT PARAMETERS:
;  none
; OUTPUTS:
; COMMON BLOCKS:
; SIDE EFFECTS:
; RESTRICTIONS:
;  None
; PROCEDURE:
; 
;  first calls rt_locate
; 
; MODIFICATION HISTORY:
;  Written 2004 May 8, Leslie Young SwRI
;  based on finterp, in locate.f
;-

function rt_fintrp, xtab, ytab, xx

    ntab = n_elements(xtab)
    i1 = rt_locate(xtab,xx)
    i2 = i1 + 1
    rr = (xx - xtab[i1]) / (xtab[i2] - xtab[i1])
    y = ytab[i1] + (rr * (ytab[i2] - ytab[i1]))
    return, y
    
end