FUNCTION entropy,p,s
;+
;H=entropy(p) returns the entropy associated with the
;probability array p. If two arrays, p and s, are provided then
;the returned value is total(p*alog(s))/alog(2). The
;calculation is done over the nonzero values of the
;array in the logarithm.
;
;-

IF n_params() EQ 1 then s=p

IF abs(total(p) - 1.0) GT 1e-6 THEN MESSAGE,'Unacceptable probability array in argument'
k=Where(s GT 0)
return,-total(p[k]*alog(s[k]))/alog(2)
end