function h=entropy(p)
% H=ENTROPY(P)  返回概率矢量P的熵函數。
if length(find(p<0))~=0,
    error('Not a prob.vector,negative component(s)')
end
if abs(sum(p)-1)>10e-10,
    error('Not aprob.vector,components do not add up to 1')
end
h=sum(-p.*log2(p));