function y=entropy(x)
% Usage: y=entropy(x)
% compute the entropy of a probability vector x
% each element of x must be non-negative, and sum(x)=1
% (C) 2006 by Yu Hen Hu
% created: Dec. 19. 2006

if min(x) < 0, error('x must contain non-negative entries only!'); end
% normalize x such that sum(x)=1
x=x/sum(x);

y=-sum(x.*log2(x));