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S=sparse(1:N,idxM,1); sumM=S.'*(M*S);

S=sparse(1:N,idxM,1); sumM=S.'*(M*S);

信用(和有用的讨论):

Credits (and useful discussion):

https://www.mathworks.com/matlabcentral/answers/407634-how-to-sum-parts-of-a-matrix-of-different-sizes-without-using-for -循环