问题描述
在Matlab中，如果仍然需要进行大量计算，并且稀疏数组要比普通数组好一点，并且该数组中约有25％是非零的，那在什么时候呢?
In Matlab, at what point is having a sparse array better than a normal array if I still have a lot of calculations to do on it, and about 25% of the array are non-zeros?
推荐答案
我个人很少会因为只有25％非零的数组而为稀疏而烦恼.如果您不相信我，请自己尝试.
Personally, I'd rarely bother with sparse for an array that is only 25% non-zeros. If you don't believe me, try it yourself.
A = sprand(2000,2000,0.25);
tic,B = A*A;toc
Elapsed time is 1.771668 seconds.
Af = full(A);
tic,B = Af*Af;toc
Elapsed time is 0.499045 seconds.
作为稀疏矩阵与此有关的额外工作花费太多，不值得打扰.现在，使用真正稀疏的矩阵进行尝试.
The extra work involved with this as a sparse matrix costs too much to be worth the bother. Now try it with a really sparse matrix.
A = sprand(2000,2000,0.005);
Af = full(A);
tic,B = A*A;toc
Elapsed time is 0.037763 seconds.
tic,B = Af*Af;toc
Elapsed time is 0.446680 seconds.
当然，您自己的问题会有所不同，但不会有太大的不同.对于使用真正稀疏矩阵的人来说，稀疏矩阵是一个真正的福音，但是在大多数情况下，只有25％的非零值不足以带来任何收益.
Of course, your own problem will be different, but it will not be that different. Sparse matrices are a true boon for the person who uses truly sparse matrices, but 25% non-zeros is simply not "sparse" enough for any gain in most cases.
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