MATLAB:以矩阵为值的accumarray(第二个输入参数)

Question

我有一个包含百分比值的矩阵，其中每一行代表一个单独的观察结果。我需要计算这些值对应于相同下标的累积乘积。我尝试使用 Accumarray 函数，只要我使用列向量作为值(而不是矩阵)，它就可以正常工作并且符合预期。我想知道在不循环遍历值矩阵的各个列的情况下解决问题的最佳方法是什么？

这是我的示例代码:

subs = [1;1;1;2;2;2;2;2;3;3;4;4;4];
vals1 = [0.1;0.05;0.2;0.02;0.09;0.3;0.01;0.21;0.12;0.06;0.08;0.12;0.05];

% This is working as expected
result1 = accumarray(subs,vals1, [], @(x) prod(1+x) -1)


vals2 = [vals1,vals1];

% This is not working as the second input parameter of accumarray
% apperently must be a vector (rather than a matrix)
result2 = accumarray(subs, vals2, [], @(x) prod(1+x) -1)

Answer 1

对于 vals，您可以将其设置为 1:size(vals2,1) 并使用它来提取 vals2 行。该函数还需要返回单元格。

result2 = accumarray(subs, 1:size(vals2,1), [], @(x) {prod(1+vals2(x,:),1)-1})

您可以连接单元格元素:

result3 = vertcat(result2{:})

或者全部写在一行:

result3 = cell2mat( accumarray(subs, 1:size(vals2,1), [], @(x) {prod(1+vals2(x,:),1)-1}))

result3 =

   0.38600   0.38600
   0.76635   0.76635
   0.18720   0.18720
   0.27008   0.27008

使用 [10000 x 200] 矩阵作为输入在 Octave 中比较三种建议方法的测试结果:

subs = randi(1000,10000,1);
vals2 = rand(10000,200);

=========CELL2MAT========
Elapsed time is 0.130961 seconds.
=========NDGRID========
Elapsed time is 3.96383 seconds.
=========FOR LOOP========
Elapsed time is 6.16265 seconds.

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