向量化"for"循环 [英] Vectorizing 'for' loops

问题描述

这一段代码可以按我的意愿工作，但是根据良好的MATLAB代码的精神，有一种方法可以对此进行矢量化(以前是k x 1矢量):

This piece of code works as I want it to, but in the spirit of good MATLAB code, is there a way to vectorize this (previous is a k x 1 vector):

start = zeros(k,1);
for i = 2:length(previous)
    if (previous(i-1) == -1)
        start(previous(i))= start(previous(i))+1;
    end    
end

通常，在MATLAB中进行矢量化代码的直观方法是什么?

What, in general, is the intuitive way to go about vectorizing code in MATLAB?

推荐答案

您可以在没有find的情况下执行此操作，以实现最佳性能:

You can do this without find, for maximum performance:

I = [false; previous(1:end-1) == -1];
idx = previous(I);
start(idx) = start(idx) + 1;

这还避免了previous(end) == -1的风险，该风险会导致替代项中的索引超出范围错误.

This also avoids the risk that previous(end) == -1, which would cause an index out-of-range error in the alternative.

请注意，如果idx包含重复的索引，则此功能与原始功能不同.

Note that this doesn't work the same as your original if idx contains duplicated indices.

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