快速列出PackedArray的产品标志? [英] Fast list-product sign for PackedArray?

问题描述

作为我先前的问题的延续，西蒙的查找PackedArray列表产品的方法很快，但是不适用于负值.

As a continuation of my previous question, Simon's method to find the list product of a PackedArray is fast, but it does not work with negative values.

可以通过Abs对其进行固定"，而花费的时间最少，但是该标志丢失了，因此我需要分别查找产品标志.

This can be "fixed" by Abs with minimal time penalty, but the sign is lost, so I will need to find the product sign separately.

我尝试过的最快方法是EvenQ @ Total @ UnitStep[-lst]

The fastest method that I tried is EvenQ @ Total @ UnitStep[-lst]

lst = RandomReal[{-2, 2}, 5000000];

Do[
  EvenQ@Total@UnitStep[-lst],
  {30}
] // Timing

Out[]= {3.062, Null}

有更快的方法吗?

推荐答案

这比您的解决方案快两倍多，并且除了使用Rule@@@提取相关术语的废话外，我发现它更清楚-它只计算每个符号的数字元素.

This is a little over two times faster than your solution and apart from the nonsense of using Rule@@@ to extract the relevant term, I find it more clear - it simply counts the number elements with each sign.

EvenQ[-1 /. Rule@@@Tally@Sign[lst]]

比较时间(和输出)

In[1]:= lst=RandomReal[{-2,2},5000000];
        s=t={};
        Do[AppendTo[s,EvenQ@Total@UnitStep[-lst]],{10}];//Timing
        Do[AppendTo[t,EvenQ[-1/.Rule@@@Tally@Sign[lst]]],{10}];//Timing
        s==t
Out[3]= {2.11,Null}
Out[4]= {0.96,Null}
Out[5]= True

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