计算与谓词匹配的元素数量的惯用方法是什么? [英] What is the idiomatic way of counting the number of elements matching predicate?

问题描述

除此之外，是否有更好的方法来计算谓词函数为真的元素的数量:

Is there a better way to count the number of elements for which the predicate function is true, other than this:

PredCount[lst_, pred_] := Length@Select[lst, pred];

我之所以这么问是因为用 Select[] 构建 lst 的子集似乎效率低下，而且因为 Count[] 只能工作有图案.在我的用例中，函数 PredCount 被多次调用，lst 很大.

I'm asking because it seems inefficient to construct a subset of lst with Select[], and because Count[] only works with patterns. In my use case, the function PredCount is called many times with a large lst.

推荐答案

通常可以通过将谓词转换为带条件的模式来实现.例如:

You can often do this by turning your predicate into a pattern with a condition. For example:

Count[list, x_/;x>5]

将计算列表中大于 5 的元素数.

would count the number of elements in list which are greater than 5.

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