在Clojure中查找与谓词匹配的元素的索引？ [英] Find index of an element matching a predicate in Clojure?

问题描述

使用Clojure，如何在此向量中找到具有正值的第一个索引 [ - 1 0 3 7 9] ？

With Clojure, how do I find the first index with a positive value in this vector [-1 0 3 7 9]?

我知道你可以使用第一个和过滤器获得更优雅的第一个结果：

I know you can get the first result of something rather elegantly with first and filter:


（first（filter pos？[-1 0 99 100 101]））


此代码返回值 99 。我想要的答案是 2 的索引。

This code returns the value 99. The answer I want is the index which is 2.

推荐答案

keep-indexed ，您可以得到满足谓词的索引序列：

Using keep-indexed you can get a sequence of indices for which a predicate is satisfied:

(defn indices [pred coll]
   (keep-indexed #(when (pred %2) %1) coll))

有了这个简单的函数，你可以用下面的表达式解决你的问题：

With this simple function you'll solve your problem with the expression

user=> (first (indices pos? [-1 0 99 100 101]))
2

注意，由于 keep-indexed （和 indices ）的惰性，整个序列不需要实现不执行无关的计算。

Note that, due to the lazyness of keep-indexed (and indices), the entire sequence need not be realized so no extraneous calculations are performed.

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