问题描述
我想使用D来计算目录中文件的数量。
I would like an easy way to count the number of files in a directory using D.
据D手册，dirEntries返回一个范围，但是没有length属性。因此，我必须使用计数器来遍历结果，或者以传统数组的形式收集名称，这些数组可以找到...的长度。是否有更好的方法？
As far as I can tell from the D manual, dirEntries returns a range, but this has no length property. I therefore have to iterate through the results with a counter, or gather up the names in a traditional array which I can find the length of... Is there a better way?
auto txtFiles = dirEntries(".", "*.txt", SpanMode.shallow);
int i=0;
foreach (txtFile; txtFiles)
i++;
writeln(i, " files found..");
推荐答案
范围通常没有 length 属性，除非可以在 O（1）中进行计算。任何不能计算其长度大于 O（n）的范围都不会具有 length 属性，因为它效率太低。获取没有 length 属性的范围的长度的惯用方式是使用。如果范围定义了 length ，它将使用 length ；否则，它只会在范围内迭代并计算有多少元素。
Ranges generally don't have a length property unless it can be calculated in O(1). Any range which can't calculate its length in better than O(n) isn't going to have a length property, because it's too inefficient. The idiomatic way to get the length of a range with no length property is to use std.range.walkLength. It uses length if the range defines length; otherwise, it simply iterates over the range and counts up how many elements there are.
您还可以使用，但是它用于计算与谓词匹配的元素数，默认情况下为一个谓词，它为每个元素返回 true ，效率低于 walkLength ，因为 walkLength 在元素上进行迭代时不会对元素进行任何调用，如果已定义，它将使用 length 来代替，而计数永远不会。
You can also use std.algorithm.count, but it's intended for counting the number of elements which match a predicate, and while it defaults to a predicate which returns true for every element, that's less efficient than walkLength, because walkLength doesn't call anything on the elements as it iterates over them, and it will use length instead if it's been defined, whereas count never will.
这篇关于使用Dlang计数目录中的文件的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！