Ruby：在每个映射，注入，each_with_index和each_with_object之间进行选择 [英] Ruby : Choosing between each, map, inject, each_with_index and each_with_object

问题描述

当我多年前开始编写Ruby时，花了一段时间才了解 each and 地图。只有当我发现所有其他 Enumerable 和 Array 方法。

在官方文档和许多的帮助下 StackOverflow

下面是我花更长的时间理解的：

• 为什么我应该使用其中一种方法？


• 有没有指导方针？
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我希望这个问题不是重复的：我对为什么？更感兴趣。比什么？或How？，我认为它可以帮助Ruby新手。 更多 tl; dr
$ b

如何在每个map，inject，each_with_index和each_with_object之间进行选择？
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• 当您需要generic时，使用 #each 迭代并且不关心结果。例如 - 你有数字，你想打印每个数字的绝对值：
  
  

    numbers.each {| number | puts number.abs} 
    




• 使用 #map 当你想要一个新的列表，其中每个元素通过变换原始元素以某种方式形成。例子 - 你有数字，你想得到他们的正方形：

    numbers.map {| number |数字** 2} 
    




• 使用 #inject 当你想以某种方式将整个列表减少到一个单一的值。例子 - 你有数字，你想得到它们的总和：
  
  

    numbers.inject（&：+）
    




• 使用 #each_with_index code> #each ，除了您还希望每个元素都有索引：
  
  

   数字.each_with_index {|数字，索引| ＃编号＃位于＃{index}位置} 
    




• #each_with_object 更受限制。最常见的情况是，如果你需要类似于 #inject 的东西，但想要一个新的集合（而不是奇异值），这不是原始的直接映射。示例 - 数字直方图（频率）：
  
  

    numbers.each_with_object（{}）{|数字，直方图|直方图[编号] =直方图[编号] .to_i.next} 
    

When I started writing Ruby many years ago, it took me a while to understand the difference between each and map. It only got worse when I discovered all the other Enumerable and Array methods.

With the help of the official documentation and many StackOverflow questions, I slowly began to understand what those methods did.

Here is what took me even longer to understand though :

• Why should I use one method or another?
• Are there any guidelines?

I hope this question isn't a duplicate : I'm more interested in the "Why?" than the "What?" or "How?", and I think it could help Ruby newcomers.

解决方案

A more tl;dr answer:

How to choose between each, map, inject, each_with_index and each_with_object?

• Use #each when you want "generic" iteration and don't care about the result. Example - you have numbers, you want to print the absolute value of each individual number:

  numbers.each { |number| puts number.abs }
  

• Use #map when you want a new list, where each element is somehow formed by transforming the original elements. Example - you have numbers, you want to get their squares:

  numbers.map { |number| number ** 2 }
  

• Use #inject when you want to somehow reduce the entire list to one single value. Example - you have numbers, you want to get their sum:

  numbers.inject(&:+)
  

• Use #each_with_index in the same situation as #each, except you also want the index with each element:

  numbers.each_with_index { |number, index| puts "Number #{number} is on #{index} position" }
  

• Uses for #each_with_object are more limited. The most common case is if you need something similar to #inject, but want a new collection (as opposed to singular value), which is not a direct mapping of the original. Example - number histogram (frequencies):

  numbers.each_with_object({}) { |number, histogram| histogram[number] = histogram[number].to_i.next }
  

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