如何将数组转换为统计各唯一元素出现次数的哈希表？

下面是几种主流编程语言的实现方式，核心逻辑都是遍历数组，累加每个元素的出现次数：

Ruby 实现 #

Ruby 2.7及以上版本自带tally方法，一行搞定：

arr = ["americans", "oath", "oath", "generation", "americans"]
count_hash = arr.tally
# 输出结果: {"americans"=>2, "oath"=>2, "generation"=>1}

如果用旧版Ruby，手动实现计数：

arr = ["americans", "oath", "oath", "generation", "americans"]
count_hash = Hash.new(0) # 初始化哈希表，默认值为0
arr.each { |item| count_hash[item] += 1 }
# 结果与上面一致

Python 实现 #

用标准库的Counter类，简洁高效：

from collections import Counter

arr = ["americans", "oath", "oath", "generation", "americans"]
count_dict = Counter(arr)
# 输出结果: {'americans': 2, 'oath': 2, 'generation': 1}

手动实现的话，用字典的get方法处理初始值：

arr = ["americans", "oath", "oath", "generation", "americans"]
count_dict = {}
for item in arr:
    # 元素不存在时默认取0，加1后存回字典
    count_dict[item] = count_dict.get(item, 0) + 1

JavaScript 实现 #

用reduce方法遍历数组并构建计数对象：

const arr = ["americans", "oath", "oath", "generation", "americans"];
const countObj = arr.reduce((acc, curr) => {
  // 累加计数，不存在的元素初始化为0再加1
  acc[curr] = (acc[curr] || 0) + 1;
  return acc;
}, {});
// 输出结果: { americans: 2, oath: 2, generation: 1 }

内容的提问来源于stack exchange，提问作者Bob Kun