问计算国家/地区的数量。(此任务与JavaScript中的多维数组和对象有关。)
EN

Stack Overflow用户

提问于 2019-11-13 14:05:37

回答 3查看 122关注 0票数 0

城市及其国家/地区以以下结构给出：

let data = [
        {
            country: 'Serbia',
            city:    'Belgrad',
        },
        {
            country: 'Ukraine',
            city:    'Kiev',
        },
        {
            country: 'Russia',
            city:    'Moscow',
        },
        {
            country: 'Russia',
            city:    'Sant-Peterborough',
        },
        {
            country: 'Ukraine',
            city:    'Lvov',
        },
        {
            country: 'China',
            city:    'Beijing',
        },
        {
            country: 'Poland',
            city:    'Cracow',
        },
    ];

我的解决方案是：

let arr =[];
let sum= 0;
for ( elem of data){
  arr.push(elem.country)
}
let  uniqueCountries = [...new Set(arr)];
console.log(uniqueCountries.length)

它起作用了。

我的问题是：

是否有可能找到更好的解决方案(例如，不使用新的集合)？

object

javascript

arrays

回答 3

Stack Overflow用户

发布于 2019-11-13 14:09:13

Set有一个可以使用的size属性。您还可以通过在set实例化中使用map来简化代码，如下所示：

let data = [
  { country: "Serbia", city: "Belgrad" },
  { country: "Ukraine", city: "Kiev" },
  { country: "Russia", city: "Moscow" },
  { country: "Russia", city: "Sant-Peterborough" },
  { country: "Ukraine", city: "Lvov" },
  { country: "China", city: "Beijing" },
  { country: "Poland", city: "Cracow" }
]

const set = new Set(data.map(o => o.country))

console.log(set.size)

我不知道你为什么不在这里使用集合，但是如果你愿意，你可以使用地图来代替：

let data = [
  { country: "Serbia", city: "Belgrad" },
  { country: "Ukraine", city: "Kiev" },
  { country: "Russia", city: "Moscow" },
  { country: "Russia", city: "Sant-Peterborough" },
  { country: "Ukraine", city: "Lvov" },
  { country: "China", city: "Beijing" },
  { country: "Poland", city: "Cracow" }
];

const map = data.reduce((a, o) => (a[o.country] = 0, a), {})
console.log(Object.keys(map).length)

票数 1

Stack Overflow用户

发布于 2019-11-13 14:10:15

Set将是一种更干净的方式。您可以使用map()而不是for循环来清理代码。在设置之前，您将使用一个对象并遍历列表并获取国家/地区。

let data = [
  { country: 'Serbia', city: 'Belgrad', },
  { country: 'Ukraine', city: 'Kiev', },
  { country: 'Russia', city: 'Moscow', },
  { country: 'Russia', city: 'Sant-Peterborough', },
  { country: 'Ukraine', city: 'Lvov', },
  { country: 'China', city: 'Beijing', },
  { country: 'Poland', city: 'Cracow', },
];

var countries = Object.keys(data.reduce((o, { country }) => o[country] = true && o, {}))
console.log(countries.length)

要使您的代码更简洁，请将for循环替换为map()

let data = [
  { country: 'Serbia', city: 'Belgrad', },
  { country: 'Ukraine', city: 'Kiev', },
  { country: 'Russia', city: 'Moscow', },
  { country: 'Russia', city: 'Sant-Peterborough', },
  { country: 'Ukraine', city: 'Lvov', },
  { country: 'China', city: 'Beijing', },
  { country: 'Poland', city: 'Cracow', },
];

let arr = data.map(o => o.country) // replaces your for loop
const mySet = new Set(arr)
console.log(mySet.size) . // no need to make an array to get length

票数 0

Stack Overflow用户

发布于 2019-11-13 14:11:49

您可以使用reduce方法统计国家/地区：

let data = [
  {
      country: 'Serbia', city: 'Belgrad',
  },
  {
      country: 'Ukraine', city: 'Kiev',
  },
  {
      country: 'Russia', city:  'Moscow',
  },
  {
      country: 'Russia', city:  'Sant-Peterborough',
  },
  {
      country: 'Ukraine', city: 'Lvov',
  },
  {
      country: 'China', city: 'Beijing',
  },
  {
      country: 'Poland', city: 'Cracow',
  },
];

const result = data.reduce((a, {country, city}) =>{
  a[country] = a[country] || {count: 0};
  a[country].count += 1;

  return a;
}, {})

console.log(result);