我有一个带有重复值的数组。我想找到任何给定值的出现次数。
例如，如果我有一个数组定义为：var dataset = [2,2,4,2,6,4,7,8];，我想找到数组中某个值的出现次数。也就是说，程序应该显示如果我有 3 次出现 value 2，出现 1 次 value 6，依此类推。
什么是最惯用/优雅的方式来做到这一点？
惯用地查找给定值在数组中出现的次数
IT技术
javascript
arrays
2021-01-29 23:07:30
6个回答
reduce在这里比filter因为它不构建仅用于计数的临时数组更合适。
var dataset = [2,2,4,2,6,4,7,8];
var search = 2;
var count = dataset.reduce(function(n, val) {
return n + (val === search);
}, 0);
console.log(count);在 ES6 中：
let count = dataset.reduce((n, x) => n + (x === search), 0);
请注意，很容易将其扩展为使用自定义匹配谓词，例如，计算具有特定属性的对象：
people = [
{name: 'Mary', gender: 'girl'},
{name: 'Paul', gender: 'boy'},
{name: 'John', gender: 'boy'},
{name: 'Lisa', gender: 'girl'},
{name: 'Bill', gender: 'boy'},
{name: 'Maklatura', gender: 'girl'}
]
var numBoys = people.reduce(function (n, person) {
return n + (person.gender == 'boy');
}, 0);
console.log(numBoys);计算所有项目，即{x:count of xs}在javascript中创建一个对象很复杂，因为对象键只能是字符串，因此您无法可靠地计算混合类型的数组。尽管如此，以下简单的解决方案在大多数情况下都能很好地工作：
count = function (ary, classifier) {
classifier = classifier || String;
return ary.reduce(function (counter, item) {
var p = classifier(item);
counter[p] = counter.hasOwnProperty(p) ? counter[p] + 1 : 1;
return counter;
}, {})
};
people = [
{name: 'Mary', gender: 'girl'},
{name: 'Paul', gender: 'boy'},
{name: 'John', gender: 'boy'},
{name: 'Lisa', gender: 'girl'},
{name: 'Bill', gender: 'boy'},
{name: 'Maklatura', gender: 'girl'}
];
// If you don't provide a `classifier` this simply counts different elements:
cc = count([1, 2, 2, 2, 3, 1]);
console.log(cc);
// With a `classifier` you can group elements by specific property:
countByGender = count(people, function (item) {
return item.gender
});
console.log(countByGender);2017年更新
在 ES6 中，您可以使用Map对象来可靠地计算任意类型的对象。
class Counter extends Map {
constructor(iter, key=null) {
super();
this.key = key || (x => x);
for (let x of iter) {
this.add(x);
}
}
add(x) {
x = this.key(x);
this.set(x, (this.get(x) || 0) + 1);
}
}
// again, with no classifier just count distinct elements
results = new Counter([1, 2, 3, 1, 2, 3, 1, 2, 2]);
for (let [number, times] of results.entries())
console.log('%s occurs %s times', number, times);
// counting objects
people = [
{name: 'Mary', gender: 'girl'},
{name: 'John', gender: 'boy'},
{name: 'Lisa', gender: 'girl'},
{name: 'Bill', gender: 'boy'},
{name: 'Maklatura', gender: 'girl'}
];
chessChampions = {
2010: people[0],
2012: people[0],
2013: people[2],
2014: people[0],
2015: people[2],
};
results = new Counter(Object.values(chessChampions));
for (let [person, times] of results.entries())
console.log('%s won %s times', person.name, times);
// you can also provide a classifier as in the above
byGender = new Counter(people, x => x.gender);
for (let g of ['boy', 'girl'])
console.log("there are %s %ss", byGender.get(g), g);的类型感知实现Counter可能如下所示（Typescript）：
type CounterKey = string | boolean | number;
interface CounterKeyFunc<T> {
(item: T): CounterKey;
}
class Counter<T> extends Map<CounterKey, number> {
key: CounterKeyFunc<T>;
constructor(items: Iterable<T>, key: CounterKeyFunc<T>) {
super();
this.key = key;
for (let it of items) {
this.add(it);
}
}
add(it: T) {
let k = this.key(it);
this.set(k, (this.get(k) || 0) + 1);
}
}
// example:
interface Person {
name: string;
gender: string;
}
let people: Person[] = [
{name: 'Mary', gender: 'girl'},
{name: 'John', gender: 'boy'},
{name: 'Lisa', gender: 'girl'},
{name: 'Bill', gender: 'boy'},
{name: 'Maklatura', gender: 'girl'}
];
let byGender = new Counter(people, (p: Person) => p.gender);
for (let g of ['boy', 'girl'])
console.log("there are %s %ss", byGender.get(g), g);
array.filter(c => c === searchvalue).length;
这是一次显示所有计数的一种方法：
var dataset = [2, 2, 4, 2, 6, 4, 7, 8];
var counts = {}, i, value;
for (i = 0; i < dataset.length; i++) {
value = dataset[i];
if (typeof counts[value] === "undefined") {
counts[value] = 1;
} else {
counts[value]++;
}
}
console.log(counts);
// Object {
// 2: 3,
// 4: 2,
// 6: 1,
// 7: 1,
// 8: 1
//}
较新的浏览器仅因使用 Array.filter
var dataset = [2,2,4,2,6,4,7,8];
var search = 2;
var occurrences = dataset.filter(function(val) {
return val === search;
}).length;
console.log(occurrences); // 3
const dataset = [2,2,4,2,6,4,7,8];
const count = {};
dataset.forEach((el) => {
count[el] = count[el] + 1 || 1
});
console.log(count)
// {
// 2: 3,
// 4: 2,
// 6: 1,
// 7: 1,
// 8: 1
// }