JavaScript30秒, 从入门到放弃之Array(四)
发布于 4 个月前 作者 hongmaoxiao 689 次浏览 来自 分享

原文地址:JavaScript30秒, 从入门到放弃之Array(四)

博客地址:JavaScript30秒, 从入门到放弃之Array(四)

水平有限,欢迎批评指正

maxN

Returns the n maximum elements from the provided array. If n is greater than or equal to the provided array’s length, then return the original array(sorted in descending order).

Use Array.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in descending order. Use Array.slice() to get the specified number of elements. Omit the second argument, n, to get a one-element array.

const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);

返回一个数组的前n个最大值,如果指定的n大于或等于指定数组的长度,那么将返回原数组(按降序排列后)。

使用Array.sort()ES6的扩展运算符来生成一个按降序排列的浅度复制数组。使用Array.slice()来截取指定个数的数组元素。若省略第二个参数n时,n=1

➜  code cat maxN.js
const maxN = (arr, n = 1) => [...arr].sort((a, b) => b - a).slice(0, n);

console.log(maxN([1, 2, 3]));
console.log(maxN([1, 2, 3], 2));

➜  code node maxN.js
[ 3 ]
[ 3, 2 ]

主要看懂这个sort就好了:

sort((a, b) => b - a)

这是降序排的方法,怎么讲?

变形一:

sort(fn(a,b))

这个fn呢有两个参数ab就是数组排序是按顺序相邻的两个数组元素。a前、b后。

变形二:

sort((a, b) => {
  if (b > a) {
    return 1;
  } else if (b < a) {
    return -1;
  }
  return 0;
})

return1表示把前面的数a放后面,后面的数b在放前面;return0表示不换位置;return-1表示前面的数a放前面,后面的数b放后面。

例子中,当b > a时把a换到b后面,意即把大数放前边了,即降序排列。反之升序排列。

slice(0, n)

排完之后slice(0, n)截取前n个元素组成的数组即为数组最大的前n个数。

minN

Returns the n minimum elements from the provided array. If n is greater than or equal to the provided array’s length, then return the original array(sorted in ascending order).

Use Array.sort() combined with the spread operator (...) to create a shallow clone of the array and sort it in ascending order. Use Array.slice() to get the specified number of elements. Omit the second argument, n, to get a one-element array.

const minN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);

返回一个数组的前n个最小值,如果指定的n大于或等于指定数组的长度,那么将返回原数组(按升序排列后)。

使用Array.sort()ES6的扩展运算符来生成一个按升序排列的浅度复制数组。使用Array.slice()来截取指定个数的数组元素。若省略第二个参数n时,n=1

➜  code cat minN.js
const maxN = (arr, n = 1) => [...arr].sort((a, b) => a - b).slice(0, n);

console.log(maxN([1, 2, 3]));
console.log(maxN([1, 2, 3], 2));

➜  code node minN.js
[ 1 ]
[ 1, 2 ]

sort((a, b) => a - b)maxN相反,命题得证!

nthElement

Returns the nth element of an array.

Use Array.slice() to get an array containing the nth element at the first place. If the index is out of bounds, return []. Omit the second argument, n, to get the first element of the array.

const nthElement = (arr, n = 0) => (n > 0 ? arr.slice(n, n + 1) : arr.slice(n))[0];

返回指定数组的第n个元素(索引从0算起)。

使用Array.slice()截取数组,使截取的数组的第一个元素就是nth对应的元素。如果索引n超过数组范围,返回空数组[]。省略第二个参数n,按n=0计。

➜  code cat nthElement.js
const nthElement = (arr, n = 0) => (n > 0 ? arr.slice(n, n + 1) : arr.slice(n))[0];

console.log(nthElement(['a', 'b', 'c'], 1));
console.log(nthElement(['a', 'b', 'b'], -3));

➜  code node nthElement.js
b
a

就是简单的用slice去截取元素,取截取后的第一个元素即可。

partition

Groups the elements into two arrays, depending on the provided function’s truthiness for each element.

Use Array.reduce() to create an array of two arrays. Use Array.push() to add elements for which fn returns true to the first array and elements for which fn returns false to the second one.

const partition = (arr, fn) =>
  arr.reduce(
    (acc, val, i, arr) => {
      acc[fn(val, i, arr) ? 0 : 1].push(val);
      return acc;
    },
    [[], []]
  );

根据提供的方法对一个数组就行调用后,按运算结果的布尔值是否为真分类。为真,归到二维数组索引为0的数组中;为假,归到二维数组索引为1的数组中。

使用Array.reduce()生成一个1x2的二维数组。使用Array.push()把指定fn运算结果为true的数组元素添加到二维数组的第一个数组中,运算结果为false的数组元素添加到二维数组的第二个数组中。

➜  code cat partition.js
const partition = (arr, fn) => arr.reduce((acc, val, i, arr) => {
    acc[fn(val, i, arr) ? 0 : 1].push(val);
    return acc;
}, [
    [],
    []
]);

const users = [{
    user: 'Pony',
    age: 47,
    active: true
}, {
    user: 'barney',
    age: 36,
    active: false
}, {
    user: 'fred',
    age: 40,
    active: true
}];

console.log(partition(users, o => o.active));

➜  code node partition.js
[ [ { user: 'Pony', age: 47, active: true },
    { user: 'fred', age: 40, active: true } ],
  [ { user: 'barney', age: 36, active: false } ] ]

acc的默认值是一个1x2的二维空数组[[], []]。随着reduce的遍历过程将把满足对应条件的元素分别push到对应的数组中。

acc[fn(val, i, arr) ? 0 : 1].push(val);

fn(val, i, arr)如果为true将会把对应的元素val添加到acc的索引为0的数组中,否则添加到索引为1的数组中。这样遍历结束就达到了分组的目的。

例子中,fno => o.active就是根据对象的active的属性是否为true进行分类,所以我们看到,userPonyfred的元素都在二维数组的索引为0的数组中,其它在二维数组的索引为1的数组中。

pull

Mutates the original array to filter out the values specified.

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in an array by resetting it’s length to zero and Array.push() to re-populate it with only the pulled values.

(For a snippet that does not mutate the original array see without)

const pull = (arr, ...args) => {
 let argState = Array.isArray(args[0]) ? args[0] : args;
 let pulled = arr.filter((v, i) => !argState.includes(v));
 arr.length = 0;
 pulled.forEach(v => arr.push(v));
};

改变原数组使其过滤掉指定的那些元素。

使用Array.filter()Array.includes()剔除数组里不需要的元素。先用Array.length = 0把原数组变成空数组,然后再通过Array.push()把过滤后剩余的元素重新填充进去。

(类似方法不改变原数组的请看without方法)

➜  code cat pull.js
const pull = (arr, ...args) => {
    let argState = Array.isArray(args[0]) ? args[0] : args;
    let pulled = arr.filter((v, i) => !argState.includes(v));
    arr.length = 0;
    pulled.forEach(v => arr.push(v));
};

let myArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(myArray, 'a', 'c');
let secondArray = ['a', 'b', 'c', 'a', 'b', 'c'];
pull(secondArray, ['a', 'c'], 'b');

console.log(myArray);
console.log(secondArray);

➜  code node pull.js
args:  [ 'a', 'c' ]
args:  [ [ 'a', 'b' ], 'c' ]
[ 'b', 'b' ]
[ 'c', 'c' ]
let argState = Array.isArray(args[0]) ? args[0] : args;

判断args的第一个元素是不是一个数组,如果是,把该数组赋值给argState作为后续排除数组元素的元数组;否则args就是元数组。

let pulled = arr.filter((v, i) => !argState.includes(v));

结合filterincludes把数组arr中包含在argState中的元素排除掉。

arr.length = 0;
pulled.forEach(v => arr.push(v));

此处,把数组长度设为0,将数组置空,然后再遍历pulled,把所有pulled的元素pusharr中,最终arr就只含有排除掉指定元素后的其他元素。

pullAtIndex

Mutates the original array to filter out the values at the specified indexes.

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in an array by resetting it’s length to zero and Array.push() to re-populate it with only the pulled values. Use Array.push() to keep track of pulled values

const pullAtIndex = (arr, pullArr) => {
 let removed = [];
 let pulled = arr
   .map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
   .filter((v, i) => !pullArr.includes(i));
 arr.length = 0;
 pulled.forEach(v => arr.push(v));
 return removed;
};

改变原数组使其过滤掉指定的那些索引值对应的元素。

使用Array.filter()Array.includes()剔除数组里不需要的元素。先用Array.length = 0把原数组变成空数组,然后再通过Array.push()把过滤后剩余的元素重新填充进去。同时使用Array.push()跟踪记录剔除掉的所有元素。

➜  code cat pullAtIndex.js
const pullAtIndex = (arr, pullArr) => {
    let removed = [];
    let pulled = arr.map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
        .filter((v, i) => !pullArr.includes(i));

    arr.length = 0;
    pulled.forEach((v) => arr.push(v));
    return removed;
};

let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtIndex(myArray, [1, 3]);

console.log('myArray: ', myArray);
console.log('pulled: ', pulled);

➜  code node pullAtIndex.js
myArray:  [ 'a', 'c' ]
pulled:  [ 'b', 'd' ]
let pulled = arr
  .map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))
  .filter((v, i) => !pullArr.includes(i));

arrmap是为了把要排除掉的元素pushremoved变量中。pullArr.includes(i) ? removed.push(v) : v这个三元运算符就是判断索引是否在要排除掉的指定索引数组pullArr中。如果在,添加到removed中,否则直接返回该元素。

接下来filterarr中匹配pullArr的索引对应元素剔除掉。

arr.length = 0;
pulled.forEach((v) => arr.push(v));
return removed;

最后把arr置空后再填入满足条件的元素,然后返回剔除掉的元素组成的数组。

pullAtValue

Mutates the original array to filter out the values specified. Returns the removed elements.

Use Array.filter() and Array.includes() to pull out the values that are not needed. Use Array.length = 0 to mutate the passed in an array by resetting it’s length to zero and Array.push() to re-populate it with only the pulled values. Use Array.push() to keep track of pulled values

const pullAtValue = (arr, pullArr) => {
  let removed = [],
    pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
    mutateTo = arr.filter((v, i) => !pullArr.includes(v));
  arr.length = 0;
  mutateTo.forEach(v => arr.push(v));
  return removed;
};

改变原数组使其过滤掉指定的那些值所匹配的元素们,返回剔除掉所有元素组成的数组。

使用Array.filter()Array.includes()剔除数组里不需要的元素。先用Array.length = 0把原数组变成空数组,然后再通过Array.push()把过滤后剩余的元素重新填充进去。同时使用Array.push()跟踪记录剔除掉的所有元素。

➜  code cat pullAtValue.js
const pullAtValue = (arr, pullArr) => {
    let removed = [],
        pushToRemove = arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v)),
        mutateTo = arr.filter((v, i) => !pullArr.includes(v));

    arr.length = 0;
    mutateTo.forEach((v) => arr.push(v));
    return removed;
};

let myArray = ['a', 'b', 'c', 'd'];
let pulled = pullAtValue(myArray, ['b', 'd']);

console.log('myArray: ', myArray);
console.log('pulled: ', pulled);

➜  code node pullAtValue.js
myArray:  [ 'a', 'c' ]
pulled:  [ 'b', 'd' ]

逻辑上和pullAtIndex差不多,差别就在一个是过滤索引,另一个是过滤

为此实现上就有了以下不同:

// pullAtIndex
arr.map((v, i) => (pullArr.includes(i) ? removed.push(v) : v))

// pullAtValue
arr.forEach((v, i) => (pullArr.includes(v) ? removed.push(v) : v))

一个用了arr.map,一个用了arr.forEach

为什么呢?

arr.maparr的元素是会改变的,但是对于要剔除掉索引来说要删除掉索引对应的值是否有变化是无关紧要的。而对于匹配值来说就不灵了,因为本来要剔除掉的值在map的过程中改变了,到filter的时候就匹配不出来了,就无法剔除了。

所以改成了arr.forEach,它是不改变数组元素的,没有副作用,不干扰后续filter。另外forEach的结果是undefined

reducedFilter

Filter an array of objects based on a condition while also filtering out unspecified keys.

Use Array.filter() to filter the array based on the predicate fn so that it returns the objects for which the condition returned a truthy value. On the filtered array, use Array.map() to return the new object using Array.reduce() to filter out the keys which were not supplied as the keys argument.

const reducedFilter = (data, keys, fn) =>
  data.filter(fn).map(el =>
    keys.reduce((acc, key) => {
      acc[key] = el[key];
      return acc;
    }, {})
  );

根据一个条件对一个数组进行过滤,同时过滤掉不需要的键。

使用Array.filter()去过滤出指定方法fn对数组元素对象调用结果为真值的元素,对过滤后的数组使用Array.map()返回一个新的对象,对象包含的键值对是由Array.reduce()根据指定keys过滤掉不需要的键而组成的。

➜  code cat reducedFilter.js
const reducedFilter = (data, keys, fn) =>
    data.filter(fn).map(el =>
        keys.reduce((acc, key) => {
            acc[key] = el[key];
            return acc;
        }, {})
    );


const data = [{
    id: 1,
    name: 'john',
    age: 24
}, {
    id: 2,
    name: 'mike',
    age: 50
}];

console.log(reducedFilter(data, ['id', 'name'], item => item.age > 24));

➜  code node reducedFilter.js
[ { id: 2, name: 'mike' } ]
data.filter(fn)

数组data根据方法fn过滤掉了不满足条件的数组元素。

keys.reduce((acc, key) => {
  acc[key] = el[key];
  return acc;
}, {})

keys是最终要保留的键的数组,reduceacc初始值是空对象{},遍历过程中,把所有的el对象中键包含于keys数组所有键值对累加到acc对象中。

map(el => fn1)

最后联合map方法可以看出,最终返回的是一个数组,数组内包含fn1方法也就是keys.reduce方法返回的acc的对象。

remove

Removes elements from an array for which the given function returns false.

Use Array.filter() to find array elements that return truthy values and Array.reduce() to remove elements using Array.splice(). The func is invoked with three arguments (value, index, array).

const remove = (arr, func) =>
  Array.isArray(arr)
    ? arr.filter(func).reduce((acc, val) => {
        arr.splice(arr.indexOf(val), 1);
        return acc.concat(val);
      }, [])
    : [];

删除数组中以指定方法调用结果为false的所有元素。

使用Array.filter()来找出数组中所有运行指定方法结果为真的元素,使用Array.reduce()配合Array.splice()删除掉不需要的元素。func函数调用有三个参数(value, index, array)

➜  code cat remove.js
const remove = (arr, func) =>
    Array.isArray(arr) ?
    arr.filter(func).reduce((acc, val) => {
        arr.splice(arr.indexOf(val), 1);
        return acc.concat(val);
    }, []) : [];

const arr = [1,2,3,4];
console.log(remove(arr, n => n % 2 == 0));
console.log(arr);

➜  code node remove.js
[ 2, 4 ]
[ 1, 3 ]
Array.isArray(arr) ? filterfun : [];

先判断给定参数arr是否是一个数组,是,执行filter函数;否,直接返回结果空数组[]

arr.filter(func).reduce((acc, val) => {
  arr.splice(arr.indexOf(val), 1);
  return acc.concat(val);
}, [])

arr.filter(func)首先过滤出func运行结果为真所有数组元素。reduce方法将filter剩余的所有数组元素以concat的方式返回结果数组。而在原数组arr中,则用splicefunc运行结果为真的所有元素剔除。

其实就最终的返回结果来说,arr.filter(func)已经可以返回正确的结果,之所以看起来多此一举的使用了reduce的原因在于必须把不需要的元素从原数组arr中剔除。

以下是我在没看代码之前根据例子运行结果先写的代码:

➜  code cat remove1.js
const remove = (arr, fn) => {
  let removed = [];
  arr.forEach(v => (fn(v) ? removed.push(v) : v));
  const left = arr.filter(v => !fn(v));

  arr.length = 0;
  left.forEach(v => arr.push(v));

  return removed;
};

const arr = [1,2,3,4];
console.log(remove(arr, n => n % 2 == 0));
console.log(arr);

➜  code node remove1.js
[ 2, 4 ]
[ 1, 3 ]

我认为代码本身应该没什么问题,但可能没那么优雅,另外就是没有做Array.isArray的前置条件判断。

sample

Returns a random element from an array.

Use Math.random() to generate a random number, multiply it by length and round it of to the nearest whole number using Math.floor(). This method also works with strings.

const sample = arr => arr[Math.floor(Math.random() * arr.length)];

返回数组中随机的一个元素。

使用Math.random()生成一个随机数,乘以数组的长度,然后再配以Math.floor()获取整数索引,进而返回该索引对应的数组元素。这个方法也同样适用于字符串。

➜  code cat sample.js
const sample = (arr) => arr[Math.floor(Math.random() * arr.length)]

console.log(sample([3, 7, 9, 11]));

➜  code node sample.js
7
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