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What you need to know about JS arrays: Array.from

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Sep 08, 2020 pm 01:21 PM

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What you need to know about JS arrays: Array.from

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Preface

I have wanted to do this series since last year when I discovered that the replace method of strings has many magical uses, but I never had the time and didn’t think of a good name, so I put it on hold. When I was doing my homework last Friday, I saw a solution in the comments, which is to use Array.from to solve it in one line, and it is quite efficient. So I read through the documents and blogs, and after I figured it out, I thought it was time to start this series. Moreover, arrays are one of the most commonly used data structures in our development. As one of the methods of generating arrays, it makes sense to start with from. Let’s just make do with the name of the series.

Basic syntax

Definition: The from() method is used to pass an object or iterable with the length property Object to return an array.

Syntax: Array.from(object, mapFunction, thisValue)

##ParametersDescriptionRequired, the object to be converted to an array. Optional, the function to be called for each element in the array. Optional, the this object in the mapping function (mapFunction).


object
mapFunction
thisValue

1. Convert class array to array

Array.from(&#39;hello&#39;)                        //["h", "e", "l", "l", "o"]
Array.from(new Set([&#39;name&#39;,&#39;age&#39;]))        //["name", "age"]
Array.from({name:&#39;lgc&#39;,age:25})            //[]
let map=new Map()
map.set(&#39;name&#39;,&#39;lgc&#39;)
map.set(&#39;age&#39;,25)
Array.from(map)                            //[["name", "lgc"],["age", 25]]
function test(){
    console.log(Array.from(arguments))
}
test(1,2,3)                                //[1, 2, 3]复制代码

This is our most commonly used function. When writing these examples, I was still wondering: why map can be converted into an array but object can only be converted into an empty array. When I looked at the novice tutorial and saw the above definition, I understood. Object has neither a length nor an iterable object. I used to think that object was also an iterable object. After all, you can use for-in. But in fact, the object of es6 is not an iterable object. I won’t go into details here. Interested students can check it out.

2. Array deep copy (one line of code)

function clone(arr){
    return Array.isArray(arr) ? Array.from(arr, clone):arr
}
let arrayA=[[1,2],[3,4]]
let arrayB=clone(arrayA)
arrayA===arrayB                                  //false
arrayA[0]===arrayB[0]                            //false复制代码

The second parameter mapFunction of Array.from is mainly used here. By default, mapFunction passes two parameters, array value and subscript.

3. Array deduplication

function unique(arr){
    return Array.from(new Set(arr))
}复制代码

This is also the most basic from and one of our most commonly used functions.

4. Other uses of from

Look at the definition of from again: the from() method is used to pass objects or objects with the

length attribute An iterable object to return an array. Just have length? Give it a try

Array.from({length:2},(val,index)=>index)                        //[0,1]复制代码

Okay, but what’s the use of it? First, like the above code, you can easily generate an array within a certain range and with certain rules

Array.from({length:3},(val,index)=>index*10)                     //[0,10,20]复制代码

Second, the initialization of the array. For example, you want to generate an object array of a specified length. What was the first reaction? fill?

let testArr=Array(3).fill({})
testArr[0]===testArr[1]                                         //true复制代码

Every object here is actually the same. If you modify one, the others will naturally change accordingly, but many times what we need is not like this.

let testArrb=Array.from({length:3},()=>({}))
testArrb[0]===testArrb[1]                                       //false复制代码

These two methods can be used according to needs.

5. Advanced usage of from

The above content is actually layer by layer to better understand the following problem-solving ideas.

LeetCode Question 867:

Given a matrix

A, return the transposed matrix of A.

The transpose of a matrix refers to flipping the main diagonal of the matrix and exchanging the row index and column index of the matrix.

Example 1: Input: [[1,2,3],[4,5,6],[7,8,9]] Output: [[1,4,7],[2, 5,8],[3,6,9]] Example 2: Input: [[1,2,3],[4,5,6]] Output: [[1,4],[2,5], [3,6]]

The first reaction was that this was not an index swap? It's so simple. So I wrote the following code

var transpose = function(A) {
    let x=A.length
    let y=A[0].length
    for(let i=0;i<x;i++){
        for(let j=0;j<y;j++){
            if(j-i>0){
                [A[i][j],A[j][i]]=[A[j][i],A[i][j]]
            }
        }
    }
    return A
};复制代码

Execution code: passed, submission: failed. Damn it? Looking at the error message, I found that the situation in Example 2 where the "length and width" are not equal was ignored. Change your thinking and flip the internal and external cycles. Each time the outermost loop is executed, one column is treated as a row. Execute, pass.

But this version looks too expensive, and the execution time is too slow. But after all, I have implemented it myself, so you can go to the comment area to find other ideas.

The following is implemented by the master in the comment area, but I didn’t understand it the first time.

var transpose = function(A) {    return Array.from({length:A[0].length},(_v,i)=>A.map(v=>v[i]))};复制代码

{length:A[0].length}, uses the width of the given matrix as the length of the transposed matrix, in order to satisfy the condition of "an object with the

length attribute" . (_v,i)=>A.map(v=>v[i]), takes the columns of the given matrix as the rows of the transposed matrix. The core idea is the same as my second version, but the implementation methods and techniques are too much. And the execution time is also shorter, worship the great god.

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