# Array.list(n)

{% tabs %}
{% tab title="💾 程式" %}
💾 [replit](https://replit.com/@pegasusroe/Arrayintegersn#index.js)

```javascript
// ⭐️ Array.list(n)
Array.list = function(n, {
    start = 0,
    step = 1,
} = {}) {

    // initialize
    let i = 0;                  // integer index
    let value = start;          // array item value
    let arr = Array(n);         // empty array

    while (i < n) {
        arr[i] = value;         // set item value
        value += step;          // next value (increment by `step`)
        i += 1;                 // next index
    }

    return arr;
};
```

💈範例：

```javascript
Array.list(5),                          // [ 0, 1, 2, 3, 4 ]
Array.list(5, { start: 2 }),            // [ 2, 3, 4, 5, 6 ]
Array.list(5, { start: 2, step: 3 }),   // [ 2, 5, 8, 11, 14 ]
Array.list(5, { step: -1 }),            // [ 0, -1, -2, -3, -4 ]
```

{% endtab %}

{% tab title="跑分" %}

| 使用方法                           | 跑分 (ms) |
| ------------------------------ | ------- |
| `Array(n).fill().map()`        | 10      |
| `[ ...Array(n).keys() ].map()` | 26      |
| `Array.from()`                 | 49      |
| `while` loop                   | 3 👑    |

💾 [replit](https://replit.com/@pegasusroe/Arrayn-vs-Arraynfill#index.js)

```javascript
// ⭐️ arr1(n): Array(n).fill().map()
function arr1(n, {
    start = 0
}={}){
    return Array(n)                     // empty array (only set {length: n})
        .fill()                         // ⭐️ set integer properties❗
        .map((_, i) => i + start);      // ⭐️ map over integer properties❗
}

// ⭐️ arr2(n): Array(n).keys()
function arr2(n, {
    start = 0
}={}){
    let arr = [ ...Array(n).keys() ];
    return start == 0 ? arr : 	        // [0, 1, 2, ..., n-1]
        arr.map(i => i + start);        // [start, start+1, ...]
}

// ⭐️ arr3(n): Array.from()
function arr3(n, {
    start=0
}={}){
    // passing map function to Array.from(), 
    // with an object with a `length` property
    return Array.from({length: n}, (_, i) => i + start);
}

// ⭐️ arr4(n): while loop
function arr4(n, {
    start=0
}={}){

    let i = 0;                  // integer index
    let value = start;          // array item value
    let arr = Array(n);         // empty array

    while(i < n){
        arr[i] = value;         // set item value
        value += 1;             // next value (increment by 1)
        i += 1;                 // next index
    }

    return arr;
}

// ⭐️ measure 
function measure(f, n){
    let t1 = new Date().getTime();
    f(n);
    let t2 = new Date().getTime();
    return t2 - t1;
    // log(`${f.name}: ${t2 - t1} ms`);
}


// ⭐️ test k times
let k = 100;

(function(times){

    let m = 100000;                          // array length
    let sum = [0, 0, 0, 0];
    let fs = [arr1, arr2, arr3, arr4];

    let t1 = new Date().getTime();

    for(let _ = 0; _ < times; _++){
        fs.forEach((f, j) => sum[j] += measure(f, m));
    }

    let t2 = new Date().getTime();
    log(`total time elapsed: ${(t2 - t1)/1000} sec`);

    let avgs = sum.map(x => x/times);
    log(avgs);
    
})(k);

// total time elapsed: 8.676 sec
//
//   arr1   arr2   arr3  arr4⭐️
// ----------------------------
// [ 9.59, 28.67, 49.25, 3.39 ]
// [ 11.1, 24.41, 48.46, 2.76 ]
```

{% endtab %}

{% tab title="📘 手冊" %}

* [array.fill()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fill#using_fill_to_populate_an_empty_array) - 沒填寫參數，表示 filled with `undefined`&#x20;
* [Array()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Array)
  {% endtab %}

{% tab title="👥 相關" %}

* [array.from](https://lochiwei.gitbook.io/web/js/val/builtin/arr/static-methods/array.from "mention")
* [fill](https://lochiwei.gitbook.io/web/js/val/builtin/arr/static-methods/fill "mention")
  {% endtab %}
  {% endtabs %}


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