76 lines
1.4 KiB
Markdown
76 lines
1.4 KiB
Markdown
|
---
|
|||
|
|
id: 56105e7b514f539506016a5e
|
|||
|
|
title: 使用 For 循環反向遍歷數組
|
|||
|
|
challengeType: 1
|
|||
|
|
videoUrl: 'https://scrimba.com/c/c2R6BHa'
|
|||
|
|
forumTopicId: 16808
|
|||
|
|
dashedName: count-backwards-with-a-for-loop
|
|||
|
|
---
|
|||
|
|
|
|||
|
|
# --description--
|
|||
|
|
|
|||
|
|
只要我們定義好合適的條件，for 循環也可以反向遍歷。
|
|||
|
|
|
|||
|
|
爲了讓每次遞減 2，我們需要改變 initialization、condition 和 final-expression。
|
|||
|
|
|
|||
|
|
設置 `i = 10`，並且當 `i > 0` 的時候才繼續循環。 我們使用 `i -= 2` 來讓 `i` 每次循環遞減 2。
|
|||
|
|
|
|||
|
|
```js
|
|||
|
|
var ourArray = [];
|
|||
|
|
for (var i = 10; i > 0; i -= 2) {
|
|||
|
|
ourArray.push(i);
|
|||
|
|
}
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
循環結束後，`ourArray` 的值爲 `[10,8,6,4,2]`。 讓我們改變初始值和最後的表達式，這樣我們就可以按照奇數從後往前兩兩倒着數。
|
|||
|
|
|
|||
|
|
# --instructions--
|
|||
|
|
|
|||
|
|
使用一個 `for`循環，把從 9 到 1 的奇數添加到 `myArray`。
|
|||
|
|
|
|||
|
|
# --hints--
|
|||
|
|
|
|||
|
|
應該使用 `for` 循環。
|
|||
|
|
|
|||
|
|
```js
|
|||
|
|
assert(/for\s*\([^)]+?\)/.test(code));
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
應該使用數組方法 `push`。
|
|||
|
|
|
|||
|
|
```js
|
|||
|
|
assert(code.match(/myArray.push/));
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
`myArray` 應該等於 `[9,7,5,3,1]`。
|
|||
|
|
|
|||
|
|
```js
|
|||
|
|
assert.deepEqual(myArray, [9, 7, 5, 3, 1]);
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
# --seed--
|
|||
|
|
|
|||
|
|
## --after-user-code--
|
|||
|
|
|
|||
|
|
```js
|
|||
|
|
if(typeof myArray !== "undefined"){(function(){return myArray;})();}
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
## --seed-contents--
|
|||
|
|
|
|||
|
|
```js
|
|||
|
|
// Setup
|
|||
|
|
var myArray = [];
|
|||
|
|
|
|||
|
|
// Only change code below this line
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
# --solutions--
|
|||
|
|
|
|||
|
|
```js
|
|||
|
|
var myArray = [];
|
|||
|
|
for (var i = 9; i > 0; i -= 2) {
|
|||
|
|
myArray.push(i);
|
|||
|
|
}
|
|||
|
|
```
|