293 lines
14 KiB
Markdown
293 lines
14 KiB
Markdown
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---
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title: Caesars Cipher
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localeTitle: 凯撒密码
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---
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![:triangular_flag_on_post:](https://forum.freecodecamp.com/images/emoji/emoji_one/triangular_flag_on_post.png?v=3 ":triangular_flag_on_post:")如果卡住,请记得使用**`Read-Search-Ask`** 。尝试配对程序![:busts_in_silhouette:](https://forum.freecodecamp.com/images/emoji/emoji_one/busts_in_silhouette.png?v=3 ":busts_in_silhouette:")并编写自己的代码![:pencil:](https://forum.freecodecamp.com/images/emoji/emoji_one/pencil.png?v=3 ":铅笔:")
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### ![:checkered_flag:](https://forum.freecodecamp.com/images/emoji/emoji_one/checkered_flag.png?v=3 ":checkered_flag:")问题说明:
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* 您需要编写一个函数,该函数将使用_Caesar密码_编码的字符串作为参数并对其进行解码。
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* 这里使用的是ROT13,其中字母的值移动了13位。例如'A' ![:left_right_arrow:](https://forum.freecodecamp.com/images/emoji/emoji_one/left_right_arrow.png?v=3 ":left_right_arrow:") 'N','T' ![:left_right_arrow:](https://forum.freecodecamp.com/images/emoji/emoji_one/left_right_arrow.png?v=3 ":left_right_arrow:") 'G'。
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* 你必须将它移回13个位置,这样'N' ![:left_right_arrow:](https://forum.freecodecamp.com/images/emoji/emoji_one/left_right_arrow.png?v=3 ":left_right_arrow:") '一个'。
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#### 相关链接
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* [String.prototype.charCodeAt](http://forum.freecodecamp.com/t/javascript-string-prototype-charcodeat/15933)
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* 使用String.fromCharCode
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## ![:speech_balloon:](https://forum.freecodecamp.com/images/emoji/emoji_one/speech_balloon.png?v=3 ":speech_balloon:")提示:1
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使用_String.charCodeAt()_将英文字符转换为ASCII。
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> _现在尝试解决问题_
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## ![:speech_balloon:](https://forum.freecodecamp.com/images/emoji/emoji_one/speech_balloon.png?v=3 ":speech_balloon:")提示:2
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使用_String.fromCharCode()_将ASCII转换为英文字符。
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> _现在尝试解决问题_
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## ![:speech_balloon:](https://forum.freecodecamp.com/images/emoji/emoji_one/speech_balloon.png?v=3 ":speech_balloon:")提示:3
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保留AZ之间不存在的任何东西。
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> _现在尝试解决问题_
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## 扰流警报!
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![警告牌](//discourse-user-assets.s3.amazonaws.com/original/2X/2/2d6c412a50797771301e7ceabd554cef4edcd74d.gif)
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**提前解决!**
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## ![:beginner:](https://forum.freecodecamp.com/images/emoji/emoji_one/beginner.png?v=3 ":初学者:")基本代码解决方案
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```javascript
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function rot13(str) {
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// Split str into a character array
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return str.split('')
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// Iterate over each character in the array
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.map.call(str, function(char) {
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// Convert char to a character code
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x = char.charCodeAt(0);
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// Checks if character lies between AZ
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if (x < 65 || x > 90) {
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return String.fromCharCode(x); // Return un-converted character
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}
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//N = ASCII 78, if the character code is less than 78, shift forward 13 places
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else if (x < 78) {
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return String.fromCharCode(x + 13);
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}
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// Otherwise shift the character 13 places backward
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return String.fromCharCode(x - 13);
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}).join(''); // Rejoin the array into a string
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}
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```
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![:rocket:](https://forum.freecodecamp.com/images/emoji/emoji_one/rocket.png?v=3 ":火箭:") [运行代码](https://repl.it/CLjU/38)
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### 代码说明:
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* 声明并初始化字符串变量`nstr`以存储已解码的字符串。
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* for循环用于循环输入字符串的每个字符。
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* 如果角色不是大写的英文字母(即它的ascii不在65和91之间),我们将保持原样并[继续](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/continue)下一次迭代。
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* 如果它是大写的英文字母,我们将从它的ascii代码中减去13。
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* 如果ascii代码小于78,当它减去13时它将超出范围,所以我们将为它添加26(英文字母中的字母数),以便在A之后它将返回Z.例如M (77) ![:left_right_arrow:](https://forum.freecodecamp.com/images/emoji/emoji_one/left_right_arrow.png?v=3 ":left_right_arrow:") 77-13 = 64(不是英文字母)+26 = 90 ![:left_right_arrow:](https://forum.freecodecamp.com/images/emoji/emoji_one/left_right_arrow.png?v=3 ":left_right_arrow:") Z(90)。
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#### 相关链接
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* [Array.prototype.map](http://forum.freecodecamp.com/t/javascript-array-prototype-map/14294)
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* [String.prototype.split](http://forum.freecodecamp.com/t/javascript-string-prototype-split/15944)
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* [Array.prototype.join](http://forum.freecodecamp.com/t/javascript-array-prototype-join/14292)
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## ![:sunflower:](https://forum.freecodecamp.com/images/emoji/emoji_one/sunflower.png?v=3 ":向日葵:")中级代码解决方案:
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```javascript
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// Solution with Regular expression and Array of ASCII character codes
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function rot13(str) {
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var rotCharArray = [];
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var regEx = /[AZ]/ ;
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str = str.split("");
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for (var x in str) {
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if (regEx.test(str[x])) {
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// A more general approach
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// possible because of modular arithmetic
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// and cyclic nature of rot13 transform
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rotCharArray.push((str[x].charCodeAt() - 65 + 13) % 26 + 65);
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} else {
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rotCharArray.push(str[x].charCodeAt());
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}
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}
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str = String.fromCharCode.apply(String, rotCharArray);
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return str;
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}
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// Change the inputs below to test
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rot13("LBH QVQ VG!");
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```
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### 代码说明:
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* 在名为`rotCharArray`的变量中创建一个空数组来存储字符代码。
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* `regEx`变量存储从A到Z的所有大写字母的正则表达式。
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* 我们将`str`拆分为一个字符数组,然后使用for循环遍历数组中的每个字符。
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* 使用if语句,我们测试以查看字符串是否仅包含从A到Z的大写字母。
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* 如果返回true,我们使用`charCodeAt()`函数和rot13转换返回正确的值,否则返回初始值。
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* 然后,我们使用`rotCharArray`变量中的字符代码返回字符串。
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### 算法说明:
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```
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ALPHA KEY BASE ROTATED ROT13
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-------------------------------------------------------------
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[A] 65 <=> 0 + 13 => 13 % 26 <=> 13 + 65 = 78 [N]
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[B] 66 <=> 1 + 13 => 14 % 26 <=> 14 + 65 = 79 [O]
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[C] 67 <=> 2 + 13 => 15 % 26 <=> 15 + 65 = 80 [P]
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[D] 68 <=> 3 + 13 => 16 % 26 <=> 16 + 65 = 81 [Q]
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[E] 69 <=> 4 + 13 => 17 % 26 <=> 17 + 65 = 82 [R]
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[F] 70 <=> 5 + 13 => 18 % 26 <=> 18 + 65 = 83 [S]
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[G] 71 <=> 6 + 13 => 19 % 26 <=> 19 + 65 = 84 [T]
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[H] 72 <=> 7 + 13 => 20 % 26 <=> 20 + 65 = 85 [U]
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[I] 73 <=> 8 + 13 => 21 % 26 <=> 21 + 65 = 86 [V]
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[J] 74 <=> 9 + 13 => 22 % 26 <=> 22 + 65 = 87 [W]
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[K] 75 <=> 10 + 13 => 23 % 26 <=> 23 + 65 = 88 [X]
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[L] 76 <=> 11 + 13 => 24 % 26 <=> 24 + 65 = 89 [Y]
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[M] 77 <=> 12 + 13 => 25 % 26 <=> 25 + 65 = 90 [Z]
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[N] 78 <=> 13 + 13 => 26 % 26 <=> 0 + 65 = 65 [A]
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[O] 79 <=> 14 + 13 => 27 % 26 <=> 1 + 65 = 66 [B]
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[P] 80 <=> 15 + 13 => 28 % 26 <=> 2 + 65 = 67 [C]
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[Q] 81 <=> 16 + 13 => 29 % 26 <=> 3 + 65 = 68 [D]
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[R] 82 <=> 17 + 13 => 30 % 26 <=> 4 + 65 = 69 [E]
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[S] 83 <=> 18 + 13 => 31 % 26 <=> 5 + 65 = 70 [F]
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[T] 84 <=> 19 + 13 => 32 % 26 <=> 6 + 65 = 71 [G]
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[U] 85 <=> 20 + 13 => 33 % 26 <=> 7 + 65 = 72 [H]
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[V] 86 <=> 21 + 13 => 34 % 26 <=> 8 + 65 = 73 [I]
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[W] 87 <=> 22 + 13 => 35 % 26 <=> 9 + 65 = 74 [J]
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[X] 88 <=> 23 + 13 => 36 % 26 <=> 10 + 65 = 75 [K]
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[Y] 89 <=> 24 + 13 => 37 % 26 <=> 11 + 65 = 76 [L]
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[Z] 90 <=> 25 + 13 => 38 % 26 <=> 12 + 65 = 77 [M]
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```
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#### 相关链接
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* [Function.apply](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply)
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* [正则表达式](https://forum.freecodecamp.com/t/regular-expressions-resources/15931)
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* [Regex.test](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/test)
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![:rocket:](https://forum.freecodecamp.com/images/emoji/emoji_one/rocket.png?v=3 ":火箭:") [运行代码](https://repl.it/CLjU/39)
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## ![:rotating_light:](https://forum.freecodecamp.com/images/emoji/emoji_one/rotating_light.png?v=3 ":rotating_light:")高级代码解决方案
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```
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function rot13(str) { // LBH QVQ VG!
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return str.replace(/[AZ]/g, L => String.fromCharCode((L.charCodeAt(0) % 26) + 65));
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}
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```
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### 算法说明:
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理解在JavaScript中象征性地表示为`%`模运算符( _有时称为模运算符_ )是理解算法的关键。
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这是一个有趣的运算符,它出现在工程的各个地方,例如密码学。
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基本上,对数字进行操作,它将数字除以给定的除数,并给出除法的余数。
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例如,
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* `0 % 5 = 0`因为`0 / 5 = 0`且余数为`0` 。
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* `2 % 5 = 2`因为`2 / 5 = 0` ,余数为`2`
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* `4 % 5 = 4`因为`4 / 5 = 0` ,余数为`4`
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* `5 % 5 = 0`因为`5 / 5 = 1`且余数为`0`
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* `7 % 5 = 2`因为`7 / 5 = 1` ,余数为`2`
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* `9 % 5 = 4`因为`9 / 5 = 1`而余数为`4`
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* `10 % 5 = 0`因为`10 / 5 = 2`且余数为`0`
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但你必须注意到这里的模式。
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您可能已经注意到,当它刚刚达到RHS值的倍数时,惊人的模运算符会覆盖LHS值。
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例如,在我们的例子中,当`LHS = 5` ,它被包裹到`0`
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要么
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当`LHS = 10` ,它再次`LHS = 10` `0` 。
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因此,我们看到出现以下模式
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```
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0 ⇔ 0
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1 ⇔ 1
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2 ⇔ 2
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3 ⇔ 3
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4 ⇔ 4
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5 ⇔ 0
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6 ⇔ 1
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7 ⇔ 2
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8 ⇔ 3
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9 ⇔ 4
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10 ⇔ 0
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```
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因此,我们得出结论,使用模运算符,可以将一系列值映射到\[ `0`到`DIVISOR - 1` \]之间的范围。在我们的情况下,我们映射\[ `5 - 9`之间\] \[ `0 - 4` \]或映射\[ `6 - 10` \]之间\[ `0 - 4` \]。
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你知道吗?
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现在让我们考虑映射`26`数字的范围,即在\[ `65 - 90` \]之间,它表示[Unicode字符集中的](http://unicode-table.com/en/alphabets/)大写\[ **英文字母** \]到\[ `0 - 25` \]之间的数字范围。
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```
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[A] 65 % 26 ⇔ 13
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[B] 66 % 26 ⇔ 14
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[C] 67 % 26 ⇔ 15
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[D] 68 % 26 ⇔ 16
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[E] 69 % 26 ⇔ 17
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[F] 70 % 26 ⇔ 18
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[G] 71 % 26 ⇔ 19
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[H] 72 % 26 ⇔ 20
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[I] 73 % 26 ⇔ 21
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[J] 74 % 26 ⇔ 22
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[K] 75 % 26 ⇔ 23
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[L] 76 % 26 ⇔ 24
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[M] 77 % 26 ⇔ 25
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[N] 78 % 26 ⇔ 0
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[O] 79 % 26 ⇔ 1
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[P] 80 % 26 ⇔ 2
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[Q] 81 % 26 ⇔ 3
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[R] 82 % 26 ⇔ 4
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[S] 83 % 26 ⇔ 5
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[T] 84 % 26 ⇔ 6
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[U] 85 % 26 ⇔ 7
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[V] 86 % 26 ⇔ 8
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[W] 87 % 26 ⇔ 9
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[X] 88 % 26 ⇔ 10
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[Y] 89 % 26 ⇔ 11
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[Z] 90 % 26 ⇔ 12
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```
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您可以注意到,\[ `65 - 90` \]范围内的每个数字都映射到\[ `0 - 25` \]之间的唯一数字。
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您可能还注意到,每个给定数字(例如`65` )映射到另一个数字(例如`13` ),该数字可用作偏移值(即`65 + OFFSET` )以获得给定数字的ROT13。
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例如`65`映射到`13` ,可以作为偏移值,并添加到`65` ,得到`78` 。
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```
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[A] 65 % 26 ⇔ 13 + 65 = 78 [N]
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[B] 66 % 26 ⇔ 14 + 65 = 79 [O]
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[C] 67 % 26 ⇔ 15 + 65 = 80 [P]
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[D] 68 % 26 ⇔ 16 + 65 = 81 [Q]
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[E] 69 % 26 ⇔ 17 + 65 = 82 [R]
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[F] 70 % 26 ⇔ 18 + 65 = 83 [S]
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[G] 71 % 26 ⇔ 19 + 65 = 84 [T]
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[H] 72 % 26 ⇔ 20 + 65 = 85 [U]
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[I] 73 % 26 ⇔ 21 + 65 = 86 [V]
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[J] 74 % 26 ⇔ 22 + 65 = 87 [W]
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[K] 75 % 26 ⇔ 23 + 65 = 88 [X]
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[L] 76 % 26 ⇔ 24 + 65 = 89 [Y]
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[M] 77 % 26 ⇔ 25 + 65 = 90 [Z]
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[N] 78 % 26 ⇔ 0 + 65 = 65 [A]
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[O] 79 % 26 ⇔ 1 + 65 = 66 [B]
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[P] 80 % 26 ⇔ 2 + 65 = 67 [C]
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[Q] 81 % 26 ⇔ 3 + 65 = 68 [D]
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[R] 82 % 26 ⇔ 4 + 65 = 69 [E]
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[S] 83 % 26 ⇔ 5 + 65 = 70 [F]
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[T] 84 % 26 ⇔ 6 + 65 = 71 [G]
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[U] 85 % 26 ⇔ 7 + 65 = 72 [H]
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[V] 86 % 26 ⇔ 8 + 65 = 73 [I]
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[W] 87 % 26 ⇔ 9 + 65 = 74 [J]
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[X] 88 % 26 ⇔ 10 + 65 = 75 [K]
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[Y] 89 % 26 ⇔ 11 + 65 = 76 [L]
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[Z] 90 % 26 ⇔ 12 + 65 = 77 [M]
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```
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### 代码说明:
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* `String.prototype.replace` [函数](http://forum.freecodecamp.com/t/javascript-string-prototype-replace/15942)允许您基于某些模式匹配(由正则表达式定义)和[转换函数](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace#Specifying_a_function_as_a_parameter) (应用于每个模式匹配)来转换`String` 。
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* [箭头函数](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions)语法用于将函数参数写入`replace()` 。
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* `L`表示单个单位,从每个模式匹配`/[AZ]/g` - 字母表中的每个大写字母,从`A`到`Z` ,存在于字符串中。
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* 箭头函数对给定字符串中存在的英文字母的每个大写字母应用`rot13`变换。
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#### 相关链接
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* [String.prototype.replace()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace)
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## ![:clipboard:](https://forum.freecodecamp.com/images/emoji/emoji_one/clipboard.png?v=3 ":剪贴板:")对贡献者的说明:
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* ![:warning:](https://forum.freecodecamp.com/images/emoji/emoji_one/warning.png?v=3 ":警告:") **请勿**添加与任何现有解决方案类似的解决方案。如果您认为它**_相似但更好_** ,那么尝试合并(或替换)现有的类似解决方案。
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* 添加解决方案的说明。
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* 将解决方案分为以下类别之一 - **基本** , **中级**和**高级** 。 ![:traffic_light:](https://forum.freecodecamp.com/images/emoji/emoji_one/traffic_light.png?v=3 ":红绿灯:")
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* 如果您添加了任何**相关的主要内容,**请仅添加您的用户名。 ( ![:warning:](https://forum.freecodecamp.com/images/emoji/emoji_one/warning.png?v=3 ":警告:") **_不要_** _删除任何现有的用户名_ )
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> 看到![:point_right:](https://forum.freecodecamp.com/images/emoji/emoji_one/point_right.png?v=3 ":point_right:") [**`Wiki Challenge Solution Template`**](http://forum.freecodecamp.com/t/algorithm-article-template/14272)供参考。
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