2018-12-27 14:10:05 +08:00
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2018-12-29 13:14:57 +08:00
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## 前言
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循环语句：通过循环语句可以反复的执行一段代码多次。
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2018-12-27 14:10:05 +08:00
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## for循环
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### for循环的结构
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for循环举例：
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```javascript
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for (var i = 1; i <= 100; i++) {
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console.log(i);
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}
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```
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上方代码的解释：
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### for循环遍历
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```javascript
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for (var i = 1; i < 13; i = i + 4) {
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console.log(i);
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}
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```
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上方代码的遍历步骤：
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```
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程序一运行，将执行var i = 1;这条语句， 所以i的值是1。
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然后程序会验证一下i < 13是否满足，1<13是真，所以执行一次循环体（就是大括号里面的语句）。
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执行完循环体之后，会执行i=i+4这条语句，所以i的值，是5。
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程序会会验证一下i < 13是否满足，5<13是真，所以执行一次循环体（就是大括号里面的语句）。
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执行完循环体之后，会执行i=i+4这条语句，所以i的值，是9。
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程序会会验证一下i < 13是否满足，9<13是真，所以执行一次循环体（就是大括号里面的语句）。
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执行完循环体之后，会执行i=i+4这条语句，所以i的值，是13。
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程序会会验证一下i < 13是否满足，13<13是假，所以不执行循环体了，将退出循环。
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最终输出输出结果为：1、5、9
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```
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接下来做几个题目。
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题目1：
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```javascript
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for (var i = 1; i < 10; i = i + 3) {
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i = i + 1;
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console.log(i);
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}
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```
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输出结果：2、6、10
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题目2：
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```javascript
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for (var i = 1; i <= 10; i++) {
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}
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console.log(i);
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```
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输出结果：11
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题目3：
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```javascript
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for(var i = 1; i < 7; i = i + 3){
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}
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console.log(i);
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```
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输出结果：7
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题目4：
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```javascript
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for (var i = 1; i > 0; i++) {
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console.log(i);
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}
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```
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死循环。
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2018-12-29 13:14:57 +08:00
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## while循环语句
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### while循环
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语法：
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```javascript
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while(条件表达式){
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语句...
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}
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```
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执行流程：
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```
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while语句在执行时，先对条件表达式进行求值判断：
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如果值为true，则执行循环体：
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循环体执行完毕以后，继续对表达式进行判断
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如果为true，则继续执行循环体，以此类推
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如果值为false，则终止循环
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```
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**如果有必要的话，我们可以使用 break 来终止循环**。
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### do...while循环
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语法：
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```javascript
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do{
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语句...
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}while(条件表达式)
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```
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执行流程：
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```
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do...while语句在执行时，会先执行循环体：
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循环体执行完毕以后，在对while后的条件表达式进行判断：
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如果结果为true，则继续执行循环体，执行完毕继续判断以此类推
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如果结果为false，则终止循环
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```
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### while循环和 do...while循环的区别
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这两个语句的功能类似，不同的是：
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- while是先判断后执行，而do...while是先执行后判断。
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也就是说，do...while可以保证循环体至少执行一次，而while不能。
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### while循环举例1
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**题目**：假如投资的年利率为5%，试求从1000块增长到5000块，需要花费多少年？
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**代码实现**：
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```html
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<!DOCTYPE html>
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<html lang="">
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<head>
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<meta>
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<meta>
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<meta>
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<title>Document</title>
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</head>
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<body>
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<script>
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/*
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* 假如投资的年利率为5%，试求从1000块增长到5000块，需要花费多少年
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*
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* 1000 1000*1.05
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* 1050 1050*1.05
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*/
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//定义一个变量，表示当前的钱数
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var money = 1000;
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//定义一个计数器
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var count = 0;
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//定义一个while循环来计算每年的钱数
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while (money < 5000) {
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money *= 1.05;
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//使count自增
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count++;
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}
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console.log(money);
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console.log("一共需要" + count + "年");
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</script>
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</body>
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</html>
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```
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打印结果：
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```
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5003.18854203379
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一共需要33年
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```
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另外，你也可以自己算一下，假如投资的年利率为5%，从1000块增长到1万块，需要花费48年：
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```
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10401.269646942128
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一共需要48年
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```
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## 我的公众号
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想学习<font color=#0000ff>**代码之外的技能**</font>？不妨关注我的微信公众号：**千古壹号**（id：`qianguyihao`）。
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扫一扫，你将发现另一个全新的世界，而这将是一场美丽的意外：
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