零基础学Python:流程控制及if语句
流程控制
控制流程(也称为流程控制)是计算机运算领域的用语,意指在程序运行时,个别的指令(或是陈述、子程序)运行或求值的顺序。
![](https://upload-images.jianshu.io/upload_images/25631092-eb45b91f20a2824c.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-f7e30d1156fd2736.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
不管是多么复杂的程序,都是由这些很多简单的流程控制构建起来的。
1.if 语句
if 语句是最简单的选择结构。如果满足条件就执行设定好的操作,不满足条件就执行其他其他操作。
判断的定义
![](https://upload-images.jianshu.io/upload_images/25631092-63952d532324a14a.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
判断语句 又被称为 “分支语句”
![](https://upload-images.jianshu.io/upload_images/25631092-cc35d3b0b772cd8e.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-99deaa1ce51b3b68.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
注意:
- if 和 else 语句以及各自的缩进部分共同是一个 完整的代码块
![](https://upload-images.jianshu.io/upload_images/25631092-b34c4711739c0068.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-ca6612f570488fab.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
多条件
![](https://upload-images.jianshu.io/upload_images/25631092-b1372b18272e9f5b.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-f36e021df52b4d9c.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-ccd72d884d6ce844.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-ce7fb1200a6fc662.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
注意
![](https://upload-images.jianshu.io/upload_images/25631092-ff58638a19a85c44.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
elif 的应用场景是:同时 判断 多个条件,所有的条件是 平级 的
![](https://upload-images.jianshu.io/upload_images/25631092-1f01c6b46200a617.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-f7e9394b12d96177.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
if 的嵌套
![](https://upload-images.jianshu.io/upload_images/25631092-dba21168d43a397c.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
总结
if 的三种形式
![](https://upload-images.jianshu.io/upload_images/25631092-c8d5cf864590da8e.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-7e2a404d065c69bd.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
![](https://upload-images.jianshu.io/upload_images/25631092-d21fb83681840d99.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)
if嵌套
![](https://upload-images.jianshu.io/upload_images/25631092-455e678c6d3ddb47.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)