98 lines
2.8 KiB
Markdown
98 lines
2.8 KiB
Markdown
---
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title: If Elif Else Statements
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localeTitle: If / Elif / Else语句
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---
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## If / Elif / Else语句
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`if` / `elif` / `else`结构是控制程序流的常用方法，允许您根据某些数据的值执行特定的代码块。如果关键字`if`后面的条件表达式返回为`True` ，则代码块将执行： 请注意，在条件检查之前和之后不使用括号，如同其他语言一样。
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```python
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if True:
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print('If block will execute!')
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```
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```python
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x = 5
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if x > 4:
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print("The condition was true!") #this statement executes
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```
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如果条件为`false` ，您可以选择添加在if代码块之后执行的`else`响应：
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```python
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if not True:
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print('If statement will execute!')
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else:
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print('Else statement will execute!')
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```
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或者你也可以看看这个例子
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```python
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y = 3
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if y > 4:
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print("I won't print!") #this statement does not execute
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else:
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print("The condition wasn't true!") #this statement executes
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```
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_请注意， `else`关键字后面没有条件 - 它执行条件为if代码块未执行的所有情况_
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可以通过在初始`if`语句之后包含一个或多个`elif`来检查多个条件，但只执行一个条件：
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```python
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z = 7
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if z > 8:
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print("I won't print!") #this statement does not execute
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elif z > 5:
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print("I will!") #this statement will execute
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elif z > 6:
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print("I also won't print!") #this statement does not execute
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else:
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print("Neither will I!") #this statement does not execute
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```
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_请注意，只有第一个计算为`true`条件才会执行。即使`z > 6`为`true` ， `if/elif/else`块`if/elif/else`在第一个真实条件之后终止。这意味着只有在else之前没有条件为`true`的情况下才会执行`else` 。_
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我们还可以创建嵌套if用于决策。在此之前请参阅前面的href ='https：//guide.freecodecamp.org/python/code-blocks-and-indentation'target ='\_ blank'rel ='nofollow'>缩进指南。
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让我们举个例子来找既是一个偶数也大于'10'的数字
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```
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python
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x = 34
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if x % 2 == 0: # this is how you create a comment and now, checking for even.
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if x > 10:
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print("This number is even and is greater than 10")
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else:
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print("This number is even, but not greater 10")
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else:
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print ("The number is not even. So point checking further.")
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```
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这只是嵌套if的一个简单示例。请随时在线浏览更多内容。
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虽然上面的示例很简单，但您可以使用[布尔比较](https://guide.freecodecamp.org/python/comparisons)和[布尔运算符](https://guide.freecodecamp.org/python/boolean-operations)创建复杂条件。
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**_内联python if-else语句_**
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我们也可以使用if-else语句内联python函数。以下示例应检查数字是否大于或等于50，如果是，则返回`True`：
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```
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python
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x = 89
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is_greater = True if x >= 50 else False
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print(is_greater)
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```
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输出
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```
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>
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True
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>
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```
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