147 lines
3.3 KiB
Markdown
147 lines
3.3 KiB
Markdown
# Python推导式
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## 列表推导式
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列表推导式主要是用于按照我们指定的内容填充，生成一个新的列表，基本语法如下：
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> new_list = [ expression for item in iterable if condition ]
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### 示例
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```python
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# 生成0~10的一个列表
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l = [i for i in range(11)]
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print(l) # 输出: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
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# 生成平方数的列表
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squares = [x ** 2 for x in range(10)]
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print(squares) # 输出: [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
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# 生成偶数的列表
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even_numbers = [x for x in range(20) if x % 2 == 0]
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print(even_numbers) # 输出: [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
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```
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推导式没有特殊之处，只是一种便利的编程方式....
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### 案例
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1. 找到嵌套列表中名字含有两个及以上‘a’的所有名字
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```python
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fruits = [['peach','Lemon','Pear','avocado','cantaloupe','Banana','Grape'],
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['raisins','plum','apricot','nectarine','orange','papaya']]
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print([name for lst in fruits for name in lst if name.count('a') >= 2])
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/
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# Output:
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['avocado', 'cantaloupe', 'Banana', 'papaya']
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```
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2. 30以内所有能被3整除的数
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```python
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multiples = [i for i in range(30) if i % 3 == 0]
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print(multiples)
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```
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## 集合推导式
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集合推导式用于生成一个新的集合。它的语法与列表推导式类似，但使用大括号 `{}`。
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```python
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new_set = {expression for item in iterable if condition}
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```
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### 示例
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```python
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# 生成平方数的集合
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squares_set = {x ** 2 for x in range(10)}
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print(squares_set) # 输出: {0, 1, 4, 9, 16, 25, 36, 49, 64, 81}
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# 生成唯一的字符集合
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unique_chars = {char for char in "hello world"}
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print(unique_chars) # 输出: {'h', 'e', 'l', 'o', ' ', 'r', 'w', 'd'}
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```
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### 案例
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计算列表中每个值的平方，自带去重功能
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```python
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l = [1,2,3,4,1,-1,-2,3]
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squared = {x**2 for x in l}
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print(squared)
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# Output:
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{16, 1, 4, 9}
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```
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## 字典推导式
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同上，我们可以使用字典推导式生成一个新的字典
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```python
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new_dict = {key_expression: value_expression for item in iterable if condition}
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```
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### 示例
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```python
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# 生成平方数的字典
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squares_dict = {x: x ** 2 for x in range(10)}
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print(squares_dict) # 输出: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
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# 生成字符及其 ascii 值的字典
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ascii_dict = {char: ord(char) for char in "abc"}
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print(ascii_dict) # 输出: {'a': 97, 'b': 98, 'c': 99}
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```
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### 案例
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1. 将一个字典的key和value对调
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```python
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dic1 = {'a':1,'b':2}
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dic2 = {dic1[k]: k for k in dic1}
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print(dic2)
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```
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2. 合并大小写对应的value值，将k统一成小写
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```python
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dic1 = {'a':1,'b':2,'y':1, 'A':4,'Y':9}
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dic2 = {k.lower():dic1.get(k.lower(),0) + dic1.get(k.upper(),0) for k in dic1.keys()}
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print(dic2)
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```
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## 生成器表达式
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把列表推导式的的`[]`换成`()`就变成了生成器表达式，相比于列表表达式，生成器表达式不会直接产生值，而是需要我们手动去迭代它
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### 案例
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```python
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gen_expr = (x ** 2 for x in range(5))
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print(gen_expr)
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```
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**手动迭代：**
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```python
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for value in gen_expr:
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print(value)
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```
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# 小练一下
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使用推导式打印出九九乘法口诀表（仅使用一行代码）
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```python
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print('\n'.join(['\t'.join([f'{i}x{j}={i * j}' for i in range(1,j+1)]) for j in range(1, 10)]))
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``` |