91 lines
2.1 KiB
Markdown
91 lines
2.1 KiB
Markdown
# 推导式
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## 推导式详细格式
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```plain
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variable = [out_exp_res for out_exp in input_list if out_exp == 2]
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out_exp_res: 列表生成元素表达式，可以是有返回值的函数。
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for out_exp in input_list： 迭代input_list将out_exp传入out_exp_res表达式中。
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if out_exp == 2： 根据条件过滤哪些值可以。
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```
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## 列表推导式
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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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30以内所有能被3整除的数的平方
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```python
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def squared(x):
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return x*x
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multiples = [squared(i) for i in range(30) if i % 3 == 0]
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print(multiples)
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```
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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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## 字典推导式
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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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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,'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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## 集合推导式
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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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```
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## 练习题
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1. 过滤掉长度小于3的字符串列表，并将剩下的转换成大写字母
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2. 求(x,y)其中x是0-5之间的偶数，y是0-5之间的奇数组成的元祖列表
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3. 将1000以内的素数放入一个列表中
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4. 推导式实现九九乘法表
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```python
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print("\n".join(["\t".join([f"{i}x{j}={i*j}" for j in range(1, i+1)]) for i in range(1,10)]))
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# 1x1=1
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# 2x1=2 2x2=4
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# 3x1=3 3x2=6 3x3=9
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# 4x1=4 4x2=8 4x3=12 4x4=16
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# 5x1=5 5x2=10 5x3=15 5x4=20 5x5=25
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# 6x1=6 6x2=12 6x3=18 6x4=24 6x5=30 6x6=36
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# 7x1=7 7x2=14 7x3=21 7x4=28 7x5=35 7x6=42 7x7=49
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# 8x1=8 8x2=16 8x3=24 8x4=32 8x5=40 8x6=48 8x7=56 8x8=64
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# 9x1=9 9x2=18 9x3=27 9x4=36 9x5=45 9x6=54 9x7=63 9x8=72 9x9=81
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```
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