3.3 Python 基础之条件和循环

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• if 语句和三元运算符
• while 语句
• for 语句
• range() 函数
• break, continue
• pass

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if 语句和三元运算符

If 语句

>>> x = int(input("Please enter an integer: "))
Please enter an integer: 42
>>> if x < 0:
...     x = 0
...     print('Negative changed to zero')
... elif x == 0:
...     print('Zero')
... elif x == 1:
...     print('Single')
... else:
...     print('More')
...
More

三元运算符

方式一：if 条件表达式

condition = True
print(2 if condition else 1)

方式二：通过使用元组来实现（不推荐使用，因为两个 statement 都会执行）

condition = True
print((1, 2)[condition])

参考

• 三元运算符 - Python 进阶

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While 语句

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for 语句

遍历数组

>>> # Measure some strings:
... words = ['cat', 'window', 'defenestrate']
>>> for w in words:
...     print(w, len(w))
...
cat 3
window 6
defenestrate 12

遍历 dict

for key in d:
    print(key)

for key, value in d.items():
    print(key, value)

range 函数

>>> for i in range(5):
...     print(i)
...
0
1
2
3
4

One more thing

当需要修改正在遍历的集合时，最好先做拷贝：

>>> for w in words[:]:  # Loop over a slice copy of the entire list.
...     if len(w) > 6:
...         words.insert(0, w)
...
>>> words
['defenestrate', 'cat', 'window', 'defenestrate']

参考：

• Iterating over dictionaries using 'for' loops