Python文档学习笔记（3）--流程控制语句 (1)

while语句

else可选

i = 0
while i <= 3:
    print("循环{}次".format(i))
    i += 1
else:
    print("结束")

if 语句

可以有0个或多个 elif 块,，并且 else 块是可选的

一个if ... elif ... elif ...序列 是对其他语言的switch or case 语句的替代方案。

>>> 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

for 语句

Python 的 for 语句可以按照元素出现的顺序迭代任何序列（列表或字符串）

 

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

如果要在循环内修改正在迭代的序列（例如，复制所选的项目），建议首先创建原序列的拷贝

import time
words = ['cat', 'window', 'defenestrate']
print(id(words))　　#id()查内存地址
b = words[:]
for w in b:  # Loop over a slice copy of the entire list.
    if len(w) > 6:
        words.insert(0, w)
        print(id(b),words)
        time.sleep(1)


[:]浅拷贝后内存地址变了
267616368200
267616357448 ['defenestrate', 'cat', 'window', 'defenestrate']

range() 函数

包含起点不包含终点，可以是负数，第三个参数可以指定一个步进值。

由 range() 返回的对象的行为就像它是一个列表，但事实上它不是。它是一个对象。当您遍历它时， 它会返回所需的序列连续项。但它并不真的生成了列表，从而节省了空间

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

range(5, 10)
   5 through 9

range(0, 10, 3)
   0, 3, 6, 9

range(-10, -100, -30)
  -10, -40, -70

要迭代序列的索引，您可以将range()和len()组合如下：

>>> a = ['Mary', 'had', 'a', 'little', 'lamb']
>>> for i in range(len(a)):
...     print(i, a[i])
...
0 Mary
1 had
2 a
3 little
4 lamb

或者是这样;
enumerate(iterable, start=0)

>>> seasons = ['Spring', 'Summer', 'Fall', 'Winter']
>>> list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]
>>> list(enumerate(seasons, start=1))
[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]

 

break和continue语句，以及循环中else子句

break语句用于跳出最近的 for 或 while 循环。

循环语句可以有一个 else 子句;当（for）循环迭代完整个列表或（while）循环条件变为假，而非由break语句终止时，就会执行这个else语句。

>>> for n in range(2, 10):
...     for x in range(2, n):
...         if n % x == 0:
...             print(n, 'equals', x, '*', n//x)
...             break
...     else:
...         # loop fell through without finding a factor
...         print(n, 'is a prime number')
...
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3

continue语句表示继续下一次迭代：

>>> for num in range(2, 10):
...     if num % 2 == 0:
...         print("Found an even number", num)
...         continue
...     print("Found a number", num)
Found an even number 2
Found a number 3
Found an even number 4
Found a number 5
Found an even number 6
Found a number 7
Found an even number 8
Found a number 9

pass 语句

pass 语句什么也不做。当语法上需要语句但程序不需要动作时，可以使用它

>>> while True:
...     pass  # Busy-wait for keyboard interrupt (Ctrl+C)
...

>>> class MyEmptyClass:
...     pass
...

>>> def initlog(*args):
...     pass   # Remember to implement this!
...