【python基础】6-控制结构

2018-02-27 11:14:25来源:https://www.jianshu.com/p/968e00a326e5作者:王诗翔人点击

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条件检查
if
for
while
continue和break



条件检查
单个和组合测试
>>> num = 5
>>> num > 2
True
>>> num > 3 and num <= 5
True
>>> 3 < num <= 5
True
>>> num % 3 == 0 or num % 5 == 0
True
>>> fav_fiction = 'Harry Potter'
>>> fav_detective = 'Sherlock Holmes'
>>> fav_fiction == fav_detective
False
>>> fav_fiction == "Harry Potter"
True

测试变量和值本身
>>> bool(num)
True
>>> bool(fav_detective)
True
>>> bool(3)
True
>>> bool(0)
False
>>> bool("")
False
>>> bool(None)
False
>>> if -1:
... print("-1 evaluates to True in condition checking")
...
-1 evaluates to True in condition checking

条件测试中in操作符的使用

对比这种检查方式


>>> def num_chk(n):
... if n == 10 or n == 21 or n == 33:
... print("Number passes condition")
... else:
... print("Number fails condition")
...
>>> num_chk(10)
Number passes condition
>>> num_chk(12)
Number fails condition

和另一种


>>> def num_chk(n):
... if n in (10, 21, 33):
... print("Number passes condition")
... else:
... print("Number fails condition")
...
>>> num_chk(12)
Number fails condition
>>> num_chk(10)
Number passes condition


(10, 21, 33)是一个元组数据类型,会在后面的章节讲解
Python文档 - 真值检验



if
#!/usr/bin/python3
num = 45
# 单个if
if num > 25:
print("Hurray! {} is greater than 25".format(num))
# if-else
if num % 2 == 0:
print("{} is an even number".format(num))
else:
print("{} is an odd number".format(num))
# if-elif-else
# 可以使用任意数目的elif
if num < 0:
print("{} is a negative number".format(num))
elif num > 0:
print("{} is a positive number".format(num))
else:
print("{} is neither postive nor a negative number".format(num))

函数代码块、控制结构等等都是通过缩进区分
推荐使用4个空格缩进
Python文档 - 编码风格

一个常见的语法错误是忘记了控制结构语句后的:
条件周围的()是可选的
缩进代码块可以有任意数目的语句,包括空行
$ ./if_elif_else.py
Hurray! 45 is greater than 25
45 is an odd number
45 is a positive number

if-else作为条件操作符


#!/usr/bin/python3
num = 42
num_type = 'even' if num % 2 == 0 else 'odd'
print("{} is an {} number".format(num, num_type))

不像其他许多语言,Python没有?:条件操作符
单行if-else的使用是一种变通方法
模拟三元操作符的更多方法
$ ./if_else_oneliner.py
42 is an even number



for
#!/usr/bin/python3
number = 9
for i in range(1, 5):
mul_table = number * i
print("{} * {} = {}".format(number, i, mul_table))

传统基于循环的迭代可以通过使用range函数实现
默认参数start=0step=1,不含stop

针对列表、元组等等变量的迭代会在后续章节讲述
Python文档 - 迭代工具
$ ./for_loop.py
9 * 1 = 9
9 * 2 = 18
9 * 3 = 27
9 * 4 = 36



while
#!/usr/bin/python3
# 持续地询问直到用户输入一个正整数
usr_string = 'not a number'
while not usr_string.isnumeric():
usr_string = input("Enter a positive integer: ")

while循环允许我们直到某个条件被满足之前不断执行语句块
Python docs - 字符串方法
$ ./while_loop.py
Enter a positive integer: abc
Enter a positive integer: 1.2
Enter a positive integer: 23
$



continue和break

continuebreak关键字用于在某些条件下改变正常的循环操作


continue - 跳过循环代码块余下的语句并进入下一次迭代


#!/usr/bin/python3
prev_num = 0
curr_num = 0
print("The first ten numbers in fibonacci sequence: ", end='')
for num in range(10):
print(curr_num, end=' ')
if num == 0:
curr_num = 1
continue
temp = curr_num
curr_num = curr_num + prev_num
prev_num = temp
print("")


continue放置在循环代码块中的任意位置而不用担心复杂的代码流
这个例子仅仅展示continue的使用,查看这里获取更加Python化的操作方式
$ ./loop_with_continue.py
The first ten numbers in fibonacci sequence: 0 1 1 2 3 5 8 13 21 34

break - 跳过循环代码块余下的语句(如果有)并进入退出循环代码块


#!/usr/bin/python3
import random
while True:
# 使用range函数注意500没有包含在内
random_int = random.randrange(500)
if random_int % 4 == 0 and random_int % 6 == 0:
break
print("Random number divisible by 4 and 6: {}".format(random_int))


while True:是常用作无限循环
randrange和range函数相似,有start, stop, step参数
Python文档 - random
$ ./loop_with_break.py
Random number divisible by 4 and 6: 168
$ ./loop_with_break.py
Random number divisible by 4 and 6: 216
$ ./loop_with_break.py
Random number divisible by 4 and 6: 24

这个while_loop.py例子可以用break语句重写


>>> while True:
usr_string = input("Enter a positive integer: ")
if usr_string.isnumeric():
break
Enter a positive integer: a
Enter a positive integer: 3.14
Enter a positive integer: 1
>>>

在嵌套循环中,continuebreak仅影响中间一层对应的循环
Python文档 - 循环中的else从句








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