Python循环技巧

Python 编程提供两种循环：for 循环和while 循环。结合break 和 continue 等循环控制语句，我们可以创建各种形式的循环。

无限循环

我们可以使用 while 语句创建无限循环。如果 while 循环的条件始终为 True，就会得到一个无限循环。

示例 #1：使用 while 的无限循环

# An example of infinite loop
# press Ctrl + c to exit from the loop

while True:
   num = int(input("Enter an integer: "))
   print("The double of",num,"is",2 * num)

输出

Enter an integer: 3
The double of 3 is 6
Enter an integer: 5
The double of 5 is 10
Enter an integer: 6
The double of 6 is 12
Enter an integer:
Traceback (most recent call last):

条件在顶部的循环

这是一个没有 break 语句的普通 while 循环。while 循环的条件在顶部，当条件为 False 时，循环终止。

条件在顶部的循环流程图

示例 #2：条件在顶部的循环

# Program to illustrate a loop with the condition at the top

# Try different numbers
n = 10

# Uncomment to get user input
#n = int(input("Enter n: "))

# initialize sum and counter
sum = 0
i = 1

while i <= n:
   sum = sum + i
   i = i+1    # update counter

# print the sum
print("The sum is",sum)

运行程序后，输出将是

The sum is 55

条件在中间的循环

这种循环可以通过无限循环以及循环体中间的条件 break 来实现。

条件在中间的循环流程图

示例 #3：条件在中间的循环

# Program to illustrate a loop with condition in the middle. 
# Take input from the user until a vowel is entered

vowels = "aeiouAEIOU"

# infinite loop
while True:
   v = input("Enter a vowel: ")
   # condition in the middle
   if v in vowels:
       break
   print("That is not a vowel. Try again!")

print("Thank you!")

输出

Enter a vowel: r
That is not a vowel. Try again!
Enter a vowel: 6
That is not a vowel. Try again!
Enter a vowel: ,
That is not a vowel. Try again!
Enter a vowel: u
Thank you!

条件在底部的循环

这种循环确保循环体至少执行一次。它可以通过无限循环以及末尾的条件 break 来实现。这类似于 C 语言中的 do...while 循环。

条件在底部的循环流程图

示例 #4：条件在底部的循环

# Python program to illustrate a loop with the condition at the bottom
# Roll a dice until the user chooses to exit

# import random module
import random

while True:
   input("Press enter to roll the dice")

   # get a number between 1 to 6
   num = random.randint(1,6)
   print("You got",num)
   option = input("Roll again?(y/n) ")

   # condition
   if option == 'n':
       break

输出

Press enter to roll the dice
You got 1
Roll again?(y/n) y
Press enter to roll the dice
You got 5
Roll again?(y/n) n