Python 简明教程

Python - Nested Loops

在 Python 中，当你编写一个或多个 loops 在循环语句中称为 nested loop 。主循环被认为是外部循环，而外部循环中的循环称为内部循环。

In Python, when you write one or more loops within a loop statement that is known as a nested loop. The main loop is considered as outer loop and loop(s) inside the outer loop are known as inner loops.

Python 编程语言允许在另一个循环内使用一个循环。循环是重复执行特定指令的代码块。有两种类型的循环，即 for 和 while，我们可使用它们创建嵌套循环。

The Python programming language allows the use of one loop inside another loop. A loop is a code block that executes specific instructions repeatedly. There are two types of loops, namely for and while, using which we can create nested loops.

Python Nested for Loop

带有或多个内部 for 循环的 for loop 称为嵌套 for 循环。for 循环用于循环任何序列（例如列表、元组或字符串）的项目，对序列的每个项目执行相同的操作。

The for loop with one or more inner for loops is called nested for loop. A for loop is used to loop over the items of any sequence, such as a list, tuple or a string and performs the same action on each item of the sequence.

Python Nested for Loop Syntax

Python 编程语言中，Python 嵌套 for 循环语句的语法如下 −

The syntax for a Python nested for loop statement in Python programming language is as follows −

for iterating_var in sequence:
   for iterating_var in sequence:
      statements(s)
   statements(s)

Python Nested for Loop Example

下面的程序使用嵌套 for 循环来访问 month 和 days 列表。

The following program uses a nested for loop to iterate over months and days lists.

months = ["jan", "feb", "mar"]
days = ["sun", "mon", "tue"]

for x in months:
  for y in days:
    print(x, y)

print("Good bye!")

当执行以上代码时，它生成以下结果 −

When the above code is executed, it produces following result −

jan sun
jan mon
jan tue
feb sun
feb mon
feb tue
mar sun
mar mon
mar tue
Good bye!

Python Nested while Loop

具有一个或多个内部 while 循环的 while loop 是嵌套 while 循环。使用 while 循环针对未知次数重复执行代码块，直到指定的布尔表达式变成 TRUE。

The while loop having one or more inner while loops are nested while loop. A while loop is used to repeat a block of code for an unknown number of times until the specified boolean expression becomes TRUE.

Python Nested while Loop Syntax

Python 编程语言中，嵌套 while 循环语句的语法如下 −

The syntax for a nested while loop statement in Python programming language is as follows −

while expression:
   while expression:
      statement(s)
   statement(s)

Python Nested while Loop Example

下面的程序使用嵌套 while 循环在 2 到 100 之间查找质数 −

The following program uses a nested while loop to find the prime numbers from 2 to 100 −

i = 2
while(i < 25):
   j = 2
   while(j <= (i/j)):
      if not(i%j): break
      j = j + 1
   if (j > i/j) : print (i, " is prime")
   i = i + 1

print ("Good bye!")

在执行时，以上代码产生以下结果 −

On executing, the above code produces following result −

2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
Good bye!