Python nested loops

The Python language allows you to embed another loop inside a loop body.

Python for loop nested syntax:

for iterating_var in sequence:

for iterating_var in sequence:

statements(s)

statements(s)

Python while loop nesting Syntax:

while expression:

while expression:

statement(s)

statement(s)

You can embed other loop bodies within the loop body, such as in a while loop in You can embed a for loop, and conversely, you can embed a while loop within a for loop.

Example:

1. Take out one at a time from the first list, and take out one at a time from the second list, and combine them into a new list. The new list contains all combinations

List1 = ['zi', 'qiang', 'xue', 'tang']

List2 = [1, 2]

new_list = []

for m in List1:

for n in List2:

                                                                          ; , 2] , ['qiang', 1], ['qiang', 2], ['xue', 1], ['xue', 2], ['tang', 1], ['tang', 2]]

2. Take out two at a time from a list and find all the combinations

List = [1, 2, 3, 4, 5]

length = len(List )

for x in range(0, length-1):

for y in range(x+1, length):

print List[x], List[y]

result :

1 2

1 3

1 4

1 5

2 3

2 4

2 5

3 4

3 5

4 5

3. The following example uses a nested loop to output prime numbers between 2 and 20:

Analysis: For a number n, if the numbers from 1 to n ** 0.5 (root n) are all divisible, Then the number n is a prime number.

3.1 Use for to implement

# -*- coding: utf-8 -*-

n = 20

for i in range(1, n):

for j in range(2, int(i**0.5)):

if i % j == 0:

break

else:

print i, 'is a prime number'

3.2 Use while to Implementation

#!/usr/bin/python

# -*- coding: utf-8 -*-

i = 2

while(i < 20):

j = 2

while(j <= (i/j)):

if not(i%j): # Or write if i % j == 0, if it means

break

j = j + 1

if (j > i/j):

print i, "is a prime number"

i = i + 1

print "Good bye!"

Explanation: i % j means the remainder after i is divided by j. For numbers, the Boolean value corresponding to 0 is false, and other values ​​are true. not (i % j) means , and the condition can only be established when i % j is 0, which means if it can be divided integers.

The output result of the above example:

2 is a prime number

3 is a prime number

5 is a prime number

7 is a prime number

11 is a prime number

13 is a prime number

17 It’s a prime number

Good bye !