python note 2 - bubble sort

foreword

During interviews, interviewers often like to ask how to perform bubble sort? This question is believed to be stumbling to a group of heroes. This article will explain in detail how to use python to perform bubble sorting.

1. Basic Principles

1. Concept:

Bubble Sort is a relatively simple sorting algorithm in the computer field.

It repeatedly walks through the sequence to be sorted, comparing two elements at a time, and swapping them if they are in the wrong order. The work of visiting the sequence is repeated until no more exchanges are needed, that is, the sequence has been sorted.

The name of this algorithm comes from the fact that larger elements will slowly "float" to the top of the sequence by swapping, hence the name.

2. Algorithm principle:

The bubble sort algorithm works as follows: (from back to front)

> Compare adjacent elements. If the first is bigger than the second, swap the two of them .

> Do the same for each pair of adjacent elements, from the first pair at the beginning to the last pair at the end. At this point, the last element should be the largest number.

>Repeat the above steps for all elements except the last one.

> Continue to repeat the above steps for fewer and fewer elements each time, until there are no pairs of numbers to compare.

 

2. Swap two numbers

1. If a = 10, b = 20, how to exchange two numbers?

2. The principle of realizing the exchange of two numbers is actually very simple, just set a temporary variable c:

> Pass the value of a to c first, then the value of c is 10, and the value of a is still 10

> Then pass the value of b to a, at this time the value of a is 20, and the value of b is still 20

> Finally pass the value of c to b, at this time the value of b is 10

3. It is not so complicated to exchange two numbers in python, just use this method:

>a , b = b,  a

 

3. Traverse and compare adjacent numbers

1. For example, a queue is: [1, 3, 10, 9, 21, 35, 4, 6]

2. You can do a traversal, compare the adjacent numbers, and exchange if the previous number is larger than the latter number, so that the first traversal can sink the largest number to the last position

3. For ease of understanding, the exchanged places are circled in red

 

4. Cyclic sinking

1. The above traversal comparison has only done one sinking, and the largest number is sinking to the last position, then the second largest number needs to sink to the penultimate position,

Loop through until the smallest number is on top.

2. Here s is to first calculate the length of the li queue, and then reverse the range function to become: [7, 6, 5, 4, 3, 2, 1, 0]

 

Five, sort () sorting

1. To tell the truth, sorting in python doesn’t have to be so troublesome, a function can do it: sort()

 

6. Reference code

# coding:utf-8
li = [1, 3, 10, 9, 21, 35, 4, 6]

s = range (len (li)) [:: - 1 ]
 print s

for i in s:
    for j in range(i):
        if li[j] > li[j + 1]:
            li[j], li[j + 1] = li[j + 1 ], li[j]

#sort function # 
li.sort ()

print li