A brief discussion on sorting in Python

A brief talk about sorting

Sorting functions are often used in programs. Python provides sort and sorted functions, one for sorting in place and the other for returning the new result after sorting

1. Parameters

##Function prototype:

sort([cmp[, key[, reverse]]])

• means that the sort method accepts three parameters, all of which can be omitted. The default is ascending order.

• The first parameter cmp is a comparison function. How to compare two parameters (elements of a list)? For comparison of built-in types such as integers, the method is very intuitive. , but for comparisons of custom types, you have to define the comparison function yourself. The function returns 0, which means the two numbers are equal, and returns a negative number, which means the first parameter is smaller, and the first parameter is ranked behind the second parameter. .

• #The second parameter key is the attribute of the comparison list element.

• The third parameter reverse is of type bool, which means whether to reverse (sort in reverse order)

①, cmp parameter example:

#cmp 函数，两个数倒过来比较 注！只能在python2.0上运行
s = [1, 2, 3, 4, 5]
s.sort(cmp=lambda a, b:cmp(b, a))
print s
# [5, 4, 3, 2, 1]

②, Common parameter key, reverse usage method , Code:

# key 指定排序方式  reverse 是否反排序

li = ['x11','abc323','e26','112ddd','fstgd2']

li.sort(key=len,reverse=True)    # 用长度进行排序，从大到小进行排序
print(li)
# ['abc323', '112ddd', 'fstgd2', 'x11', 'e26']

li.sort(key=lambda x:x[-1])     # key可以指定lambada函数x为列表中每个元素
print(li)                       # 元素的最后一个字符进行排序
# ['x11', 'fstgd2', 'abc323', 'e26', '112ddd']

li = zip(range(10),range(10)[::-1])  # 列表中元素为元祖是排序
print(li,type(li))
# <zip object at 0x000000E7F75504C8> <class &#39;zip&#39;>
li = list(li)
print(li)
# [(0, 9), (1, 8), (2, 7), (3, 6), (4, 5), (5, 4), (6, 3), (7, 2), (8, 1), (9, 0)]
li.sort(key=lambda x:x[-1])
print(li)
# [(9, 0), (8, 1), (7, 2), (6, 3), (5, 4), (4, 5), (3, 6), (2, 7), (1, 8), (0, 9)]

#**注！默认sort也是会对列表中元祖进行排序的
li.sort()
print(li)
# (0, 9), (1, 8), (2, 7), (3, 6), (4, 5), (5, 4), (6, 3), (7, 2), (8, 1), (9, 0)]

The parameter key can be: key=int, key=len, key=lambda...　

2. Sorting

①. How to output the keys in the dict from small to large according to value- value?

dic = {'z':1, 'y':4,'x':2,'g':3,'sg':3}

dic= sorted(dic.items(),key=lambda x:x[1])
print(dic)
# [('z', 1), ('x', 2), ('sg', 3), ('g', 3), ('y', 4)]

Convert to dictionary after sorting:

from collections import OrderedDict

dic = {'z':1, 'y':4,'x':2,'g':3,'sg':3}
dic= OrderedDict(sorted(dic.items(),key=lambda x:x[1]))

print dic
# OrderedDict([('z', 1), ('x', 2), ('sg', 3), ('g', 3), ('y', 4)])
for k,v in dic.items():
    print k,v
# z 1
# x 2
# sg 3
# g 3
# y 4

②,Given a string containing only uppercase and lowercase letters and numbers, sort it to ensure:

• All lowercase letters come before uppercase letters

• All letters come before numbers

• All odd numbers come before even numbers

s = "Sorting1234"

def sort_str(x):     # x 传入的每个元素
    if x.isdigit():
        if int(x) % 2 == 0:
            return (4,x)    # 返回的是元祖，元祖可进行排序
        return (3,x)
    elif x.islower():
        return (0,x)
    elif x.isupper():
        return (1,x)

li = sorted(s,key=sort_str)
print(li)
# ['g', 'i', 'n', 'o', 'r', 't', 'S', '1', '3', '2', '4']
string = ''.join(li)
print(string)
# ginortS1324

More concise code:

s = "Sorting1234"

s ="".join(sorted(s, key=lambda x: (x.isdigit(), x.isdigit() and int(x) % 2 == 0, x.isupper(), x.islower(), x)))
print(s)
# ginortS1324

　

　

The above is the detailed content of A brief discussion on sorting in Python. For more information, please follow other related articles on the PHP Chinese website!