[Python知识库]python基础算法笔记

1. 基础知识

基础公式

// 除数取整、%除数取余、

字符串截取：

[1:]--获取从位置1开始后面的字符（默认首位是0）

[:-1]--删除位置为-1的字符（也就是获取从位置0带位置-1之间的字符）

[-1:]--获取位置-1的字符相当于 [-1]

[::-1]--倒序

lst2 = list('ajsgd')

lst2[5::-1] #?['d', 'g', 's', 'j', 'a']，倒序输出具体的值

列表倒序：

list.reverse()

使用空格连接列表中的词

' '.join(string)返回的就是中间是空的一段

.append()

.extend([1,2,3])#添加的是列表里面的数

.insert(1,56)

.remove(56)#去掉遍历列表见到的第一个56

lst1.pop()

lst1.index(2)

函数式编程：lambda,[],{}，map,filter,zip,enumerate

(lambda x,y:x+y)(3,5)#8 一次性调用时不需要赋值
f2 = lambda pair: pair[0]*pair[3]#输入列表
f2([2,3,6,4,6])

output1 = [x**2 for x in range(10)]

worlist = ["red","blue","yellow"]
output = [len(x) for x in worlist]
lst=[x for x?in range(10)]
lst1 = list(zip(lst,worlist))#zip的运用，遍历lst和worlist，并对应，后面没有对应上的就舍弃

output = [(x,y) for x in lst for y in worlist if x<len(y)]#两个参数的筛选

sent = "show something, show me your code"
sent = sent.upper()
sent = sent.split(" ")##返回list
sentlist = list(sent)
sentset = set(sent)
senttuple = tuple(sent)
sent,sentlist,sentset,senttuple
(['SHOW', 'SOMETHING,', 'SHOW', 'ME', 'YOUR', 'CODE'],
 ['SHOW', 'SOMETHING,', 'SHOW', 'ME', 'YOUR', 'CODE'],
 {'CODE', 'ME', 'SHOW', 'SOMETHING,', 'YOUR'},
 ('SHOW', 'SOMETHING,', 'SHOW', 'ME', 'YOUR', 'CODE'))

output = [x[0].upper() for x in list]#只大写第一个字母str可以用[]选取字母
['A', 'O', 'P']

dics = {k:sentlist.count(k) for k in sentlist}
#字典的推导式，之前是放在list里的，现在放在字典里，只要按照格式来写就行

dics = {k:list(senttuple).count(k) for k in senttuple}#tuple也是可以转换成list,count的

#map
output = list(map(lambda x:x.upper(),wordlist))#map遍历
print(output)#map不能直接输出，是一个函数，输入list

output = filter(lambda x:len(x)<5,wordlist)
#使用filter，过滤，和if相似操作，lambda后面表示的是条件
print(list(output))


def fun1(f,x):
    y = x**2
    return f(y,10)
def fun1(f,x):
    y = x**2
    return f(y,10)

f1 = lambda x,y:x+y
f2 = lambda x,y:x*y

fun1(f1,3)##19


output = collections.defaultdict(list)

for k,v in data:
    output[k].append(v)

output：
defaultdict(list,
            {'fall': [2],
             'spring': [0, 4, 5],
             'summer': [1, 6, 7],
             'winter': [3, 8]})

output = collections.defaultdict(int)
for k,v in data:
    output[k]+=v
print(output)
defaultdict(<class 'int'>, {'spring': 7, 'summer': 20, 'fall': 6, 'winter': 3})


sensons = ["winter","spring","summer","fall"]
x = enumerate(sensons,10)#可以从10开始计数
list(x)#enumerate,map,filter,zip都需要list来输出



WHAT'S YOUR NAME? LIHUA
WHAT'S YOUR TEL? 1123
WHAT'S YOUR ADD? GREEN

2. 简单案例

词频统计

s = 'aaajshjsd'
output = {k:s.count(k) for k in set(s)}#快捷获得词频统计，字符

output = collections.Counter(sent)#快捷获得词频统计，字符
print(output)

output = {k:sent.count(k) for k in set(sent)}#快捷获得词频统计

#统计字符串中字符的个数
str = 'All statistical techniques that simultaneously analyze multiple'
lstt = list(str)
lstt.count('e')

# 统计字符串中字符出现的次数，计入dictt中
strr = 'All statistical techniques that simultaneously analyze multiple measurements on individuals or objects under investigation'
lstt = list(strr)
dictt = {}
for i in set(strr):
    num = lstt.count(i)
    dictt[i] = num


#累加器，看一下实验即可
def fun(num):
    return lambda y:num+y
f = fun(1)
f(2)

cont = f.read()#指针的移动
print(cont)
f.readline()#返回str
f.readlines()#读出所有的行，返回到list

#设置密码
pwd = ""
for i in range(8):
    pwd += random.choice(s1)#字符str可以直接加一起


sent = "hello world,lukin is a chinese "
wdlst = sent.split(" ")
#1
wsfre = collections.defaultdict(int)#词频统计，词
for wd in wdlst:
    wsfre[wd] += 1
print(wsfre)
#2
output = collections.Counter(wdlst)

#3
output = {k:sent.count(k) for k in set(sent.split(" "))}#快捷获得词频统计

#tuple解决fib
def fib(n):
    a,b = 0,1
    for i in range(n):
#        print(i,a)
        print(b)
        a,b = b,a+b    
fib(10)

3.关于类

class student:#类型定义
    movie = "cartoon" #类的属性，直接可以用变量名称
    def __init__(self,ids='000',nm="unkonwn",sco=98):#构造函数
        self.id = ids#实例属性
        self.name = nm
        self.score = sco
        
    def greet(self):#函数必须是self
        print(self.id,self.name,"say hello to you!")
    
    def __repr__(self):#输出相应的对象信息，可以直接调用该函数的
        return "{0},{1} score:{2}".format(self.id,self.name,self.score)

stu1 = student("001","Tom",95)#创建一个student对象，实例 object
stu2 = student("002","sarah",98)

4.关于函数

def auth(quote, **speaker_info):
    print(">", quote)
    print("-"*(len(quote)+2))
    print(type(speaker_info))
    print(speaker_info)
    for k, v in speaker_info.items():
        print(k, "----", v)

auth("hello", name="mary",age=3, grade="pre",home="noland")
> hello
-------
<class 'dict'>
{'name': 'mary', 'age': 3, 'grade': 'pre', 'home': 'noland'}
name ---- mary
age ---- 3
grade ---- pre
home ---- noland