Python 基础语法复习

由于选修了《人工智能模式识别》的课程，要求用phthon来实现算法，乘着周三晚上没课，就来回顾一下python的主要语法。

环境：   Anaconda

　　　   Python3.6

1.变量：

• 在python中，变量是不需要提前声明类型的

1 #data type
2 str_test = "China"
3 int_test = 123
4 float_test = 122.5
5 
6 print(str_test)
7 print(int_test)
8 print(float_test)  

• 类型可以转换，用type查看变量类型

#转换
str_eight = str(8)
eight = 8
str_eight_two = str(eight)

str_eight = "8"
int_eight = int(str_eight)

print(int_eight)
print(type(int_eight))

2.list类型

• list类型里的元素类型可以不同，添加元素时可用list.apppend(),添加时自动为其设置下标index,可使用下标访问list中的元素

 1 countries = []
 2 temperatures = []
 3 
 4 countries.append("China")
 5 countries.append("India")
 6 countries.append("United States")
 7 
 8 temperatures.append(30.5)
 9 temperatures.append(25.0)
10 temperatures.append(15.1)
11 
12 print(countries)
13 print(temperatures)
14 
15 china = countries[0]
16 china_temperature = temperatures[0]
17 print(china)
18 print(china_temperature)

• 计算list长度，切片操作，值得注意的是list[-1] == list[length-1], 即下标是循环的

 1 int_months = [1,2,3,4,5,6,7,8,9,10,11,12]
 2 length = len(int_months)
 3 print(length)
 4 index = len(int_months)-1
 5 last_value = int_months[index]
 6 print(last_value)
 7 print(int_months[-1])       #倒数也可
 8 #切片
 9 two_four = int_months[2:4]    #取头不取尾
10 print(two_four)
11 tree_last = int_months[3:]
12 print(tree_last)

3.程序的结构

• loop>>for,  range()的用法值得注意

1 #loop
2 cities = ["Austin","Dallas","Houston"]
3 for city in cities:
4     print(city)
5 for i in range(10):
6     print(i)

• loop>>while

1 i = 0
2 while i < 3:
3     i += 1
4     print(i)

• 双重循环

1 cities = [["Austin","Dallas","Houton"],["Haerbin","Shanghai","Beijing"]]
2 print(cities)
3 #for city in cities:
4     #print(city)
5     
6 for i in cities:
7     for j in i:
8         print(j)

• 判断语句

#if statements
sample_rate = 700
greater = (sample_rate > 5)
if greater:        #也可以为表达式
    print(sample_rate)
else:               #不写else也可以
    print('less than')

• list中查找的简写

1 #find a value 
2 animals = ["cat","dog","rabbit"]
3 for animal in animals:
4     if animal == "cat":
5         print("Cat found")
6 if "cat" in animals:
7     print("2 is also right")

4.dictionary类型

• 字典的创建、初始化和赋值

 1 students = {}
 2 students["Tom"] = 60
 3 students["Jim"] = 70
 4 print(students)
 5 
 6 students = {}
 7 students = {
 8     "Tom": 60,
 9     "Jim": 70
10 }
11 print(students)

• 字典的应用----统计个数

#统计
pantry = ["apple", "orange", "grape", "apple", "orange", "apple", "tomato", "potato", "grape"]
pantry_counts = {}

for item in pantry:
    if item in pantry_counts:
        pantry_counts[item] = pantry_counts[item] + 1
    else:
        pantry_counts[item] = 1
print(pantry_counts)

5.文件处理

• 文件的读与写

1 f = open("test_write.txt","w")  #不存在的文件自动新建
2 f.write("123456")
3 f.write("
")
4 f.write("234567")
5 
6 f.close()

 1 #File
 2 #打开
 3 f = open("test.txt","r")
 4 
 5 #处理
 6 g = f.read()
 7 print(g,type(g))
 8 
 9 #关闭
10 f.close()     #不要忘了关闭

• 文件csv的操作举例以及split()的使用

 1 weather_data = []
 2 f = open("weather.csv",'r')
 3 data = f.read()
 4 #print(data)
 5 rows = data.split("
")
 6 #print(rows)
 7 for row in rows:
 8     split_row = row.split(",")
 9     print(split_row)
10     weather_data.append(split_row[0])
11 print(weather_data)
12 f.close()

操作该部分时，我先建立了一个xlsx文件，随后将名称改为了csv文件，打不开文件，使用“rb”操作后显示乱码。随后发现另存为csv文件可以很好的解决这个问题，对csv文件的构成也有了深刻的了解。

6.函数的操作

 1 def printHello():
 2     print("hello python")
 3     
 4 def printNum():
 5     for i in range(0,10):
 6         print(i)
 7     return
 8 def add(a,b):
 9     return a+b
10 printHello()
11 printNum()
12 add(1,2)

与C/C++不同，传入参数不需要申明类型。