python in C language assignment and assignment operator in the huge difference

 

First, let us look at a simple C program:

1 a = 8;
2 b = a;
3 b = 6;
4 printf("a = %d, b = %d\n",  a, b);

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I believe as long as learned C language, do not know the program will run the program will be able to print out a = 8, b = 6.

Now let's take a look at this program python version

1 a = 8
2 b = a
3 b = 6
4 print(a, b)

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Let's look at the results:

 

 

Everything seems normal.

 

No, if really so simple, it is up to me why! ! !

 

Look, the program again. . .

1 name = ['fujian', 'beijing', 'shanghai']
2 name2 = name
3 name2[1] = 'tainjing'
4 print(name)
5 print(name2)

python program

The result is still not what we thought it would:

[ 'Fujian', 'beijing', 'shanghai']

[ 'Fujian', 'tainjing', 'shanghai']

 

The reality is:

 

 

 Why！Why！Why！

 

For the following reasons:

　　1. c language variable is like a box, a = 8 8 like the case of this data into the named a; and the python in this respect can be regarded as a big difference with the C language;

　　　 python variables is somewhat similar to the C language pointer, a = 8 in python like a data point to this 8.

　　2. python in each assignment is the variable to point to a new data (position)

 

Now we come back to take a look at these two python programs,

　　One:

　　　　　　a = 8

　　　　　　b = a

　　　　　　b = 6

　　首先语句a = 8说明变量a指向了8这个数据；

　　接着语句a = b说明变量b指向了a，也就等同与a与b一起指向了8这个数据；

　　最后语句b = 6，说明此时变量b的指向已经发生了改变， 但是变量a的指向并没有改变，变量b现在指向了6这个数据，变量a还是指向原来的数据8；

　　所以最后会打印出a = 8,  b = 6.

 

 

　　二：

　　　　name = ["fujian", "beijing", "shanghai"]

　　　　name2 = name

　　　　name2[1] = "tianjing"

　　　　 

　　　　同样我们可以类似上一个程序那样分析:

　　　　首先第一条语句说明变量name指向了一个列表；

　　　　第二条语句说明变量name2与name指向了同一个列表；

　　　　第三条语句的分析非常重要：   

　　　　请注意

　　　　name[1]其实也可以看成是一个‘变量’，只不过这个变量是属于name所指向的列表的一部分，它原先是指向“beijing”这个字符串，

　　　　现在经过第三条语句后，变量name[1]就改变了指向，指向了“tianjing"这个字符串了；这样也恰好改变了列表中的内容。

　　　　再加上最终name与name2还是指向最初的那个列表（只不过这个列表‘更新’了），

　　　　所以现在问题不就解决了！！！

 

 

　　　　如果您发现以上信息有问题，请及时留言，谢谢！