2.1 use of variables

The use of variables is like naming you . In data analysis , We need to process a lot of data , But the data itself is not easy to remember , And there are a lot of the same data , For example, two identical height data . So for the convenience of , We can give these different data different names .

stay Python In language , The variable definition method is very simple , Is the variable name plus the initial value assigned to .

num = 1

This means that a variable is defined , The name is num, Its initial value is 1. As you can imagine , We put 1 This integer is put into a called num Drawers . Relative to variables , This 1 We call it a constant . therefore , The initial value of variables can generally be set through constants .

Many beginners feel very twisted , I think why the variable assignment uses the equal sign ？ These are actually habits , Some computer languages adopt such as “：=” Methods such as . This is why you need to practice more , You will get used to it after more practice .

It is called constant , Because 1 Cannot be changed , If it changes , That becomes another constant , Unlike variables , Different values can be stored as needed .

num = 1
print(num)
num = 2
print(num)

The operation interface is ：

The second output of the program is 2, Express num The value of the variable has been assigned to 2 Has been changed , The original 1 Has been lost , The value of the variable has changed .

Another example is that we define two heights ：

height1 = 1.80
height2 = 1.76

Similar to just now , Just this time we put a decimal into the variable . Decimals are sometimes called floating point numbers , The reason is English decimal float It also means floating .

ad locum , Although there are all kinds of data , But in Python There are not many common types of data in , Except integers and decimals , Another common one is character type , Also called string . We have also explained to you before ：

name = ' College students' '

Please note that , String constants need to be preceded by single quotation marks or double quotation marks , Otherwise, and variable name （ The variable name can be regarded as a string without single quotation marks , stay Python It only indicates special purposes such as variable name and function name ） It's hard to distinguish , And then there are problems .

There are also Booleans , That is, the variables represented by true and false .

isStudent = True

Be careful , there True It must be written strictly in case , Said really , Use False Said the false . These special names represent the two constants true and false .

Many students like to use Chinese input , But almost all computer languages stipulate that except variable names and string constants , All symbols need to use English half width characters , Please pay attention to switching input methods in time . For example, the equals sign here is actually Chinese full angle , Run prompt illegal characters ：

At the same time, pay attention to the difference between case ！

num = 0
print(NUM)

The operation interface is ：

The error prompt says NUM No definition （not defined）, Because uppercase and lowercase are treated as different variable names . 

There is also a common beginner's mistake , Just use it first （ Read ） Variable , Then define （ Or the variable has not been assigned at all ）

print(num)
num = 0

The operation interface is ：

At this time, the running error still indicates num No definition . It makes sense , If possible , Excuse me, the first line num What is the value ？ After all, it has not been assigned . therefore , Variables can only be used if they are created first , Not yet created , Of course not . General creation , In fact, it's assignment , That is, assign values first and then , Can be used . It's like saving money before withdrawing it .

Variable names cannot be arbitrarily named , It is generally recommended to use letters 、 Numbers and underscores , At the same time, don't start with a number . in addition , Some special names are in Python It has a special purpose , We call it keyword or reserved word （ Later, we will continue to contact ）, Don't use them as variable names , Such as ：

For example, there is a problem with this code ,for Cannot be the name of a variable ：

The previous article has explained when to define variables , If you need to use a data repeatedly , Or if you need to use the same data in different places , Then you should define variables , And save this data .

Let's do an exercise , Let the user enter two numbers , Add it up and output ：

num1 = input()
num2 = input()
print(num1 + num2)

The reason why two variables are defined here , Just to store two different inputs . In the operation interface , I have entered 1 and 2 Two figures ：

But do you think there is a problem ？ Actually , This is not a mistake . By default ,input Only strings can be entered , For Strings , The plus sign means connection , Of course 1+2=12. 

In order to be able to calculate integers in a real sense , We have to convert data types ：

num1 = input()
num2 = input()
print(int(num1) + int(num2))

Here we use int function , It means converting a string to an integer . Here we notice that we put num1 and num2 We separate int In parentheses after the function .

Function represents a function , Generally, if there is nothing in parentheses , Its functions are often fixed . But if you pass data inside , As an argument to a function , The function will show different results according to the different parameter data passed . For example, the string passed in can be converted to the corresponding integer . The data in parentheses can be variables , It can also be a constant .

Many functions have parameters , such as ：

num1 = input(' please enter an integer 1：')
num2 = input(' please enter an integer 2：')
print(int(num1) + int(num2))

At this time input With string parameters , This prompt message can be automatically displayed before input , better . The operation interface is ：

Actually , At this time print Functions also have parameters , Is the sum of two integers , So the parameters are different every time , The output results are different .

Let's put it another way ：

num1 = int(input(' please enter an integer 1：'))
num2 = int(input(' please enter an integer 2：'))
print(num1 + num2)

This effect is actually the same , But there are also differences . Notice that , here int The function is to input The function inputs the string obtained as its own parameter , The return is the corresponding integer , therefore ,num1 and num2 It always stores integers .

In the code just now ,num1 and num2 It's always a string , It is not temporarily converted to an integer until output . therefore , The difference between the two is mainly reflected in the different data types of variables . You must always pay attention to the data type of variables . meanwhile , For questions like this , Since we want to num1 and num2 Express integer , It is better to convert it to an integer immediately after accepting the input , This can avoid many unnecessary temporary data conversion later .

Of course , It is also possible to write in a separate way ：

num1 = input(' please enter an integer 1：')
num1 = int(num1)
num2 = input(' please enter an integer 2：')
num2 = int(num2)
print(num1 + num2)

here , First line num1 It's a string , But the second line becomes an integer . No problem with that ,Python Variables are very flexible , What to store , What kind of variable is .

Of course , The definition of variables can also be flexibly selected according to needs ：

num1 = int(input(' please enter an integer 1：'))
num1 = num1 + int(input(' please enter an integer 2：'))
print(num1)

This code defines only one variable , It realizes the same function as just now . The key here is the second line . First , We must not understand the assignment method of adopting the equal sign according to the equality judgment of Traditional Mathematics , It's just an assignment mark , The second line means to num1 out , At this point, it should be the result of converting an integer after the first input , Then let the user input a string and convert it to an integer , Add the two together , And put it back in again num1 in , obviously ,num1 The final integer addition result is still saved .

But because we only use one variable , Therefore, the integer entered for the first time is lost after accumulation , So this also reflects the value of variable definition again , If you still need to use two different input data later , Then you should create variables separately to save .

For multiple variables , We can also use some simpler assignment methods ：

num1, num2 = 1, 2
print(num1, num2)

It is equivalent to ：

num1 = 1
num2 = 2
print(num1, num2)

For num1 and num2 Assignment , Is to assign two constants to two variables respectively ,print The output of the function using commas also means that two variables are output , Default space separation .

Sometimes some interesting functions can be realized by using this writing method ：

num1, num2 = 1, 2
num1, num2 = num2, num1
print(num1, num2)

The operation interface is ：

Output is 2 1, Using this method, the exchange between the values of different variables can be realized .

Of course, for the exchange of variable values , We can also use other variable transfer methods to achieve ：

num1, num2 = 1, 2
num3 = num1
num1 = num2
num2 = num3
print(num1, num2)

The output is 2 1. You can think about the specific numerical exchange process by yourself . Here is a diagram , You can think about it yourself ：

Supporting learning resources 、 MOOC video ：

https://www.njcie.com/python/https://www.njcie.com/python/