[Python 1-6] Python tutorial 1 -- number

A number is a written symbol used to represent a number ：
Chinese numeral writing 37 、 Thirty seven
Roman numeral writing XXXVII
Arabic numeral binary writing 100101
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Numbers

In programming , Numbers are often used to record game scores 、 Represent visual data 、 Storage Web Application information, etc .Python Deal with numbers in different ways according to their usage . We usually use integers , Let's take a look at Python How to manage integers .

Integers

stay Python in , You can add integers + reduce - ride * except / operation .

>>> 2 + 3 
5
>>> 3 - 2 
1
>>> 2 * 3 
6
>>> 3 / 2 
1.5

At terminal ( Command line ) In the session ,Python Directly return the result of operation .Python Use two multipliers to denote the multiplication operation :

>>> 3 ** 2 
9
>>> 3 ** 3 
27
>>> 10 ** 6 
1000000

Python It also supports the order of operations , So you can use multiple operations in the same expression . Of course, we can also use parentheses to modify the order of operations , Give Way Python Perform operations in the order you specify , As shown below :

>>> 2 + 3*4
14
>>> (2 + 3) * 4 
20

Floating point numbers

Python All numbers with decimal points are called floating point numbers . Most programming languages use this term , It points out that A fact : The decimal point can appear anywhere in the number .
Every programming language has to be carefully designed , To handle floating-point numbers properly , Make sure that no matter where the decimal point appears , The behavior of numbers is normal .
To a large extent , When you use floating-point numbers, you don't have to think about their behavior . You just type in the numbers you want to use ,Python They are usually handled the way you want them to be :

>>> 0.1 + 0.1
0.2
>>> 0.2 + 0.2 9 0.4
>>>2 * 0.1
0.2
>>>2 * 0.2
0.4

But it should be noted that , The number of decimal places contained in the result may be uncertain :

>>> 0.2 + 0.1 
0.30000000000000004 
>>> 3 * 0.1 
0.30000000000000004

All languages have this problem , There is nothing to worry about .Python Will try to find a way , As much as possible accurate To express the result , But because of the way numbers are represented inside computers , It's hard in some cases . For now , Just ignore the extra decimal places .

Using functions str() Avoid typos

You often need to use variable values in messages . for example , Suppose you want to wish someone a happy birthday , You might write code like this ( Save the following code as birthday.py):

age = 23
message = "Happy " + age + "rd Birthday!"
print(message)

You might think , The above code will print a simple birthday greeting :Happy 23rd birthday!. But if you run the code , They will be found to cause errors :

Traceback (most recent call last):
File "birthday.py", line 2, in <module>
message = "Happy " + age + "rd Birthday!"
TypeError: Can't convert 'int' object to str implicitly

It's a typological mistake , signify Python Unable to identify the information you are using . In this example ,Python Find out that you make Used a value as an integer (int) The variable of , But it doesn't know how to interpret the value ( see ).Python know , This change A quantity may be a numerical value 23, It could also be a character 2 and 3. When you use integers in a string like above , It needs to be explicitly referred to Out of your hope Python Use this integer as a string . So , Callable functions str(), It makes Python Represents a non string value For the string :

age = 23
message = "Happy " + str(age) + "rd Birthday!"
print(message)

such ,Python You know you're going to put the number 23 Convert to string , And then display the characters in the birthday message 2 and 3. the After the above treatment , The message you expect will be displayed , It doesn't lead to mistakes :

Happy 23rd Birthday!

Most of the time , stay Python It's very simple to use numbers in . If the result is unexpected , Please check Python Press or not The way you want to interpret numbers as numbers or strings .

Homework
6-1 To write 4 Expression , They use addition respectively 、 Subtraction 、 Multiplication and division , But the results are all numbers 8.
For the use of print Statement to display the result , Be sure to enclose these expressions in brackets , in other words , You should write 4 Line is similar to the following code :
print(5 + 3)
The output should be 4 That's ok , Each row contains only numbers 8. Name it eight.py
6-2 Store your favorite number in a variable , Use this variable to create a message , Point out your favorite number , And then print this message out . Name it number.py

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