day2

You bury the mud pin bone under the spring , I'm sending snow all over the world .

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Language gossip

Tracing the roots

feng · Neumann structure

Nowadays, most computer hardware systems are still based on Feng · Neumann structure established .
feng · Neumann structure mainly has two aspects

• The storage device is separated from the central processing unit
• Data is encoded in binary

therefore , The hardware system of a computer is usually composed of five parts , Include ： Arithmetic unit , controller , Memory , Input devices and output devices . among , The combination of arithmetic unit and controller is what we usually call a central processor , Its function is to execute various operation and control instructions and process data in computer software . What we call a program is a set of instructions , The program is to A series of instructions are combined in some way , Then use these instructions to control the computer to do what we want it to do .

Variables and types

Variables are essentially a piece of memory space of a computer , Generally, it has read and write operations . There are many kinds of data processed by computers , In addition to values , It also includes text , graphics , Audio , Video and other data .python There are many types of data in , Like any other language , Users can also customize new data types . Next in , First introduce some new partners .

• integer ：python Can handle integer of any size ,(python2 There is int and long Two types of integers distinguish , But this distinction lies in python3 Disappeared in , In the end, there are only integers left int Big brother stayed , So when writing large integers plus, minus, multiply and divide in the future , Know what to use ), stay python In binary , octal , Decimal system , The hexadecimal representation is as follows ( It is unified here 16)

Base number	Representation
Binary system	With 0b start ,0b10000
octal	With 0o start ,0o20
Decimal system	Just write it directly ,16
Hexadecimal	With 0x start ,0x10

• floating-point ： Floating point numbers are decimals , Relative to the fixed number , Fixed point number means that the decimal point is fixed , Floating point numbers are decimal points that are not fixed , Because the representation of floating-point numbers adopts scientific counting , Therefore, the position of the decimal point can be changed .

a = 1.234 #  Fixed-point number 
b = 12e34 #  Floating point numbers 

• String type ： Strings are in single quotes or Double quotes or Any text enclosed in three quotation marks , such as 'abc',"abc",'''abc''',"""abc""", Of course, it also includes the original string r'\n'（ At this time, the output is not line feed , But the original text content \n, That is to say, the original content will be output , Instead of the content after escape ）, Byte string ,Unicode If you are interested in strings, you can understand them by yourself . Note that only three quotation marks support multiline form , Others cannot .
• Boolean type ：python There are only True and False Two values . Pay attention to the case here ,python It's a case sensitive language . Of course, you can also assign values through expressions that return Boolean values , such as 1 < 2 perhaps 1 == 2 etc. .
• Plural ：1 + 1j, This form is the representation of the plural , Note that the imaginary units here use j Rather than using i, Other expressions match the mathematical knowledge learned before .

Variable naming

Your name is the shortest love poem I have ever read

The naming of variables follows the following hard rules ( Give big brother interpreter a face )：

• Variable names are made up of letters ( The generalized Unicode character , Excluding special characters )、 Numbers and underscores , The number can't start
• Case sensitive ( uppercase A And lowercase a It's a different variable )
• Don't follow keywords ( Words with special meanings ) And system reserved words ( Such as function 、 Name of module, etc ) Conflict

Artificial requirements are as follows （ You can't abide by , Just suggest ）

• Spell in lowercase , More than one word is underlined
• Protected instance properties start with a single underscore
• Private instance properties start with two underscores

Of course, now cooperative programming is the best , Therefore, it is also very important to know the meaning of variable naming .

Use of variables

python Different from other languages , There is no need to worry about not remembering variable types , It automatically matches variable types . That is, the definition and declaration of variables are direct a = 123 Is to define an integer variable a, There is no need to add int. therefore , If a = 1, b = 2, a/b stay python The output of is 0.5 It's not like C Medium 0.

a = 321
b = 123
print(a + b)
print(a - b)
print(a * b)
print(a / b)

stay python Can be used in type Function to check the type of the variable . The concept of function here is general , The author doesn't say .

a = 100
b = 12.345
c = 1 + 5j
d = 'hello, world'
e = True
print(type(a))
print(type(b))
print(type(c))
print(type(d))
print(type(e))
"""
output:
<class 'int'>
<class 'float'>
<class 'complex'>
<class 'str'>
<class 'bool'>
"""

Do type conversion , have access to python The built-in functions in convert variable types . Pay attention to python The variable type in is not what we define , So if it is

a = 123
b = 12.3
a = b

It's not like C Cast in , It's about putting a Also become 12.3, therefore python It is recommended to use python Built in type conversion function , Otherwise, it is easy to make some strange mistakes .

function	effect
int()	Convert a numeric value or string to an integer , You can specify base ( Here, the string specifies what base is converted to decimal )
float()	Convert a string to a floating point number
str()	Converts the specified object to string form , You can specify the code
chr()	Convert the integer to the string corresponding to the encoding （ A character ）
ord()	The string （ A character ） Convert to the corresponding code （ Integers ）
bool()	Convert to Boolean
complex()	Convert to plural type

Here comes the stuffed duck ：input() It means reading data from the keyboard , String can be filled in brackets , Indicates output to the output device , As a reminder

a = int(input('a = '))
b = int(input('b = '))
print('%d + %d = %d' % (a, b, a+b))
print('%d - %d = %d' % (a, b, a-b))
print('%d * %d = %d' % (a, b, a*b))
print('%d / %d = %f' % (a, b, a/b))
print('%d // %d = %d' % (a, b, a//b))
print('%d %% %d = %d' % (a, b, a%b))
print('%d ** %d = %d' % (a, b, a**b))

Be careful ： notice input() You'll know python Preferential treatment for strings . The first is to convert the information entered on the keyboard into string form for storage , If there is no type conversion , So the default is str type , So use input Generally, you need to use the preceding type conversion function . The placeholder in front of this side , and C/C++ Similar , No more details here . Be careful python Format of the formatted output statement in , It's through % To segment the , No ,, Commas are used to separate statements , The data needed to occupy the space is written in parentheses , Separate by comma .

ps： there // yes python The product of division and division ,// For division ,/ Represents normal division ,** Represents a power operation .

Operator

The commonplace operator priority table of the great demon level ( Can we use brackets to distinguish priorities these days )

Operator	describe
[],[:]	Subscript , section
**	Index
~,+,-	According to the not , The sign
*,/,%,//	ride , except , model , to be divisible by
+,-	Add , reduce
>>,<<	Move right , Move left
&	Bitwise AND
^,|	Bitwise XOR , Press bit or
<=,<,>,>=	Less than or equal to , Less than , Greater than , Greater than or equal to
==,!=	be equal to , It's not equal to
is,is not	Identity operator
in,not in	member operator
not,or,and	Logical operators
=,+=,-=,*=,/=,%=,//=,**=,&=,|=,^=,>>=,<<=	Various assignment operators

It is suggested to use parentheses to ensure the execution order of operations

Assignment operator

Is to assign the value of the right to the left , Also distinguish = and == The beginning of

a = 10
b = 3
a += b
a *= a + 2
print(a)

a *= a + 2 because + Has a higher priority than *=, Can't , The assignment operator has the lowest priority , The most humble . So this statement is actually equivalent to a = a * (a + 2)

Comparison operators and logical operators

The value of the expression composed of comparison operators is Boolean , Include ==,!=,<,<=,>,>=, Note that it's not mathematical to judge whether it's equal here =, It is ==.

There are three logical operators ,and,or and not.

• and Is and operation , Only the left and right Boolean values are True The expression will return True, Otherwise return to False. If and The Boolean on the left is False When , So no matter what value is on the right , The final result is False, Therefore, the value on the right will be skipped when doing the operation ( Short circuit treatment ), namely and The value on the left is False When , The expression on the right does not execute at all , This and C/C++ Very similar .
• or Or operation , Only the left and right Boolean values are False When , The final result will be False, Otherwise, it's all for True.or There is also short-circuit operation , As long as the Boolean value on the left is True, Then the expression on the right will not execute at all .
• not Is non operation . It just adds a Boolean value after it , That is, unary operator , The function is to return the Boolean value opposite to the following Boolean value , For example, the back is True, Return is False.

a = 1==1
b = 3 > 2
c = 2 < 1
d = a and c
e = a or c
f = not(1 != 2)
print(a)
print(b)
print(c)
print(d)
print(e)
print(f)
'''
output:
True
True
False
False
True
False
'''

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Practice after class

• here %.1f and C/C++ The placeholder in has the same meaning , Also output decimals , Keep one decimal place
• About print(f'{f:.1f}') Explanation of output ： Add... Before the string f Represents a formatted string , That is, at this time, our original placeholder can be replaced with braces , The area surrounded by braces is the area to be replaced .
• Writing code across lines can help () You can also use '\', But for \\ There must be an operator in front, otherwise it will report an error or get the wrong answer , Because at this time, the system does not think that you are writing expressions across lines

About str Type of supplement

function	function
len()	Gets the length of the string ,len(str)
title()	Capitalize the first letter of each word in the string ,str.title()
upper()	Turn all the letters in the string into uppercase ,str.upper()
lower()	Change all the letters in the string to lowercase ,str.lower()
startwith(str1)	Is the string in str1 At the beginning ,str.startwith(str1)
endwith(str1)	Is the string in str1 At the end of the ,str.endwith(str1)

Here, strings and strings can be spliced directly , namely str1 + str2 That's all right. , If str * int, So that is str repeat int All over