Decimal and binary

Decimal system ---> Binary system , except 2 The remainder is inverted
Like the decimal system 5, Represented as binary 101 python Realization ：

d = 5
bin(5)
#"0b101"

# Convert the current binary data , To integer 
int("0b101",base=2) # 5
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The fractional part , Then take 2 Round and align
Like the decimal system 0.5, Represented as binary 0.1

Binary system ----> Decimal system , Just weighted sum
Such as binary 101, Its decimal system is expressed as 1*2^2^+0*2^1^+1*2^0^=5

Decimal and octal

Decimal system ----> octal , except 8 The remainder is inverted
Like the decimal system 8 , Expressed in octal 0o10

d = 8
oct(d)
#0o10
int("0o10",base=8)
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octal ---> Decimal system , Similarly, weighted summation

Decimal system ----> Hexadecimal ,same

d = 17
hex(d)
#'0x11'
int("0x11",base=16)  # Hexadecimal to decimal , To sum by weight 
#1*16^1 + 1*16^0
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Binary representation of characters

The character has a corresponding ASCII clock , The corresponding decimal number , Change to binary

# Get the binary of English characters 
bin(ord("a"))
#'0b1100001', One byte 
chr
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1byte = 8bit 1Kb = 1024bytes 1Mb = 1024Kb ...

Chinese and binary

Chinese characters have corresponding GBK/GB2312/GB18030 Encoding table , Corresponding decimal system , Change to binary .

A Chinese character takes up two bytes , Now the mainstream is GBK code .

Universal coding method Unicode, Correspond all characters to binary , But each character takes == Two bytes == Of memory , Than ASCII Double the code , Temporarily stored in memory can also , Saving to disk or network transmission is a waste of space .

So there is a utf-8 The coding ,Unicode Transform Format, Save disk 、 Space during transmission .

Data in memory -->Unicode code ----2bytes/per char
Data on disk -->UTF-8----1byte for english, 3bytes for chinese

linux Type... On the command line locale, View supported encoding formats

py2 And py3 The coding

py2 In memory is ASCII code , If you use Chinese, you need to declare

linux/macOS The system uses utf-8

#！/usr/bin/python2
# -*- coding:utf-8 -*-
print(" chinese ")
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windows System use gbk code

py3 In memory is Unicode code

hash Hash

Put different inputs , Hash calculation yields a fixed length output .

characteristic ： Irreversible Fast calculation , Even if the file is big , It won't affect the calculation speed too much The type of data entered , Must be immutable

purpose ： Password encryption File check

hash("xxxxx")

from hashlib import md5
m = md5()
m.update("xxx".encode())
m.hexdigest()
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python Based on hash Data type of

dict/set Why is dictionary query fast ？ In the dictionary , Every key Have to go through hash Calculation , For all the key Of hash Value to sort . Looking for some key The value of , First calculate its hash value , According to its hash Values are in the already ordered hash Binary search in value sequence . fast

According to the query hash value , Get the corresponding value The index of .

Why? set Automatic weight removal ？ When saving value , Calculating the hash value , Then store the data in the corresponding position , If there is no data at this location , The insert ; Otherwise do not insert .

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