[Tutor] bit shifting

[Ian D]

But what is the purpose of ANDing with 255?


Would this not just return the same value?


eg 1010 and with 11111111 would just return 00001010 or 1010




----------------------------------------
> From: duxbuz at hotmail.com
> To: tutor at python.org
> Date: Thu, 1 May 2014 09:53:15 +0000
> Subject: Re: [Tutor] bit shifting
>
> Ok I am getting somewhere with this now.
>
>
> A bitshift followed by ANDing the result of the shift!
>
>
> So I think n>> 24 & 0xFF
>
>
> is
>
>
> shift n 3 bytes right, save results in n and then AND n with 255 decimal?
>
> ----------------------------------------
>> From: duxbuz at hotmail.com
>> To: tutor at python.org
>> Date: Thu, 1 May 2014 08:08:17 +0000
>> Subject: [Tutor] bit shifting
>>
>> I am trying to follow some code. It is basically a python scratch interfacing script.
>>
>>
>>
>> Anyway part of the script has this code.
>>
>>
>>
>> Searching google for>> greater than signs in code with python has its issues.
>>
>>
>>
>> Can anyone clarify this stuff.
>>
>>
>>
>> I know its about 4 bytes of data. It looks like its setting all bits HIGH to me?
>>
>>
>>
>> n = len(cmd)
>> a = array('c')
>> a.append(chr((n>> 24) & 0xFF))
>> a.append(chr((n>> 16) & 0xFF))
>> a.append(chr((n>> 8) & 0xFF))
>> a.append(chr(n & 0xFF))
>>
>>
>>
>>
>>
>> More code for context for python version 2.7:
>>
>>
>>
>> from array import array
>> import socket
>> import time
>> import sys
>>
>> from Tkinter import Tk
>> from tkSimpleDialog import askstring
>> root = Tk()
>> root.withdraw()
>>
>> PORT = 42001
>> HOST = askstring('Scratch Connector', 'IP:')
>> if not HOST:
>> sys.exit()
>>
>> print("Connecting...")
>> scratchSock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>> scratchSock.connect((HOST, PORT))
>> print("Connected!")
>>
>> def sendScratchCommand(cmd):
>> n = len(cmd)
>> a = array('c')
>> a.append(chr((n>> 24) & 0xFF))
>> a.append(chr((n>> 16) & 0xFF))
>> a.append(chr((n>> 8) & 0xFF))
>> a.append(chr(n & 0xFF))
>> scratchSock.send(a.tostring() + cmd)
>>
>> while True:
>> msg = askstring('Scratch Connector', 'Send Broadcast:')
>> if msg:
>> sendScratchCommand('broadcast "' + msg + '"')
>>
>>
>>
>> Another for Python 3:
>>
>> import socket
>>
>> HOST = 'localhost'
>> PORT = 42001
>>
>> s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>> s.connect((HOST, PORT))
>>
>> def sendCMD(cmd):
>> n = len(cmd)
>> a = []
>> a.append((n>> 24) & 0xFF)
>> a.append((n>> 16) & 0xFF)
>> a.append((n>> 8) & 0xFF)
>> a.append(n & 0xFF)
>> b = ''
>> for i in list(range(len(a))):
>> b += a[i]
>> s.send(bytes(b+cmd,'UTF-8'))
>>
>> sendCMD('broadcast"hello"')
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