1. CS2911 Week 2, Class 2 Today Review Muddiest Point Quiz
12 July 2019
CS2911 Week 2, Class 2
Today
Review Muddiest Point
Quiz
Supplementary Python app
Encoding in Python
Muddiest Point
Parsing in Python
17q ,2,4,6,7,24
17q1
v (started 1_1) Recall the raw representations of the ASCII characters '\r' (CR) and '\n' (LF)
v(started 1_1) Recall the raw representations of the ASCII characters A, B, C, D, E, a, b, c, d, e, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9
v(started 1_2) Convert a many-digit binary number to its big-endian and little-endian formats
v(started 1_2) Give examples of common systems using the big-endian and little-endian formats
v Describe the difference between a compiled an an interpreted language
v Write a list literal in Python
v Write a tuple literal in Python
v Index into a tuple or list in Python
v Describe the difference between lists and tuples
v Write if-statements in Python
v Write a counting (for) loop in Python
v Write a sentinel-value (while) loop in Python
Write code that manipulates raw bytes in Python using ord(), chr(), int.from_bytes(), and i.to_bytes()
Write code that interprets and formats strings using str() and int()
(Write exceptions in Python)
Declare a Python method
Write a string literal in Python
Write a bytes literal in Python
CS2911
Dr. Yoder
Dr. Josiah Yoder

2. Muddiest Point 2-1 and Lab 2
CS2911
12 July 2019
Muddiest Point 2-1 and Lab 2
It's very unclear what we actually have to code in Lab 3.
Lab 3
So what exactly is ord
ord/chr
should the pre-lab be printed? And today wasnt too muddy. Im getting better at converting between binary and hex
Lab 2
everything was clear for me
none
Using the keyword arguments
keyword arguments
Do you want our predictions for the lab printed out, or is a digital format sufficent
Why is 5 stored as 53?
ASCII decimal
Dr. Josiah Yoder

3. CS2911
12 July 2019
Quiz 1
Dr. Josiah Yoder

4. Supplementary Python materials
CS2911
12 July 2019
Supplementary Python materials
Learn Python app from SoloLearn
Available for Android and iPhone
Supplementary – If you learn something here that is missing in class, please tell me
Quizzes and interactive environment may be helpful
Alternative: Redo an SE1011 lab with Python
Note: We are not doing OO in Python this quarter
Thanks to a student for recommending this app
Dr. Josiah Yoder

5. Questions about Lab 3? Design process How to implement in Python?
CS2911
12 July 2019
Questions about Lab 3?
Design process
How to implement in Python?
Protocol definition …
Dr. Josiah Yoder

6. Exercise: Brainstorm steps (large or small) needed to complete Lab 3
CS2911
12 July 2019
Exercise: Brainstorm steps (large or small) needed to complete Lab 3
Dr. Josiah Yoder

7. Python 3 types int 123 integer (any size) str '123' Unicode string
CS2911
12 July 2019
Python 3 types
int 123 integer (any size)
str '123' Unicode string
bytes b'123' raw bytes or ASCII string
UPDATED NEXT LECTURE
Dr. Josiah Yoder

8. How integers are stored conceptually in Python
CS2910
12 July 2019
How integers are stored conceptually in Python
i = 5
101
i = 255
i = 1000
i =
(Length is also stored, and bigger numbers are internally broken up into chunks)
= 2^31-1 is the Java's Integer.MAX_VALUE. But Python 3 integers can be as large as you want them to be.
class 1-2: Slide 21
Dr. Josiah Yoder

9. How strings are conceptually stored in in Python
CS2910
12 July 2019
How strings are conceptually stored in in Python
s = '0123'
(Actually, 32 bits are stored for each character, but only the first 21 bits are used.)
class 1-2: Slide 27
Dr. Josiah Yoder

10. Arbitrary byte literals
CS2910
12 July 2019
Arbitrary byte literals
5 = 0x05 b'\x05' # = 0x0e b'\x0e' # = 0b = 0x64 b'\x64' #
These really do store just one byte, even though you use four characters to type them in your code:
b = '\x05'
len(b)
# 1
Dr. Josiah Yoder

11. How bytes are stored in Python
CS2910
12 July 2019
How bytes are stored in Python
(Length is also stored)
s = b'ABC'
s = b'0123'
s = b'\r\n'
Dr. Josiah Yoder

12. ord() and chr() i = 5 # int literal s = chr(i) # 101
CS2910
12 July 2019
ord() and chr()
i = 5 # int literal
# 101
s = chr(i) #
s = 'a' # str literal #
i = ord(s) #
Strings are actually stored with 32 bits, but only the first 2 bits are used for a Unicode code-point, because so far, those are the only Unicode code-points defined.
Class 1-2: Slide 28
Dr. Josiah Yoder

13. bytes can be sliced like strings
CS2910
12 July 2019
bytes can be sliced like strings
b = b'ABC' # b[1:3] # # b'BC' But not indexed like strings! b[0] # … # 65 (an integer)
Dr. Josiah Yoder

14. Converting between strings and bytes
CS2910
12 July 2019
Converting between strings and bytes
# Use UTF-8 for the byte string # If a non-ASCII characters is used in s, # multiple bytes in b will replace it. s = bytes.decode(b) b = s.encode() # Use ASCII for the byte string # Only ASCII characters can be used in s s = bytes.decode(b,'ASCII') b = s.encode('ASCII')
Dr. Josiah Yoder

15. CS2910
12 July 2019
int() and str()
s = b'1234' # # # #
i = int(s) #
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16. CS2910
12 July 2019
int() and str()
i = 999 # s = str(i) #
Dr. Josiah Yoder

17. CS2910
12 July 2019
int(…,16)
s = '1AFF' # # # i = int(s,16) #
Dr. Josiah Yoder

18. CS2910
12 July 2019
format(…,'x')
i = 0xAFF # s =format(i,'04x') # # # # ('0aff')
hex(i)[2:] is popular, but doesn't have a way to include padding zeros. It also doesn't work for negative numbers. format(0xAFF,'+05x') is quite predictable. format(0xAFF,'x') also works nicely if you don't care how many digits there are.
Note that you can also use 'b', 'o' bases
Dr. Josiah Yoder

19. CS2910
12 July 2019
.to_bytes()
i = 0xAFF = # b = i.to_bytes(2,'big') # # b'\n\xff' i2 = int.from_bytes(b,'big') # class method
Dr. Josiah Yoder

20. CS2910
12 July 2019
Keyword Arguments
You can include the names of the parameters as "keywords" on your arguments: b = i.to_bytes(length=2, byteorder='big') # length – number of bytes to produce in b i2 = int.from_bytes(bytes=b, byteorder='big') # This can help make code self-documenting
Dr. Josiah Yoder

21. Review b'\xbe\xef' = 1011 1110 1110 1111 (as bytes)
CS2910
12 July 2019
Review
b'\xbe\xef' = (as bytes)
0xbeef = (as int)
(…).to_bytes(…,byteorder='big')
int.from_bytes(…,byteorder='big')
ord(…)
chr(…)
int(…)
format(…,'x')
Dr. Josiah Yoder

22. CS2910
12 July 2019
Question
Why might int(…) and format(…) be less efficient than the others for creating a raw encoding?
Why are int(…) and chr(…) less natural?
Dr. Josiah Yoder

23. https://www.polleverywhere.com/free_text_polls/zIp2tH2IVWjXv4H CS2910
12 July 2019
What was the muddiest point?
Dr. Josiah Yoder

24. Exercise: Work on Lab 3 Write method headers with documentation
CS2911
12 July 2019
Exercise: Work on Lab 3
Write method headers with documentation
Decide who will write each method
Try to figure out what the "hard" parts will be
Dr. Josiah Yoder

25. Python byte lists and strings
CS2910
12 July 2019
Python byte lists and strings
Python 2.7
Python 3.x
str bytearray unicode
bytes # ASCII string, array of bytes bytearray # Mutable list of bytes str # Unicode string
Dr. Josiah Yoder

26. CS2911
12 July 2019
Acknowledgement
This course is based on the text
Computer Networking: A Top Down Approach 7th edition Jim Kurose, Keith Ross Addison-Wesley
Dr. Josiah Yoder