Programming for Engineers in Python

1. Programming for Engineers in Python Recitation 13

2. Plan • Error detection • Luhn Algorithm • RAID • Data structures • Queue • Stack • Nested lists • Matrices • Recursion • Even and Odd • Recursive directory tree walk

3. Teaching Survey • Please answer the teaching survey: https://www.ims.tau.ac.il/Tal/ • This will help us to improve the course • Deadline: 4.2.12

4. Checksums: Check Digit • A check digit is a form of redundancy check used for error detection • It consists of a single digit computed from the other digits in the message. • Detect simple errors in the input of a series of digits • Detect a single mistyped digit or some permutations of two successive digits. • http://en.wikipedia.org/wiki/Check_digit

5. LuhnAlgorithm – “mod 10” Algorithm • Simple algorithm • Used to validate: • Credit Card numbers • IMEI numbers (GSM SIM card identifiers) • Israeli ID Cards • Checks for mistyping errors – not for forgery! • Designed by Hans Peter Luhnwhile working in IBM during the 1950s • http://en.wikipedia.org/wiki/Luhn_algorithm

6. Luhn: Calculating the check digit • Double every second digit, starting from the last • Sum the digits of the doubling product • Sum the other digits as well • Multiply results by 9 (67*9=603) • Return results mod 10 (603%10=3)

7. Luhn: Checking number validity • Do the same process, add the check digit, and the result should be 0 mod 10 • Code: Luhn.py

8. XOR – Exclusive Or • Logic operator on 2 arguments • “One or the other but not both“ • Results in True: • If one of the arguments is True and one if False • Returns False: • If both arguments are True or both arguments are False • Examples: • I’m happy XOR I’m sad • It’s raining XOR it’s not raining • http://en.wikipedia.org/wiki/Exclusive_or

9. XOR – Exclusive Or • “One or the other but not both“ • Bitwise operation (Python ^): • Returns 1 if the bits are different, 0 if bits are identical • 1 XOR 1 = 0 • 1 XOR 0 = 1 • 0 XOR 1 = 1 • 0 XOR 0 = 0 • 1110 XOR 1001 = 0111 • Equivalent to addition without carry • Useful to calculate parity bit: • 1 XOR 1 XOR 0 XOR 1 XOR 0 = 1

10. RAID Parity • RAID = redundant array of independent/inexpensive disks • Storage unit combining multiple drives • One drive serves as the parity drives and allows the recovery of lost data • http://en.wikipedia.org/wiki/RAID#RAID_Parity

11. RAID Parity Data is written to drives #1-4 Drive #6 is updated to store the parity of drives #1-4: 00101010 XOR 10001110 XOR 11110111 XOR 10110101 = 11100110

12. RAID Parity When drive #3 fails, the data can be recovered by using the same parity operation with drives #1,2,4,6 00101010 XOR 10001110 XOR 11100110 XOR 10110101 = 11110111 Recovered data is stored in drive #5, the Hot Spare When drive #3 is fixed or replaced, it will be the new Hot Spare

13. RAID in Python • We will see a simplistic implementation • Our ‘RAID’ is a matrix of integers • We will be able to declare a matrix row as ‘faulty’ without losing its data • Code: RAID.py

14. Data Structure: Queue • A Queue is a FIFO data structure • Items enter in the bottom of the queue and are served from the top of the queue • Like standing in line at the bank • Useful for many applications: • Phone calls queue in call center or helpdesk • Passing “tasks” from GUI to computing process • Code: Queue.py

15. Queue Class classQueue: def__init__(self): self.data = [ ] defpush(self, item): self.data.append(item) defpop(self): returnself.data.pop(0)

16. Data Structure: Stack • A Stack is a LIFO data structure • Items enter in the top of the queue and are served from the top of the queue • Like a pile of dirt clothes… • Useful for many applications: • Parsing hierarchical data structures like XML or HTML • Towers of Hanoi • Quicksort • All we do is change the pop method of Queue!

17. Stack Class classStack: … defpop(self): returnself.data.pop(-1)

18. Flatten Lists • Assume that we have a hierarchal categorization of objects. For example: Cherry Red Fruit Strawberry Food Yellow Banana Milki Milk Milkshake

19. Flatten Lists • This hierarchy is represented in nested lists as: ['Food', ['fruit', ['Red','[Cherry','Strawberry']], ['Yellow',['Banana']], ['Milk', ['Milki','Milkshake']] ] • We want to flatten this structure so that all items appear in one list ['Food', 'fruit', 'Red', 'Cherry', 'Strawberry', 'Yellow', 'Banana', 'Milk', 'Milki', 'Milkshake']

20. Flatten Lists - Code defflatten(lst): printlst flat_lst = [ ] forx inlst: ifisinstance(x, list): flat_lst.extend(flatten(x)) else: flat_lst.append(x) returnflat_lst

21. Nested Lists • A list of lists • Example – matrix • Matrix multiplication: defmult(A,B): n = len(A) k = len(B) m = len(B[0]) iflen(A[0]) != k: raiseValueError("Matrices must be of appopriate sizes") C = [ [None]*m for x inrange(n)] for i inrange(n): for j inrange(m): C[i][j] = sum( [ A[i][x]*B[x][j] for x inrange(k) ] ) return C

22. defprint_matrix(M): for row in M: for entry in row: print entry, print print Nested Lists >>> A = [ [1,1], [1,0] ] >>> B = [ [0,1], [1,1] ] >>> print_matrix(A) 1 1 1 0 >>> print_matrix(B) 0 1 1 1 >>> print_matrix(mult(A,B)) 1 2 0 1

23. Simple Recursion Example • Check if a number is even or odd defeven(x): ifx == 0: returnTrue else: returnodd(x - 1) defodd(x): ifx == 0: returnFalse else: returneven(x - 1) http://en.wikipedia.org/wiki/File:Screenshot_Recursion_via_vlc.png http://paulbutler.org/archives/tail-recursion-in-python/

24. Simple Recursion Example >>> even(100) True >>> even(101) False >>> even(1000) RuntimeError: maximum recursion depth exceeded • This was expected…

25. Recursion – find files • We want to find all mp3 files on the disk • We’ll do a recursive search on the disk for such files • First some os commands: • os.listdir(dir): returns all files and directories in the directory dir • os.path.isdir(dir): returns True is dir is a name of a directory, False otherwise • os.path.join: joins a list of directory names and a file name to a path, separating the names with a separator suitable for this OS Windows: c:\examples\python Unix: /examples/python

26. Recursive search on directory tree • Recursion base: files • If it’s an mp3 file, we return its name • Otherwise we don’t • Recursion step: directories • We call the function for each subdirectory and merge the results http://www.mediacollege.com/internet/images/tree.gif

27. Recursive search on directory tree • Recursion base deffind_files(location, postfix=‘.mp3’): if not isdir(location): # not a directory – a file iflocation.endswith(postfix): return [location] else: return [ ]

28. Recursive search on directory tree • Recursion step else: results = [] for name inlistdir(location): new_path = join(location, name) results += find_files(new_path, postfix) return results • We’ll now run the function on this computer • Code: find_files.py

29. Exam Stuff • Preparation class • Yoav: Wed. 22.2, 13-15, Room 102 TOCHNA • Noga: Thu. 23.2, 10-12, Room 020 Auditorium • Send questions via email before 20.2 • We will go over the sample exam • Exam material – everything from lectures, recitations, exercises • You may bring a double sided A4 page, hand-written by yourself, no photo-copying – write on it whatever you want • This will be your very own Python reference in the future