Fundamentals of Programming. Strings. Anil Ada

Transcription

1 Fundamentals of Programming Strings Anil Ada September 16th, 2014

2 TOC week Microsoft is a good friend. Phones and Kinect devices available for term projects. Kinect hack day: Oct 25th Interested in borrowing phones for term projects?

3 TOC week Summer internship interviews in January A special chance to meet Microsoft: 5pm: GHC 5th floor

4 Reminders - Read the course notes (slides are extra) - Read Piazza announcements - Solve on paper before coding - Top-down design. - Debugging: find simplest buggy example, print variables - Be careful about styling: don t add it later. - Come to my office hours. - Quiz tips

5 Basics of Handling Data Builtin Data Types Python name Description Values NoneType absence of value None bool (boolean) Boolean values True, False int (integer) integer values 2 63 to long large integer values all integers float fractional values e.g complex complex values e.g. 1+5j str (string) text e.g. Hello World!...

6 Basics of Handling Data Builtin Data Types Python name Description Values NoneType absence of value None bool (boolean) Boolean values True, False int (integer) integer values 2 63 to long large integer values all integers float fractional values e.g complex complex values e.g. 1+5j str (string) text e.g. Hello World!...

7 Basics of Handling Data In fact, pretty much everything is treated as data. import math def f(): pass print type(f) print type(abs) print type(math) <type 'function'> <type 'builtin_function_or_method'> <type 'module'> f is an object of type function

8 String A sequence (string) of characters. String literals: print USA basketball team wins gold medal. print Hillary Clinton is running for president. print Hillary to Iowa: I m back. print Miley Cyrus gains 0.5 pounds!!!

9 String A sequence (string) of characters. String literals: print USA basketball team wins gold medal. print Hillary Clinton is running for president. print Hillary to Iowa: I m back. print Miley Cyrus \ gains 0.5 pounds!!!

10 String A sequence (string) of characters. String literals: print USA basketball team wins gold medal. print Hillary Clinton is running for president. print Hillary to Iowa: I m back. print Miley Cyrus \ngains 0.5 pounds!!!

11 String A sequence (string) of characters. String literals: print USA basketball team wins gold medal. print Hillary Clinton is running for president. print Hillary to Iowa: I m back. print Miley Cyrus \tgains 0.5 pounds!!!

12 String A sequence (string) of characters. String literals: print USA basketball team wins gold medal. print Hillary Clinton is running for president. print Hillary to Iowa: I m back. print Miley Cyrus gains \\0.5 pounds!!!

13 String A sequence (string) of characters. String literals: print USA basketball team wins gold medal. print Hillary Clinton is running for president. print Hillary to Iowa: I m back. print Miley Cyrus gains \ 0.5\ pounds!!!

14 Manipulating/processing strings String representation in memory Built-in string operations Built-in functions and constants related to strings Built-in string methods String formatting

15 String representation in memory Every data in a computer is represented by numbers (binary numbers) Each character in a string is a number. print ord( a ) print chr(97) print a < b print a < A prints 97 prints a prints True prints False

16 String representation in memory

17 Example Write a function that takes a string as input, and returns the string with every character capitalized. def capitalize(s): result = for c in s: result += touppercase(c) return result def touppercase(c): if ( a <= c <= z ): return chr(ord(c) - (ord( a ) - ord( A ))) return c

18 Manipulating/processing strings String representation in memory Built-in string operations Built-in functions and constants related to strings Built-in string methods String formatting

19 String chopping Indexing G o T a r t a n s! s = Go Tartans! print s[0] length = 11 print s[5], s[length-1], s[3] expression that should evaluate to an integer

20 String chopping Indexing G o T a r t a n s! s = Go Tartans! print s[-1] print len(s) print Yabadabaduuu! [5]

21 s = Go Tartans! String chopping Slicing G o T a r t a n s! print s[3:len(s)] print s[0:len(s)] print s[3:] print s[:1] print s[:]

22 s = Go Tartans! String chopping Slicing G o T a r t a n s! print s[0:len(s):2] print s[::] print s[::-1] # print s[0:len(s):1] # prints s reversed (WEIRD!) print s[len(s)-1:0:-1] # doesn t work

23 s = Go Tartans! Strings are immutable!!!!! Slicing G o T a r t a n s! s[3] = t # ERROR s += haha # Works! Why? s = s[:3] + t + s[4:]

24 String chopping Example Given a number from 1 to 12, return first three letters of the corresponding month. e.g. 1 returns Jan, 2 returns Feb, etc... def meaningfulfunctionname(monthnum): months = JanFebMarAprMayJunJulAugSepOctNovDec pos = (monthnum-1) * 3 return months[pos:pos+3]

25 String chopping Example Looping through a string with an index def indexof(char, s): for index in xrange(len(s)): if (s[index] == char): return index return -1

26 String gluing Concatenation print Hello + World +! print Hello World! s = Hello print s World! # ERROR

27 String gluing Repetition print SPAM!!! * 20 print 20 * SPAM!!! print 20 * SPAM!!! * 20

28 Manipulating/processing strings String representation in memory Built-in string operations Built-in functions and constants related to strings Built-in string methods String formatting

29 Built-in functions print len( SPAM * 25) print ord( A ) print chr(85) print str(85) userinput = raw_input( How are you doing? )

30 Built-in constants import string print string.ascii_letters print string.ascii_lowercase print string.ascii_uppercase print string.digits print string.punctuation print string.printable print string.whitespace print \n in string.whitespace (in: built-in operator)

31 Manipulating/processing strings String representation in memory Built-in string operations Built-in functions and constants related to strings Built-in string methods String formatting

32 Built-in string methods Method: a function applied directly on an object print hey you!.capitalize() print hey you!.upper() print HEY YOU!.lower() print hey you!.title() print 314.isdigit() print hey you!.find( you ) print hey hey you!.count( hey ) Hey you! HEY YOU! hey you! Hey You! True 4 2 print LeBron is the real number 23.replace( LeBron, MJ )

33 Example: Cryptography I will cut your throat encryption loru23n8uladjkfb!#@ decryption I will cut your throat

34 Example: Caesar shift Encrypt messages by shifting each letter a certain number of places. Example: shift by 3 a d b e c f... x a y b... A D B E... X A Y B... (other symbols stay the same) Rocks my world Urfvn pb zruog Write methods to encrypt and decrypt messages. (message and shift given as input)

35 Example: Caesar shift def encrypt(message, shiftnum): result = "" for char in message: result += shift(char, shiftnum) return result def shift(char, shiftnum): if (char not in string.ascii_letters)): return char alphabet = string.ascii_lower if (char.islower()) \ else string.ascii_upper shiftnum %= 26 shifted_alphabet = alphabet[shiftnum:] + alphabet[:shiftnum] return shifted_alphabet[alphabet.find(char)]

36 Example: Caesar shift def shift2(char, shiftnum): shiftnum %= 26 if('a' <= char <= 'Z'): if(ord(char) + shiftnum > ord('z')): return chr(ord(char) + shiftnum - 26) else: return chr(ord(char) + shiftnum) elif('a' <= char <= 'z'): if(ord(char) + shiftnum > ord('z')): return chr(ord(char) + shiftnum - 26) else: return chr(ord(char) + shiftnum) else: return char Code repetition

37 Example: Caesar shift def shift3(char, shiftnum): shiftnum %= 26 if(not (('A' <= char <= 'Z') or ('a' <= char <= 'z'))): return char if ( ((char <= 'Z') and (ord(char) + shiftnum > ord('z'))) or ((char <= 'z') and (ord(char) + shiftnum > ord('z'))) ): return chr(ord(char) + shiftnum - 26) else: return chr(ord(char) + shiftnum) Requires a comment

38 Tangent: Public Key Cryptography

39 Tangent: Public Key Cryptography I will cut your throat slkjlsjnb o$vns3a I will cut your throat

40 Tangent: The factoring problem If there is an efficient program to solve the factoring problem can break public key cryptographic systems Fun fact: Quantum computers can factor large numbers efficiently!

41 Tangent: What is a quantum computer? Information processing using quantum physics.

42 Manipulating/processing strings String representation in memory Built-in string operations Built-in functions and constants related to strings Built-in string methods String formatting

43 String formatting team = Steelers numsb = 6 s = The + team + have won + numsb + Super Bowls.

44 String formatting team = Steelers numsb = 6 s = The + team + have won + str(numsb) + Super Bowls. team = Steelers numsb = 6 s = The %s have won %d Super Bowls % (team, numsb) string decimal print The %s have won %d Super Bowls % (team, numsb)

45 String formatting print Miley Cyrus gained %f pounds! % 2**(-5) float Miley Cyrus gained pounds! print Miley Cyrus gained %.2f pounds! % 2**(-5) Miley Cyrus gained 0.03 pounds! print Miley Cyrus gained %10.2f pounds! % 2**(-5) Miley Cyrus gained 0.03 pounds! print Miley Cyrus gained %-10.2f pounds! % 2**(-5) Miley Cyrus gained 0.03 pounds!

46 String formatting print Miley Cyrus gained %-10.2f pounds! % 2**(-5) Miley Cyrus gained 0.03 pounds! % [-] [minwidth] [.precision] type optional