CS 111: Program Design I Lecture 5: Strings (cont.) & heading to Caesar Robert H. Sloan & Richard Warner

University of Illinois at Chicago September 6, 2016

Survey!

n  Survey URL: Available from class Blackboard site

Alternate Problem if not taking survey: Evaluate in your head; check with computer when done: 1.  5 ** 2 2.  9 * 5 3.  15 / 12 4.  12 / 15 5.  15 // 12 6.  12 // 15 7.  5 % 2 8.  9 % 5

9.  15 % 12 10.  12 % 15 11.  6 % 6 12.  0 % 7

Review: print() function, types

n  At the end of this code right below:

print(3*5)print("3*5")what will have been printed?

A.3*53*5B.1515C.15 D. 3*53*5 15

Review: Indexing (including negative)

>>>s="Registertovote!">>>len(s)

17>>>s[7]'r'>>>s[17]error!>>>s[len(s)-1]'!'>>>s[-1]'!'

012345678911111110123456

Review of find: This will print?

hi="HelloWorld"hey="Heyworld!"print(hi.find('o'),hey.find('o'))A.  44B.  45C.  55D.  56

find() String method works for substrings too hi="HelloWorld"hey="Heyworld!"print(hi.find('ello'),hey.find('ello'))A.  2-1B.  1-1C.  11D.  22E.  Error

Slicing: Getting part of a string

n  Substring of a string is called slice n  s[m:n] returns characters from index m

(counting from 0 as always) up to but not including character n

cj="ChiefJusticeRoberts"

012345678901234567890

cj[0:5]à'Chief'cj[6:10]à'Just'

Slicing: omission = start/end

cj="ChiefJusticeRoberts"012345678901234567890

cj[:13]à'ChiefJustice'cj[14:]à'Roberts'and sort of silly one: cj[:]à'ChiefJusticeRoberts'

Another slice example

>>>s="Registertovote!">>>s[9:11]'to'>>>s[12:20]error!

012345678911111110123456

Slice can also use negative indices

>>>s="Registertovote!">>>s[len(s)-1]'!'>>>s[-1]'!'>>>s[1:-1]'egistertovote'

012345678911111110123456

Fancy slicing you'll never use

n  You are allowed to give 3 indices, which are interpreted as start:end:step

cj="ChiefJusticeRoberts"012345678901234567890

cj[4:17:3]

Fancy slicing you'll never use

n  You are allowed to give 3 indices, which are interpreted as start:end:step

cj="ChiefJusticeRoberts"012345678901234567890

cj[4:17:3]à'fuib'

Fancy slicing you'll never use except for this one trick for Homework 2 n  A weird way to reverse a string s (when we

haven't yet talked about loops) is s[::-1] q  Give me the whole string, stepping 1 backwards

each time!

n  >>> firstChief[::-1] n  'yaJ'

Some people objects just never change

n  In Python, integers are immutable q  Cannot assign to integer object q  I.e., cannot write 1 = 0

n  Probably not a surprise

n  Important: strings are immutable too! firstJustice="Jay"firstJustice[0]="H"Illegal;ERROR!

But Professor

n  Can write n=1n=0n  Is that changing an integer?

But Professor

n  Can write n=1n=0n  Is that changing an integer? n  NO! Changing which object variable name n

is assigned to n  Could reassign (entire object of) firstJusticetoo

Pictures

1

'Jay'

Some immutable objects in memory

n firstJustice

Pictures

0

'Jay'

Some immutable objects in object space (memory)

n firstJustice

n = 0 doesn't change immutable integer 1, just the assignment of a variable to an object

Pictures

0

'Jay'

Some immutable objects in object space (memory)

n firstJustice

firstJustice = 0 doesn't change immutable string 'Jay', just the assignment of variable to an object

Problem with firstJustice[0] = "H" after firstJustice = "Jay"

'Jay'

Some immutable objects in object space (memory)

firstJustice

firstJutice[0] = "H" is illegal because we cannot change the contents of any of the yellow boxes. Those objects are immutable

Coming attractions: Mutable types

n  Numbers, Booleans, and strings are all immutable

n  Python does have some mutable types q  Two very important ones we will see are q  Lists and dictionaries

GETTING READY FOR CAESAR'S INVASION NEXT WEEK

Towards Caesar Cipher

At the heart of the cipher is shifting letters n  Every letter is shifted k to left or right in

alphabet, where combination of k and left or right is encryption key

n  First design decision: Encode key (have inputs to shift function) as q  2 inputs: positive integer and "left" or "right q  1 input, integer, with sign indicating left/right

n  And if 1 input, is negative shift left or right

At the heart of the cipher is shifting letters n  Every letter is shifted k to left or right in

alphabet, where combination of k and left or right is encryption key

n  First design decision: Encode key (have inputs to shift function) as q  2 inputs: positive integer and "left" or "right q  1 input, integer, with sign indicating left/right

n  And if 1 input, is negative shift left or right q  All possible answers seem reasonable; so pick

one and see how it works

At the heart of the cipher is shifting letters n  Every letter is shifted k to left or right in

alphabet, where combination of k and left or right is encryption key

n  First design decision: Encode key (have inputs to shift function) as q  2 inputs: positive integer and "left" or "right q  1 input, integer, with sign indicating left/right

n  And if 1 input, is negative shift left or right q  Our arbitrary answer: + = shift right/up in alphabet:

n  + 3 means shift A to C

Example: +3 Shift

ABCDEFGHIJKLMNOPQRSTUVWXYZà+3SHIFTDEFGHIJKLMNOPQRSTUVWXYZ???What do we do when we run off the end?

Wrap around! ABCDEFGHIJKLMNOPQRSTUVWXYZà+3SHIFTDEFGHIJKLMNOPQRSTUVWXYZABC

How do we implement k shift?

n  We need letter number i transformed to letter i + k, assuming i + k is not past z, and to wrap otherwise

n  How do we find what letter a given character is in the alphabet?

A.  find B.  Indexing C.  String addition D.  Using the remainder function %

Finding letter in alphabet

ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'>>>ALPHABET.find('A')A.  0B.  1C.  ErrorD.  Non-errorotherthan0or1

find() gives position in alphabet

n  Using Python 0 to 25 numbering! n  And adding k to go forward just works

q  if we don't fall off the end n  Trick you will repeatedly as a computer

scientist: remainder (also called mod) operator % works for "wrapping around"

n  E.g., Z forward 3 should give C n  (25 + 3) % 26 à 2

q  Recall ALPHABET[25] à Z

% for rotating, wrapping around

n  Wikipedia and others describer Caesar cipher as using shift of letters

n  Computer scientists often speak of rotation n  In your head, think of 26 letters of alphabet in

a circle not in a line

You have the pieces of shift_letter

defshift_letter(character,k):'''Returnsashiftofcharacterkpositionsrightfork>0,wrappingaroundifpastz'''ALPHABET="ABCDEFGHIJKLMNOPQRSTUVWXYZ"position=ALPHABET.find(character)returnALPHABET[(position+k)%len(ALPHABET)]

But we need some more Python

n  How do we q  Handle left shifts

n  Julius Caesar used a shift left of 3 n  And need opposite direction shift to decrypt

q  Encrypt a whole string, such as "ATTACK" or "RETREAT" instead of just 1 character

q  Handle mixed case and spaces, as in "ATTACK AT DAWN"

Types: Booleans

>>>2+2==4True>>>'cat'=='cat'True>>>2==4False>>>2!=4True

Note operator is == (two = symbols)

String Boolean Operators

2 symmetric arguments: == equals != is not equal to 2 arguments; order matters: in tells if lhs is substring of rhs

e.g., "c" in "cat" à true

Unary operator: not negates (flips True/False)

Just FYI: Later in the semester

Also > greater than >= greater than or equal to < less than <= less than or equal to

Which of the following will evaluate to True after first line? x="ATTACKATDAWN"A.  "AT"==xB.  "AT"inxC.  x==6D.  x!=3E.  Morethanoneoftheabove

Why Booleans?

n  To take one action if some condition holds.

n  E.g., if message m to encrypt has length 1 we can use the function that just shifts 1 letter to encrypt it with Caesar