Strings Digital Medicine I Lists, strings, loops Repetition - - PowerPoint PPT Presentation

Departement Informatik

Digital Medicine I

Lists, strings, loops

Hans-Joachim Böckenhauer Dennis Komm

Autumn 2020 – October 15, 2020

Strings

Repetition Strings

Strings are “lists of characters” (there are differences) Characters correspond (mostly) to keys on keyboard Strings are written in quotation marks Access to single characters using brackets String word = "HELLO WORLD"

word[0] is first character word[1] is second character

. . .

word[len(word)-1] is last character

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Strings – Operators

Addition Zahlen: Arithmetische Operation Listen und Strings: Zusammenfügen (bei Strings existiert kein append)

daten = [1, 4, 6] daten2 = daten + [5] satz = "Guten Tag" satz2 = satz + ", Urs" satz3 = satz2 + "."

Multiplikation Zahlen: Arithmetische Operation Listen und Strings: Zusammenfügen

daten = [2, 3] * 5 satz = "HA" * 3

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Characters – The Unicode Table

0–18 19–37 38–56 57–75 76–94 95–113 114–127 Dec. Char. Dec. Char. Dec. Char. Dec. Char. Dec. Char. Dec. Char. Dec. Char. NUL 19 DC3 38 & 57 9 76 L 95 _ 114 r 1 SOH 20 DC4 39 ’ 58 : 77 M 96 ‘ 115 s 2 STX 21 NAK 40 ( 59 ; 78 N 97 a 116 t 3 ETX 22 SYN 41 ) 60 < 79 O 98 b 117 u 4 EOT 23 ETB 42 * 61 = 80 P 99 c 118 v 5 ENQ 24 CAN 43 + 62 > 81 Q 100 d 119 w 6 ACK 25 EM 44 , 63 ? 82 R 101 e 120 x 7 BEL 26 SUB 45

• 64

@ 83 S 102 f 121 y 8 BS 27 ESC 46 . 65 A 84 T 103 g 122 z 9 HT 28 FS 47 / 66 B 85 U 104 h 123 { 10 LF 29 GS 48 67 C 86 V 105 i 124 | 11 VT 30 RS 49 1 68 D 87 W 106 j 125 } 12 FF 31 US 50 2 69 E 88 X 107 k 126 ~ 13 CR 32 SP 51 3 70 F 89 Y 108 l 127 DEL 14 SO 33 ! 52 4 71 G 90 Z 109 m . . . 15 SI 34 "’ 53 5 72 H 91 [ 110 n 16 DLE 35 # 54 6 73 I 92 \ 111

• 17

DC1 36 $ 55 7 74 J 93 ] 112 p 18 DC2 37 % 56 8 75 K 94 ˆ 113 q Digital Medicine I – Lists, strings, loops Autumn 2020 Böckenhauer, Komm 3 / 40

Characters – The Unicode Table

Use functions ord() and chr()

• rd(x) returns position of character x in Unicode table

chr(y) returns character at position y in Unicode table

x = input("Enter a character: ") print("The character", x, "is at position", ord(x))

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Exercise – Print Characters

Write a program that

• utputs the first 26 uppercase letters

uses a for-loop to this end Recall The letter A is located at position 65 in the Unicode table

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Exercise – Print Characters

for i in range(65,91): print(chr(i))

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Caesar Encryption Insecure channel

Gallia est omnis divisa in partes tres, quarum unam incolunt Belgae, aliam Aquitani, tertiam, qui ipsorum lingua Celtae, nostra Galli appellantur. Digital Medicine I – Lists, strings, loops Autumn 2020 Böckenhauer, Komm 7 / 40

Symmetric Encryption

Sender Recipient

plaintext Encryption ciphertext Communication medium

(messenger, internet, . . . )

ciphertext Decryption plaintext

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Caesar Encryption

Situation Parties A and B want to communicate over an insecure channel, this time using Caesar encryption Shared key k as number between 1 and 25

A encrypts message by adding k to each character A sends encrypted message to B B decrypts message by subtracting k from each character

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Caesar Encryption

Shift characters by fixed value k by adding k Example A B C D E F G H I J K L M W X Y Z A B C D E F G H I N O P Q R S T U V W X Y Z J K L M N O P Q R S T U V Plaintext: HELLO WORLD Ciphertext: DAHHK SKNHZ

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Caesar Encryption

• 1. Entered letter is Unicode character between A and Z

A B . . . W X Y Z 65 66 . . . 87 88 89 90

• 2. Subtract 65 so that the result is between 0 and 25

A B . . . W X Y Z 1 . . . 22 23 24 25

• 3. Now add key (for instance, 3) and compute modulo 26

A B . . . W X Y Z 3 4 . . . 25 1 2

• 4. Finally add 65 to the result

A B . . . W X Y Z 68 69 . . . 90 65 66 67

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Euclidean Division (Modulo Operation)

Using “%, ” we obtain the residue of the integer division Analogously, “//” the part before the decimal point

10 % 3 = 1,

because 9 = 3 · 3

10 % 4 = 2,

because 8 = 4 · 2

11 // 5 = 2,

because 10 = 5 · 2

23 // 4 = 5,

because 20 = 4 · 5

12 % 3 = 0,

because 12 = 4 · 3 If x % y == 0, x is divided by y

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Exercise – Caesar Encryption

Write a program that runs through a given string decrypts each letter with a key k tries out each key k uses the following formula e = (v − 65 − k) % 26 + 65 Decrypt the ciphertext

DLUUQLTHUKSHBAOLBYLRHYBMANPIALZZJOVNNP

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Exercise – Caesar Encryption

for k in range(0, 26): for item in ciphertext: print(chr((ord(item) - 65 - k) % 26 + 65), end="") print() for k in range(0, 26): for i in range(0, len(ciphertext)): print(chr((ord(ciphertext[i]) - 65 - k) % 26 + 65), end="") print()

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Changing the Step Size

Loops over Lists – Larger Steps

Traverse a list with steps of length 2

data = [5, 1, 4, 3] for i in range(0, len(data), 2): print(data[i])

Output All elements at even positions from 0 up to at most len(data) are output

➯ 5,4

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The Syntax of range

for i in range(start, end, step)

Iteration over all positions from start up to end-1 with step length of step Shorthand notation

for i in range(start,end) ⇐ ⇒ for i in range(start,end,1)

Another shorthand notation

for i in range(end) ⇐ ⇒ for i in range(0, end)

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Improvement of Caesar Encryption

Use two keys alternatingly for even and odd positions

k = int(input("First key: ")) l = int(input("Second key: ")) x = input("Text (only uppercase, even length): ") for i in range(0, len(x), 2): print(chr((ord(text[i])-65 + k) % 26 + 65), end="") print(chr((ord(text[i+1])-65 + l) % 26 + 65), end="") print()

Still Caesar encryption remains insecure

➯ Project 1

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Logical Values

Boolean Values and Relational Operators Boolean Values and Variables

Boolean expressions can take on one of two values F or T F corresponds to “false” T corresponds to “true”

George Boole [Wikimedia]

Boolean variables in Python represent “logical values” Domain {False, True} Example

b = True # Variable with value True

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Relational Operators

x < y

(smaller than)

x >= y

(greater than)

x == y

(equals)

x != y

(unequal to) number type × number type → {False, True}

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Logical Values

Boolean Functions and Logical Operators Boolean Functions in Mathematics

Boolean function

f : {F, T}2 → {F, T}

F corresponds to “false” T corresponds to “true”

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a ∧ b

“logical and”

f : {F, T}2 → {F, T}

F corresponds to “false” T corresponds to “true” a b a ∧ b

F F F F T F T F F T T T

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Logical Operator and

a and b

(logical and)

{False, True} × {False, True} → {False, True} n = -1 p = 3 c = (n < 0) and (0 < p) # c = True

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a ∨ b

“logical or”

f : {F, T}2 → {F, T}

F corresponds to “false” T corresponds to “true” a b a ∨ b

F F F F T T T F T T T T

The logical or is always inclusive: a or b or both

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Logical Operator or

a or b

(logical or)

{False, True} × {False, True} → {False, True} n = 1 p = 0 c = (n < 0) or (0 < p) # c = False

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¬b

“logical not”

f : {F, T} → {F, T}

F corresponds to “false” T corresponds to “true” b ¬b

F T T F

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Logical Operator not

not b

(logical not)

{False, True} → {False, True} n = 1 a = not (n < 0) # a = True

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Logical Values

Precedences Precedences

not b and a

• (not b) and a

a and b or c and d

• (a and b) or (c and d)

a or b and c or d

• a or (b and c) or d

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Precedences

7 + x < y and y != 3 * z or not b 7 + x < y and y != 3 * z or (not b)

The unary logical operator not

➯ provides a stronger binding than binary arithmetic operators ➯ These bind stronger than relational operators ➯ and these bind stronger than binary logical operators

Some parentheses on the previous slides were actually redundant, but should still be used

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DeMorgan Rules

not (a and b) == (not a or not b) not (a or b) == (not a and not b)

Examples (not black and not white) == not (black or white) not (rich and beautiful) == (poor or ugly)

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Application – either . . . or (XOR)

(a or b) and not (a and b)

a or b, and not both

(a or b) and (not a or not b)

a or b, and one of them not

not (not a and not b) and not (a and b)

not none and not both

not ((not a and not b) or (a and b))

not: both or none

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Control Structures

Control Flow

So far. . . Up to now linear (from top to bottom)

for loop to repeat blocks x = int(input("Input: ")) for i in range(1, x+1): print(i*i)

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Control Structures

Selection Statements

Selection Statements

Implement branches

if statement if-else statement if-elif-else statement (later)

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if Statement

if condition:

statement

x = int(input("Input: ")) if x % 2 == 0: print("even")

If condition is true, then statement is executed statement:

arbitrary statement body of the if-Statement

condition: Boolean expression

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if-else Statement

if condition:

statement1

else:

statement2

x = int(input("Input: ")) if x % 2 == 0: print("even") else: print("odd")

If condition is true, then statement1 is executed,

• therwise statement2 is executed

condition: Boolean expression statement1: body of the if-branch statement2: body of the else-branch

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Layout

x = int(input("Input: ")) if x % 2 == 0: print("even") else: print("odd")

Indentation Indentation

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Control Structures

while Loops while Loops

while condition:

statement

Indentation

statement:

arbitrary statement body of the while loop

condition: Boolean expression

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while Loops

while condition:

statement condition is evaluated True: iteration starts

statement is executed False: while loop ends

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while Loops

s = 0 i = 1 while i <= 2: s = s + i i = i + 1 i

condition

s i = 1

true

s = 1 i = 2

true

s = 3 i = 3

false

s = 3

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Incrementation of Variables

Use simplified syntax for changing values of variables

n = n + 1 is written as n += 1 n = n + i is written as n += i n = n - 15 is written as n -= 15 n = n * j is written as n *= j n = n ** 4 is written as n **= 4

. . .

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The Jump Statements break

break

Immediately leave the enclosing loop Useful in order to be able to break a loop “in the middle”

s = 0 while True: x = int(input("Enter a positive number, abort with 0: ")) if x == 0: break s += x print(s)

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