1

Dictionaries, Sets, and Comprehension

Intro2CS – week 4 part II

2

Complex Data Types you have seen

StringList

RangeTuple

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Other Complex Data Types in Python

• Dictionary

• Set

ages = {“dov”:12, “shir”:13}

rooms = {212, 215, 367}

4

Dictionaries

5

Phone book application

Accepts a sequence of commands of the following possible forms:– store name number: add this contact to the phone

book– phone name: print the number for this name– exit: exit

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Phone Book

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Sets

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Unique Elements

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MutabilityType Mutable?

String No

Tuple No

List Yes

Dictionary Yes

Set Yes

Frozenset No

Bool, Int, Float No

Rule: Elements in sets and keys in dictionaries must be immutable

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List Comprehension

Mathematical notation: S = { x2 | x = 1…N}

Python Notation:

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List comprehension implementation

l = [exp(i) for i in seq if cond(i)]

l = []for i in seq: if cond(i): l.append(exp(i))

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Factors

Perfect Numbers: [6, 28, 496, 8128]Prime numbers: [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

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Dictionary and Set comprehension

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Substitution CodesSubstitution codes:

abcdefghijklmnopqrstuvwxyzqwertyuiopasdfghjklzxcvbnm

hello itssg

Ceaser’s original code: shift by 3abcdefghijklmnopqrstuvwxyzxyzabcdefghijklmnopqrstuvw

Rot13: shift by 13

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Our implementation

• A code is a dictionary: letter letter

• ceaser_code(shift) code

• Encrypt(plaintext, code) ciphertext

• Decrypt(ciphertext, code) plaintext

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Ceaser Code

jul qvq gur puvpxra pebff gur ebnq? ==> why did the chicken cross the road?