CSE 231 Lab 8 Topics to cover - PowerPoint Presentation

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CSE 231

Lab 8

Topics to cover

Dictionaires

vs

lists

Creating Dictionaries

Dictionaries

–

Methods

Dictionaries - Visualizing

Insertion-ordered collection of Python objects (Python >=3.6)

What is a key? What is a value?

Should be immutable

Any objects is allowed

Dictionaries vs Lists

What happens?

Given a dictionary, D, where:

D[“key”] = “value”

print(D[“key”])

“value”

Create and update a dictionary

Are dictionaries mutable?

Creating a Dictionary

M = {}

M = dict() M = { 200: "EE", 100: "ME"}Adding/updating a value in a DictionaryM[500] = "CS"

Patterns

Ints

:

x = 0 # initialize at zero

x += 1 # increment the counterStrings:s = ‘’ # initialize with empty strings += ch # concatenate characters Lists:L = [] # initialize with empty listL.append

(value) # append values to the list

Patterns

Dictionaries:

D = { } # initialize with

empty dictionary

S = “aabacdbacd” # we have a string, we want to count all the charactersfor ch in S: if

ch in D: # check to see if the key exists in the dictionary D[ch

] += 1 # increment the value if it exists else: D[ch] = 1 # set the value to 1 if it doesn’t exist

Dictionaries

–

Methods

Use this one most of the time:

.items() # Returns a set-like object of all the key-value pairs.Other options:

.keys() # Returns a set-like object of all the keys.

.values() # Returns a set-like pbject of all the values.

Additional Useful Methods & functions

D =

M.get

( 200 )

returns the value if key exists, else returns NoneE = M.pop( "Mike", None )pops (and returns) the value if the key exists, returns the second argument otherwisedel M[600]

deletes if the key exist, KeyError

if it doesn'tD = dict(L) # assuming L = [('a',1),('b',2),('c',3)]D = {'a': 1, 'b': 2, 'c': 3}

Iteration

for key, value in

M.items

(): # use most of the time

for key in M.keys():for key in M: # same as M.keys()for value in M.values():

What happens?

Given a dictionary D,

D = {“one” : 1, “two” : 2, ”three” : 3}

for

key,value in D.items

():print(

key,value)

one 1

two 2

three 3

What happens?

Given a dictionary D,

D = {}

Print(D[‘a’])

KeyError

Additional useful methods and function

D = {“one” : 1, “two” : 2, ”three” : 3}

list(D)

# returns a list of all the keys used in the dictionary

['one', 'two', 'three']sorted(D)

 # returns a sorted list of all the keys in the dictionary ['one', 'three', 'two']

Other useful methods and function

D = {“one” : 1, “two” : 2, ”three” : 3}

sorted(

D.items

()) # returns a sorted list of dictionary's (key, value) tuple pairs

[('one', 1), ('three', 3), ('two', 2)]dict

(sorted(D.items())) # in insertion order (if you want it sorted, get the list of tuple pairs and just use sorted)

{'one': 1, 'three': 3, 'two': 2}