Dictionaires vs lists Creating Dictionaries Dictionaries Methods Dictionaries Visualizing Insertionordered collection of Python objects Python gt36 What is a key What is a value ID: 904536
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Slide1
CSE 231
Lab 8
Slide2Topics to cover
Dictionaires
vs
lists
Creating Dictionaries
Dictionaries
–
Methods
Slide3Dictionaries - 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
Slide4Dictionaries vs Lists
Slide5What happens?
Given a dictionary, D, where:
D[“key”] = “value”
print(D[“key”])
“value”
Slide6Create 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"
Slide7Patterns
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
Slide8Patterns
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
Slide9Dictionaries
–
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.
Slide10Additional 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}
Slide11Iteration
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():
Slide12What 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
Slide13What happens?
Given a dictionary D,
D = {}
Print(D[‘a’])
KeyError
Slide14Additional 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']
Slide15Other 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}