CS 177 Week 11 Recitation Slides

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Dictionaries, Tuples

Announcements

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Why Do We Use Dictionary 1/3

• Suppose we have the following table, that associates an identifier to a student name:

• This association is a relationship between the identifier and the student name

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StudentID StudentName

S1 Kate

S2 Brittany

S3 Alice

Why Do We Use Dictionary 2/3

Can we represent this relationship using lists? Yes, we can. Look at the following:

– myList = [[‘S1’, ‘Kate’], [‘S2’, ‘Brittany’], [‘S3’, ‘Alice’]]

– However, we have to access the elements of the list above by using integer indexes:

– >>> myList[0]– ['S1', 'Kate']– >>> myList[0][1]– 'Kate‘– >>> myList[0][0]– 'S1'

CS177 - Spring 2011 404/21/23

Why Do We Use Dictionary 3/3

But we can not “ask” our Python list: please give me back the name of the student associated to the identifier ‘S1’

If we need to do that, we have to use a new Python construct: the dictionary

CS177 - Spring 2011 504/21/23

Using Dictionaries in Python

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• Python dictionaries store data element as a pairs of key/value data in the form of key:value

• “associative lists” surrounded by { }, rather than [ ] in lists

KEY VALUE

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StudentID StudentName

S1 Kate

S2 Brittany

S3 Alice

Creating Dictionaries

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• We can store the data in the above table by using python dictionary as follows

>>> StuDict = {‘S1’:‘Kate’, ‘S2’:‘Brittany’, ‘S3’:‘Alice’}• We can also do the following, >>> StuDict = {}

>>> StuDict[‘S1’] = ‘Kate’ >>> StuDict[‘S2’] = ‘Brittany’ >>> StuDict[‘S3’] = ‘Alice’

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Creating Dictionaries

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• We can also create a dictionary from a list

>>> myList = [[‘S1’, ‘Kate’], [‘S2’, ‘Brittany’], [‘S3’, ‘Alice’]]

>>> StuDict = dict(myList)>>> mydict{'S3': 'Alice', 'S2': 'Brittany', 'S1': 'Kate'}

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KEY in Dictionary vs. INDEX in List

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• Type– Key (dictionary): Any immutable type, usually strings or

integers– Index (list): Integers

• An example of a string used as a dictionary key >>> StuDict = {'S1':'Kate', 'S2':'Brittany', 'S3':'Alice'}>>>print (StuDict['S1']) ‘Kate’

• An example of an integer used as a dictionary key>>> StuDict = {1:'Kate', 2:'Brittany', 3:'Alice'}>>>print (StuDict[1]) ‘Kate’

KEY in Dictionary vs. INDEX in List

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• Definition– Key (dictionary): specified by users, not based on ordering – Index (list): provided by system, implicitly based on list ordering

• Example– Dictionary works without ordering>>> StuDict = {‘S1’:‘Kate’, ‘S2’:‘Brittany’, ‘S3’:‘Alice’}>>>print StuDict[‘S3’] ‘Kate’>>> StuDict = {‘S3’:‘Kate’, ‘S2’:‘Brittany’, ‘S1’:‘Alice’}>>>print StuDict[‘S3’] ‘Kate’– List works with implicit ordering:>>>StudList = [[‘S1’, ‘Kate’], [‘S2’, ‘Brittany’], [‘S3’, ‘Alice’]] – We cannot change element’s index in a list

Dictionary Operations

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>>> inventory = {'pears': 217, 'apples': 430, 'oranges': 525, 'bananas': 312}

• Getting an element from the dictionary

>>> inventory['oranges']

525• Remove the entry from the dictionary

>>> del inventory['pears']

>>> print(inventory)

{'apples': 430, 'oranges': 525, 'bananas': 312}

Dictionary Operations

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• The len function also works on dictionaries; it returns the number of key-value pairs:

>>> len(inventory)

3

Dictionary Methods

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• Getting all keys of a dictionary

>>> list(inventory.keys())

['oranges', 'apples', 'pears', 'bananas']

• Getting all values from a dictionary

>>> list(inventory.values())

[525, 430, 217, 312]

Dictionaries and for loops

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>>> for key in inventory:

print("Value : ", inventory[key])

Value : 525

Value : 430

Value : 217

Value : 312

Dictionary Example

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def readFile():

file = open("names.txt", "r")

contents = file.read()

file.close()

lines = contents.split("\n")

phonebook = {}

for line in lines:

items = line.split(":")

phonebook[items[0]] = items[1]

return phonebook

Dictionary Example

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def findPhoneNumber(name):

phonebook = readFile()

for key in phonebook:

if(key.lower().find(name.lower()) != -1):

print("Phone number for ", key, " is", phonebook[key])

>>> findPhoneNumber('parsoN')

Phone number for Andrew Parson is 8806336

>>>

Populating Dictionaries We can populate dictionaries with list

>>>dict([(x, chr(x+97)) for x in range(10)])

{0: 'a', 1: 'b', 2: 'c', 3: 'd', 4: 'e', 5: 'f', 6: 'g', 7: 'h', 8: 'i', 9: 'j'}

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Tuples

A tuple consists of a number of values separated by commas, surrounded by () (x, y) coordinate pairs Student records in the database

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Constructing Tuples

We construct a tuple corresponding to each row of the following table:

>>>Tup1 = (‘S1’, ‘Kate’) >>>Tup2 = (‘S2’, ‘Brittany’) >>>Tup3 = (‘S3’, ‘Alice’)

We can also build a tuple of tuples… >>>StuTup = (Tup1, Tup2, Tup3)

>>>print (StuTup) ((‘S1’, ‘Kate’), (‘S2’, ‘Brittany’), (‘S3’, ‘Alice’))

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StudentID StudentName

S1 Kate

S2 Brittany

S3 Alice

Constructing Tuples

Getting an element of a Tuple in the same way we get an element of a list:

>>> StuTup[1]('S2', 'Brittany')>>> StuTup[1][0]'S2'>>> StuTup[1][1]'Brittany'

Tuples are immutable >>>StuTup[0] = ((‘S5’, ‘Jane’)) error message -- no assignment

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Tuple Assignment

A tuple of variables on the left can be assigned values from a tuple on the right: >>> a = 10

>>> b = 20

>>> (a, b) = (b, a)

>>> a

20

>>> b

10

Number of variables on the left and the number of values on the right must be the same! >>> (a, b, c, d) = (1, 2, 3)

ValueError: need more than 3 values to unpack

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Tuples as Return Valuesdef function(a, b):

sum = a + b

product = a * b

return(sum, product)

>>> (a, b) = function(10, 20)

>>> a

30

>>> b

200

>>> result = function(50, 100)

>>> result[1]

5000

>>> result[0]

150

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Now Creating a Dictionary from a List of Tuples

Given a list of tuples as follows

[(‘S1’, ‘Kate’), (‘S2’, ‘Brittany’), (‘S3’, ‘Alice’)] Let’s create a dictionary with the dict function

dict([(‘S1’, ‘Kate’), (‘S2’, ‘Brittany’), (‘S3’, ‘Alice’)]) A tuple in the list is a pair of key:value data in the

dictionary

>>> dict([(‘S1’, ‘Kate’), (‘S2’, ‘Brittany’), (‘S3’, ‘Alice’)])

{‘S1’:‘Kate’, ‘S2’:‘Brittany’, ‘S3’:‘Alice’}

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QUESTIONS?

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