CS 177 Week 11 Recitation Slides 1 1 Dictionaries, Tuples

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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 33 StudentIDStudentName S1Kate S2Brittany S3Alice

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 /18/2015

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 /18/2015

Using Dictionaries in Python 6 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 6 StudentIDStudentName S1Kate S2Brittany S3Alice

Creating Dictionaries 7 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’ 7

Creating Dictionaries 8 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'} 8

KEY in Dictionary vs. INDEX in List 99 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 10 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 11 >>> 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 12 The len function also works on dictionaries; it returns the number of key-value pairs: >>> len(inventory) 3

Dictionary Methods 13 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 14 >>> for key in inventory: print("Value : ", inventory[key]) Value : 525 Value : 430 Value : 217 Value : 312

Dictionary Example 15 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 16 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 >>>

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'} 17

Tuples A tuple consists of a number of values separated by commas, surrounded by ()  (x, y) coordinate pairs  Student records in the database 18

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’)) 19 StudentIDStudentName S1Kate S2Brittany S3Alice

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 20

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 21

Tuples as Return Values def 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]

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’} 23

QUESTIONS? 24