CS 177 Week 11 Recitation Slides

1. CS 177 Week 11 Recitation Slides Dictionaries, Tuples 1

2. Announcements 2

3. 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 3

4. 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

5. 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

6. Using Dictionaries in Python • 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 • KEYVALUE 6

7. Creating Dictionaries • 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

8. Creating Dictionaries • 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

9. KEY in Dictionary vs. INDEX in List • 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’ 9

10. KEY in Dictionary vs. INDEX in List • 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 10

11. Dictionary Operations • >>> 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} 11

12. Dictionary Operations • The len function also works on dictionaries; it returns the number of key-value pairs: • >>> len(inventory) • 3 12

13. Dictionary Methods • 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] 13

14. Dictionaries and for loops • >>> for key in inventory: • print("Value : ", inventory[key]) • Value : 525 • Value : 430 • Value : 217 • Value : 312 14

15. Dictionary Example • 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 15

16. Dictionary Example • 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 • >>> 16

17. 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'}

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

19. 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’))

20. 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

21. 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

22. 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] 150

23. 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’}

24. QUESTIONS? 24