1. COSC 1306 COMPUTER SCIENCE AND PROGRAMMING
Jehan-François Pâris
Fall 2016 1 1

2. THE ONLINE BOOK CHAPTER XII DICTIONARIES

3. Motivation Suppose we have a list of phone numbers such as:
[[ 'CS', ' '],[ 'UHPD' : ' ']]
or inventory of soda cans such as:
[['Coke', 12], ['Diet Coke', 8], ['Coke Zero', 2]]
Finding the phone number of UHPD or the number of cans of 'Coke Zero' we have in stock requires searching through the list
Too slow

4. Dictionaries (I)
Store pairs of entries called items { 'CS' : ' ', 'UHPD' : ' '}
Each pair of entries contains
A key
A value
Key and values are separated by a colon
Pairs of entries are separated by commas
Dictionary is enclosed within curly braces

5. Dictionaries (II) { 'CS' : '713-743-3350', 'UHPD' : '713-743-3333'}
Can also be built step by step:
number = {}
number['CS'] = ' '
number['UHPD'] = ' '

6. Accessing values
If we have age = {'Alice' : 25, 'Bob': 28} then age['Alice'] is 25 and age['Bob'] is 28

7. A problem What is printed by the following program?
mydict = {"cat":12, "dog":6, "horse":23}
print(mydict["dog"]) 12 6 23
Error

8. Dictionaries are mutable
>>> age = {'Alice' : 25, 'Bob' : 28}
>>> saved = age
>>> age['Bob'] = 29
>>> age {'Bob': 29, 'Alice': 25}
>>> saved {'Bob': 29, 'Alice': 25}

9. Keys must be unique
>>> age = {'Alice' : 25, 'Bob' : 28, 'Alice' : 26}
>>> age {'Bob': 28, 'Alice': 26}

10. Operations on dictionaries
>>> age = {'Bob': 28, 'Alice': 26}
>>> len(age) 2
>>>age['Bob'] += 1
>>>age
{'Bob': 29, 'Alice': 26}
>>>age['Bob'] + age ['Alice'] 55

11. Adding contents age = {} age['Alice'] = 25
Cannot do the same with lists
alist=[]
alist[10]= 5 # NO NO

12. Global dictionary methods
Arguments
Description
keys()
none
Returns a view of the keys in the dictionary
values()
Returns a view of the values in the dictionary
items()
Returns a view of the items (KV pairs) in the dictionary

13. Examples >>> age = {'Alice' : 25, 'Carol': 'twenty-two'}
>>> age.items() dict_items([ ('Alice', 25), ('Carol', 'twenty-two')])
>>> age.keys() dict_keys([ 'Alice', 'Carol'])
age.values() dict_values([25, 'twenty-two'])

14. Views and lists (I) To represent mydictionary as a list of KV tuples
mydictionary.items()
To get the keys
mydictionary.keys()
To get the values
mydictionary.values()
Cannot index these three representations
They are views, no lists

15. Views and lists (II) Can convert view into lists of tuples
list() function
age = {'Alice' : 25, 'Carol': 22}
>>> age.items()
dict_items([('Carol', 22), ('Alice', 25)])
>>> list(age.items())
[('Carol', 'twenty-two'), ('Alice', 25)]

16. More dictionary methods
Arguments
Description
get()
key
Returns value associated with the key or none otherwise
update()
KV pair
Updates/adds a KV pair
popitem()
Returns value associated with the key and removes the KV pair from the list
none
Same for arbitrary KV pair

17. Updating dictionaries
>>> age = {'Alice': 26 , 'Carol' : 22}
>>> age.update({'Bob' : 29}) >>> age {'Bob': 29, 'Carol': 22, 'Alice': 26}
>>> age.update({'Carol' : 23}) >>> age {'Bob': 29, 'Carol': 23, 'Alice': 26}
>>> age['Bob'] = 30 >>> age {'Bob': 30, 'Carol': 23, 'Alice': 26}

18. Returning a value >>> age = {'Bob': 29,'Liz': 23,'Ann': 26}
>>> age.get('Bob') 29
>>> age['Bob'] 29

19. Removing a specific item (I)
>>> a = {'Ann' : 24, 'Liz' : 'twenty-two'}
>>> a {'Liz': 'twenty-two', 'Ann': 24}
>>> a.pop('Liz') 'twenty-two'
>>> a {'Ann': 24}

20. Removing a specific item (II)
>>> a.pop('Alice') 26
>>> a {}
>>>

21. Remove a random item
>>> age = {'Bob': 29, 'Liz': 22, 'Ann': 27}
>>> age.popitem() ('Bob', 29)
>>> age
{'Liz': 22, 'Ann': 27}
>>> age.popitem() ('Liz', 23)
>>> age {'Ann': 27}

22. Traversing a dictionary
Suppose we have an inventory such as:
cans = {'Coke':12, 'Diet Coke':8, 'Coke Zero' : 2}
How can we compute the total number of cans we have?
Must go through all items in the dictionary
Can use
for akey in cans.keys()
to go though all keys

23. Example cans = {'Coke':12, 'Diet Coke':8, 'Coke Zero':2} total = 0
for akey in cans.keys() :
total += cans[akey]
print(total, 'cans of soft drink')
will print
22 cans of soft drink

24. Algebra matrices A matrix such as 𝐴= 0 1 2 10 11 12 20 21 22
can be represented as a list of lists
a=[[0, 1, 2],[10, 11, 12],[20, 21, 22]]
with row and column indices starting at zero
The element a[i][j] of list a then corresponds to the element 𝑎 𝑖𝑗 of the matrix

25. Sparse matrices (I) Consider a matrix like D= 1 0 0 0 1 0 0 0 1
that has few non-zero elements
The conventional Python representation
d = [[1, 0, 0], [0, 1, 0], [0, 0, 1]]
wastes a lot of space
Becomes a problem for very large matrices

26. Sparse matrices (II) We could use a dictionary
that would only contains the non-zero elements of the matrix
We would then use d[(i, i)] instead of d[i][i] to access the non-zero elements of the diagonal matrix

27. How to access all elements
d[(i, i)] only works for non-zero elements on the diagonal
Use alternate version of get() method
dict.get(key, alternate)
where alternate specifies the value to return if key is not in dictionary dict
d.get((i, j), 0)
works for all elements of the matrix