Lecture Notes

Examples below come from Chapter 9 Video

List – a linear collection of ordered values.

Dictionary – an assortment of unordered labeled values, similar to a database

Dictionaries

Different names for Dictionaries:

• Associative Arrays – Perl/Php
• Properties of Map or HashMap – Java
• Property Bag – C#/.Net

An example of working with Dictionaries (9A):

purse = dict()  

purse['money'] = 12  
purse['candy'] = 3  
purse['tissues'] = 75  
print purse  

Output:
{‘money’:12,’tissues’:75,’candy’:3} The output shows both the key(‘money’) and the value (’12’). The value can be modified, like with lists.
For example (9B):

purse['candy'] = purse['candy'] + 2  

Will change the value of ‘candy’ to 5.

Dictionaries are like Lists except that they use keys(‘money’) instead of numbers to look up values. (Lists are ordered, Dictionaries are not and so rely on keys for lookup)

Dictionary literals (constants) use curly braces {} and have a list of key : value pairs. As usual for dictionaries, the list is unordered.

Using Dictionary to tally/count items

The “in/not in” operators can be used to see if a key exists in the dictionary – without resulting in a traceback error.

Example (9C):

counts = dict()  
names = ['csev','owen','csev','zqian','cwen']  
for name in names:  
  if name not in counts:  
    counts[name]= 1  
  else:  
    counts[name] = counts[name] + 1  
print counts  

The above operation is so common, that the method ‘get’ does it for us.

print counts.get(name,0)  

Performs the same operation as:

if name in counts:  
  print counts[name]  
else:  
  print 0  

So, example 9C can be simplified down to (9D):

counts = dict()  
names = ['csev','owen','csev','zqian','cwen']  
for name in names:  
  counts[name] = counts.get(name,0) + 1  
print counts  

Few more methods to iterate a dictionary using example 9C’s dictionary

Iterate over keys: counts.keys():

for name in counts.keys():
  print name

Iterate over the values in a dictionary: counts.values():

for occurrence in counts.values():
  print occurrence

Iterate over the elements ((key, value) pairs) in a dictionary : counts.items():

for name,occurrence in counts.items():
  print name, occurrence

Exercises previously posted here have been moved and can be accessed through the Chapter 9 Exercises link below.

Chapter 9 Exercises

The above page is intended as a place for students to work out solutions and answers to the exercises from the textbook. Please do not post answers to exercises that are actual graded assignments.

More Resource Topics

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Python Dictionaries – https://www.jeffknupp.com/blog/2015/08/30/python-dictionaries/