Lesson 16 Dictionaries

Python Mini-CourseUniversity of Oklahoma

Department of Psychology

Lesson 16Dictionaries

5/10/09Python Mini-Course: Lesson 161

Lesson objectives

1. Describe the characteristics of the dictionary data structure in Python

2. Perform basic operations with dictionaries including creation, copying, updating, and traversing

3. Use dictionaries in functions


The dictionary data structure

In Python, a dictionary is mapping between a set of indices (keys) and a set of valuesThe items in a dictionary are key-value pairs


The dictionary data structure

Keys can be any Python data typeBecause keys are used for indexing, they should be immutable

Values can be any Python data typeValues can be mutable or immutable


Creating a dictionary

eng2sp = dict()

print eng2sp

eng2sp['one'] = 'uno'

print eng2sp

eng2sp['two'] = 'dos'

print eng2sp


Creating a dictionary

eng2sp = {'one': 'uno', 'two': 'dos',

'three': 'tres'}

print eng2sp

• In general, the order of items in a dictionary is unpredictable

• Dictionaries are indexed by keys, not integers


Dictionary indexing

print eng2sp['three']

print eng2sp['five']

* If the index is not a key in the dictionary, Python raises an exception


Dictionary indexing

if 'five' in eng2sp: print eng2sp['five']

print eng2sp.get('five')


The in operator

• Note that the in operator works differently for dictionaries than for other sequences• For offset indexed sequences (strings, lists,

tuples), x in y checks to see whether x is an item in the sequence

• For dictionaries, x in y checks to see whether x is a key in the dictionary


Keys and values

The keys method returns a list of the keys in a dictionary

print eng2sp.keys()

The values method returns a list of the values

print eng2sp.values()


Keys and values

The items method returns a list of tuple pairs of the key-value pairs in a dictionary

print eng2sp.items()


Example: histogram.py

def histogram(seq):

d = dict()

for element in seq:

if element not in d:

d[element] = 1

else:

d[element] += 1

return d

h = histogram('brontosaurus')

print h


Example: histogram2.py

Add the following code to histogram.py:

def print_hist(hist):

for key in hist:

print key, hist[key]


print_hist(h)


Example: histogram2.py

Change the print_hist function:


for key, value in hist:

print key, value


print_hist(h)


Sorting the keys

Change the print_hist function:


keys = hist.keys()

keys.sort()

for key in keys:

print key, hist[key]


print_hist(h)


Using lists as values: invert.py

Add the following code to histogram.py:def invert_dict(d):

inv = dict()

for key in d:

val = d[key]

if val not in inv:

inv[val] = [key]

else:

inv[val].append(key)

return inv


Using lists as values: invert.py

Add the following code to histogram.py:

hist = histogram('parrot')

print hist

inverted = invert_dict(hist)

print inverted


Using tuples as keys: troupe.py

troupe = {('Cleese', 'John'): [1,2,3],

('Chapman', 'Graham'): [4,5,6],

('Idle', 'Eric'): [7,8,9],

('Jones', 'Terry'): [10,11,12],

('Gilliam', 'Terry'): [13,14,15,16,17,18],

('Palin', 'Michael'): [19,20]}

for last, first in troupe:

print first, last, troupe[last, first]


Next session

Handling program errors (exceptions)

Reading and writing data filesSharing data with other programs (Excel, SAS, etc.)


Suggested exercises

Exercise 12.4The Case Study in chapter 13


Documents

Lesson 16 Dictionaries