Python Dictionaries

Question:

In Python, the map concept appears as a built-in data type called a dictionary. A dictionary contains key-value pairs. Dictionaries might soon become your favorite data structure in Python—they're extremely easy to use and useful. Here's a sample of setting up a dictionary

udacity = {}
udacity['u'] = 1
udacity['d'] = 2
udacity['a'] = 3
udacity['c'] = 4
udacity['i'] = 5
udacity['t'] = 6
udacity['y'] = 7

print udacity
# {'u': 1, 'd': 2, 'a': 3, 'c': 4, 'i': 5, 't': 6, 'y': 7}

In this case, the letters in "udacity" were each keys in our dictionary, and the position of that letter in the string was the value. Thus, I can do the following:

print udacity['t']
# 6

This statement is saying "go to the key labeled 't' and find it's value, 6".

Dictionaries are wonderfully flexible—you can store a wide variety of structures as values. You store another dictionary or a list:

dictionary = {}
dictionary['d'] = [1]
dictionary['i'] = [2]
dictionary['c'] = [3]
dictionary['t'] = [4]
dictionary['i'].append(5)
dictionary['o'] = [6]
dictionary['n'] = [7]
dictionary['a'] = [8]
dictionary['r'] = [9]
dictionary['y'] = [10]
print udacity
# {'d': [1], 'i': [2, 5], 'c': [3], 't': [4], 'o': [6], 'n': [7], 'a': [8], 'r': [9], 'y':[10]}

You can learn even more about dictionaries in the Python documentation .

Start Quiz:

alg_dictionaries.py

"""Time to play with Python dictionaries!
You're going to work on a dictionary that
stores cities by country and continent.
One is done for you - the city of Mountain 
View is in the USA, which is in North America.

You need to add the cities listed below by
modifying the structure.
Then, you should print out the values specified
by looking them up in the structure.

Cities to add:
Bangalore (India, Asia)
Atlanta (USA, North America)
Cairo (Egypt, Africa)
Shanghai (China, Asia)"""

locations = {'North America': {'USA': ['Mountain View']}}

"""Print the following (using "print").
1. A list of all cities in the USA in
alphabetic order.
2. All cities in Asia, in alphabetic
order, next to the name of the country.
In your output, label each answer with a number
so it looks like this:
1
American City
American City
2
Asian City - Country
Asian City - Country"""

Solution:

Here's the solution:

locations = {'North America': {'USA': ['Mountain View']}}
locations['North America']['USA'].append('Atlanta')
locations['Asia'] = {'India': ['Bangalore']}
locations['Asia']['China'] = ['Shanghai']
locations['Africa'] = {'Egypt': ['Cairo']}

print 1
usa_sorted = sorted(locations['North America']['USA'])
for city in usa_sorted:
    print city

print 2
asia_cities = []
for countries, cities in locations['Asia'].iteritems():
    city_country = cities[0] + " - " + countries 
    asia_cities.append(city_country)
asia_sorted = sorted(asia_cities)
for city in asia_sorted:
    print city

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