Exercise 07: Files and dictionaries
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Introduction
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Imagine a list as being a numbered sequence of data.
fish = ["shark", "ray", "halibut", "tuna", "squid"]
The first element is shark, and you access it by its index number:
print fish[0]
=> shark
To access the nth element, its index number is n-1. The fifth element is accessed thusly:
print fish[4]
=> squid
What if, instead of a numbered index, we could use a string as an index?
print fish["deep ocean"]
=> anglerfish
print fish["shallow river"]
=> candiru
We would then need another way of specifying a list where each element is named.
fish = {"deep ocean": "anglerfish", "amazon river": "candiru",
"lake": "bass", "shallow river": "trout"}
This is called a dictionary in python. It's also called a hashtable, or a hashmap, or very non-specifically, a map. A dictionary is a collection of 'key-value pairs'. The key 'deep ocean' _maps_ to the value 'anglerfish'.
Imagine you were writing a program to keep track of user scores in a game. If you only had arrays, you might do something like this:
names = ["Bob", "Joe", "Jack", "Jane"]
scores = [ 10, 3, 6, 15]
To find Joe's score, first you'd have to find which position "Joe" is in, then use that position to look up his score.
index_joe = names.index("Joe")
print "Joe's score is %d"%(scores[index_joe])
=> Joe's score is 3
This is unwieldy and complicated. With dictionaries, we could instead do the following:
scores = {"Bob": 10, "Joe": 3, "Jack": 6, "Jane": 15}
print "Joe's score is %d"%(scores['Joe'])
=> Joe's score is 3
Dictionaries have a method called 'get' which allows you to have a default value in case a key does not exist beforehand.
scores = {"Bob": 10, "Joe": 3, "Jack": 6, "Jane": 15}
print scores.get("Bob", 0) # The second argument is the fallback number if the key doesn't exist
=> 10
print scores.get("Billy", 0) # Billy doesn't exist in the dictionary, so return the fallback instead
=> 0
Resources:
* http://learnpythonthehardway.org/book/ex15.html
* http://learnpythonthehardway.org/book/ex16.html
* http://learnpythonthehardway.org/book/ex17.html
* http://docs.python.org/tutorial/inputoutput.html#methods-of-file-objects
* http://learnpythonthehardway.org/book/ex39.html
* http://docs.python.org/tutorial/datastructures.html#dictionaries
* http://stackoverflow.com/a/3437070
Description
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In this directory, you will find a text file, scores.txt, containing a series of local restaurant ratings. Each line looks like this:
Restaurant Name:Rating
Your job is to write a program named 'sorted_data.py' reads the file, then spits out the ratings in alphabetical order by restaurant
Sample output:
Meringue:Exercise07 chriszf$ python sorted_data.py
Restaurant 'Andalu' is rated at 3.
Restaurant "Arinell's" is rated at 4.
Restaurant 'Bay Blend Coffee and Tea' is rated at 3.
Restaurant 'Casa Thai' is rated at 2.
Restaurant 'Charanga' is rated at 3.
Restaurant 'El Toro' is rated at 5.
Restaurant 'Giordano Bros' is rated at 2.
Restaurant "Irma's Pampanga" is rated at 5.
Restaurant 'Little Baobab' is rated at 1.
Restaurant 'Pancho Villa' is rated at 3.
Restaurant 'Taqueria Cancun' is rated at 2.
Restaurant 'Urbun Burger' is rated at 1.