Exercise-6

Exercise 06: Files and dictionaries

##Dictionaries

###What is a dictionary?

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'.

###Why use a dictionary? 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

###Some useful methods: ####get() 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

####iteritems() Dictionaries can also be iterated entry-by-entry, using the method iteritems().

For example:

my_dict = {'a': 1, 'b': 2, 'c': 3}
for key, value in my_dict.iteritems():
    print "Key == %r, value == %r" % (key, value)

Prints:

Key == 'a', value == 1
Key == 'b', value == 2
Key == 'c', value == 3

This introduces two loop variables, 'key' and 'value', that will store the key and value elements of each dictionary entry in turn.

###Dictionary Exercises (do these first):

• http://learnpythonthehardway.org/book/ex39.html (Stop at "Making Your Own Dictionary Modules")
• http://docs.python.org/tutorial/datastructures.html#dictionaries
• http://www.learnpython.org/en/Dictionaries
• http://docs.python.org/library/stdtypes.html#mapping-types-dict

##File Parsing

###Read lines from a file In exercise 5 you read a file and processed one character at a time. Files can also be iterated line-by-line, using a for loop on the file directly.

For example:

twain = open('twain.txt')
for line in twain:
    # Do something

The loop variable line will store each line of the file in turn.

File manipulation Exercises (and sometimes strings):

• 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://docs.python.org/library/stdtypes.html#string-methods
• http://stackoverflow.com/a/3437070

Description

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

I wrote e 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.