1. COMPSCI 107 Computer Science Fundamentals
Lecture 06
Tuples
Reading and Writing Text Files Dictionaries

2. Learning outcomes
At the end of this lecture, students should be able to:
Assign tuples to variables
Pass tuples are parameter and return values
Access elements of tuples
Read text files
Write text files
Create dictionaries
Obtain values associated with keys in a dictionary
Iterate through views of a dictionary
COMPSCI Computer Science Fundamentals

3. Tuples A tuple is a sequence, much like a list A tuple is immutable
Enclosed in brackets (x, y)
>>> point = (1, 2)
>>> point
(1, 2)
>>> point[0] 1
>>> point + (3, 4)
(1, 2, 3, 4)
COMPSCI Computer Science Fundamentals

4. Returning multiple values
def translate(point, vector):
"""Translate a point by a given vector
Arguments:
point – a tuple representing an (x, y) point on
the cartesian plane
vector – a tuple representing a translation of
(dx, dy) on the cartesian plane
>>> translate((0, 0), (4, 5))
(4, 5)
"""
x = point[0] + vector[0]
y = point[1] + vector[1]
return (x, y)
>>> new_x, new_y = translate((1, 2), (3, 4))
COMPSCI Computer Science Fundamentals

5. Exercise: Difference between two points
Write a function that calculates the difference between two points and returns a vector (dx, dy) to translate from one to the other.
dx)
(x1, y1) dy
(x2, y2)
COMPSCI Computer Science Fundamentals

6. Reading a file Get a reference to the file on disk Modes
open(filename, mode)
Modes
‘r’ = read
‘w’ = write
‘r+’ = read and write
Default is text mode, append ‘b’ for binary
f = open(filename)
f = open(filename, 'r')
f = open(filename, 'wb')
COMPSCI Computer Science Fundamentals

7. Reading a text file Different ways to read the contents of the file
When the end of file is reached, empty string is returned
f.read() # returns the entire file as a string
f.readlines() # returns a list of lines (strings) from the file
f.readline() # returns a single line (string) from the file
for line in f:
print(line) #do something with the line
COMPSCI Computer Science Fundamentals

8. Reading data from a file
def read_data(filename):
'''Reads data from a file
Given the name of a file, reads all the data from a file and
return the data as a list where each element in the list is a
single piece of datum from the file.
test2.txt contains the text:
This is a small test
file with a few words.
>>> read_data('test2.txt')
['This', 'is', 'a', 'small', 'test', 'file', 'with', 'a', 'few', 'words.']
'''
f = open(filename)
contents = f.read()
data = contents.split()
return data
COMPSCI Computer Science Fundamentals

9. Reading numbers from file
Read data as text
Convert text to number
Functions that change types
int()
float()
str()
for line in f:
number = int(line) #one value per line
COMPSCI Computer Science Fundamentals

10. Exercise Write a function that returns the word count for a text file.
Write a function that returns the number of unique words in a text file (where words are defined as a sequence of characters separated by white space).
COMPSCI Computer Science Fundamentals

11. Writing data to a file Similar to reading data Write all the data
Close the file after all the data is written.
f.write( string_to_write )
f.close()
COMPSCI Computer Science Fundamentals

12. Exercise
Write a function called word_list that accepts the name of a text file as an argument and produces a list of all the words in that file, sorted in order with one word per line.
COMPSCI Computer Science Fundamentals

13. Dictionary An unordered set of key : value pairs
keys must be unique (within a given dictionary)
keys must be immutable
my_dict = { 'alux001' : 'Andrew',
'afer023' : 'Adriana',
'rshe001' : 'Robert' }
>>> my_dict['alux001']
'Andrew'
>>> my_dict['rshe001']
'Robert'
>>> my_dict['afer023']
'Adriana'
COMPSCI Computer Science Fundamentals

14. Adding entries to a dictionary
Variables are created when you assign a value for the first time
Assigning a value to a new dictionary entry creates that entry
Assigning a value to an existing dictionary entry changes that entry
my_dict = {}
my_dict['alux001'] = 'Andrew'
my_dict['rshe001'] = 'Robert'
my_dict['afer023'] = 'Adriana'
my_dict['alux001'] = 'Andrew Luxton-Reilly'
COMPSCI Computer Science Fundamentals

15. Deleting an element from a dictionary
Use the del operator
Use the pop method
del my_dict['alux001']
my_dict.pop('alux001')
COMPSCI Computer Science Fundamentals

16. Iterating through the dictionary
Order of items in a dictionary is undefined
Can get a "view" of the keys, values, or (key, value) pairs
Can iterate through the "view"
Views are dynamic, so they change when the dictionary changes
for k in my_dict.keys():
print(k)
for v in my_dict.values():
print(v)
for i in my_dict.items():
print(i)
COMPSCI Computer Science Fundamentals

17. Getting a value from the dictionary
Get a value associated with a given key
Use the index, use get, or use get with default value
>>> my_dict['alux001']
'Andrew'
>>> my_dict['ALUX999']
KeyError: 'ALUX999'
>>> my_dict.get('alux001')
'Andrew'
>>> my_dict.get('ALUX999')
>>>
>>> my_dict.get('alux001', 'Not valid')
'Andrew'
>>> my_dict.get('ALUX999', 'Not valid')
'Not valid'
COMPSCI Computer Science Fundamentals

18. Example: Sparse Matrix
A sparse matrix is a matrix with few non-zero values.
The most obvious way to represent the matrix is a list of lists:
matrix = [ [0, 0, 0, 0, 0, 0, 0],
[0, 1, 0, 0, 0, 0, 0],
[0, 0, 0, 3, 0, 0, 0],
[0, 0, 7, 0, 0, 0, 4],
[0, 0, 0, 0, 0, 2, 0] ]
COMPSCI Computer Science Fundamentals

19. Example: Sparse matrix
Alternative implementation
Use a dictionary
Use the location as the key
Store location as a tuple (x, y)
matrix = { (1, 1) : 1,
(2, 3) : 7,
(3, 2) : 3,
(5, 4) : 2,
(6, 3) : 4 }
COMPSCI Computer Science Fundamentals

20. Accessing the sparse matrix
Use the location (x, y) as the index
Use the location (x, y) as the index, but use a default value
matrix = { (1, 1) : 1,
(2, 3) : 7,
(3, 2) : 3,
(5, 4) : 2,
(6, 3) : 4 }
>>> matrix[(3, 2)] 3
>>> matrix[(0, 0)]
KeyError: (0, 0)
>>> matrix.get((4, 3), 0)
COMPSCI Computer Science Fundamentals

21. Summary Files need to be opened before you can read or write to them
f = open( filename[, mode] )
Files should be closed when you finish with them.
f.close()
Reading text from a file
f.read()
f.readline()
Writing text to a file
f.write( string )
COMPSCI Computer Science Fundamentals

22. Summary
Dictionaries are also known as associative arrays or hash tables
Consist of key : value pairs
keys must be immutable
Syntax is {a : b, c : d, …}
Adding an item
d[key] = value
Deleting an item
d.pop(key)
Getting an item
d.get(key, default)
COMPSCI Computer Science Fundamentals