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Lecture 05 – Sequences of data
 Previously, we looked at:
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how our programs can have unexpected consequences for society
the importance of writing tests before code
documenting code after it is written
using doctest to integrate testing and documentation
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 At the end of this lecture, students should be able to:
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Use index values to refer to elements of a sequence
Use the “slice” syntax to refer to a subsequence
Use operations that apply to sequences
Use the for loop syntax to iterate over a sequence
Use list comprehensions to create a list
Visualise how lists are stored in memory
Create multi-dimensional lists
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 Sequences allow you to store values in an organized fashion.
 Built-in types: strings, lists, tuples
 http://en.wikibooks.org/wiki/Python_Programming/Sequences
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 Strings are a sequence of characters
>>> name = 'Andrew'
>>> name[0]
'A'
>>> 'd' in name
True
>>> len(name)
6
>>> name + '_' + 'Luxton-Reilly'
'Andrew_Luxton-Reilly'
 Strings have a number of methods that are useful
 split() is especially useful
 help(str)
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 Write a function that determines whether a given string of text
contains a vowel or not. The function should return True if the text
contains a vowel.
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 Lists are a built-in type in Python
 Use square brackets to signify a list
 Can contain any type of data, or any mixture of types
 How do you visualise the following list?
my_list = [1, 2, 3]
my_list
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 Lists are a built-in type in Python
 Use square brackets to signify a list
 Can contain any type of data, or any mixture of types
 How do you visualise the following list?
my_list = [1, 2, 3]
2
1
3
my_list
0
1
2
3
4
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 Numerous list functions are supported
 Use help(list) to see a complete list of functions
 Examples:
my_list = [1, 2, 3]
len(my_list)
3
2
1
3
my_list
0
1
2
3
4
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 Numerous list functions are supported
 Use help(list) to see a complete list of functions
 Examples:
my_list = [1, 2, 3]
my_list = my_list + [4]
4
2
1
3
my_list
0
1
2
3
4
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 Numerous list functions are supported
 Use help(list) to see a complete list of functions
 Examples:
my_list.append(7)
7
4
2
1
3
my_list
0
1
2
3
4
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 Numerous list functions are supported
 Use help(list) to see a complete list of functions
 Examples:
True
3 in my_list
4
2
1
3
my_list
0
1
2
3
4
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 Numerous list functions are supported
 Use help(list) to see a complete list of functions
 Examples:
1
my_list[0]
4
2
1
3
my_list
0
1
2
3
4
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 Numerous list functions are supported
 Use help(list) to see a complete list of functions
append(item)
insert(i, item)
pop()
pop(i)
sort()
reverse()
del( my_list[i] )
index(item)
count(item)
remove(item)
Make sure you are
comfortable with these
functions (i.e. you can
use the Python docs to
call them and
understand how they
work)
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 Write a function that sums the elements of a list that contains
numbers.
 Write a function that accepts a list and returns a new list with the
same contents, but in reverse order.
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 Since a list element can be of any type, it can actually be a list
my_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
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 Since a list element can be of any type, it can actually be a list
my_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
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 Since a list element can be of any type, it can actually be a list
my_list = [[1, 2, 3], [4, 5, 6], [7], [8, 9]]
The online Python Tutor has a useful visualisation
engine:
http://www.pythontutor.com/visualize.html
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 A piece of a sequence can be obtained using the following syntax:
 sequence_name[start : stop]
 Where start is the index of the first element, and stop is the index after the last element
>>> name = "Andrew"
>>> values = [5,8,2,3,7,1]
>>> print(name[1:2])
n
>>> print(values[2:5])
[2, 3, 7]
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 Actually, the syntax allows for a third value, used to define the step
size between elements included in the slice. If a value is omitted, it
defaults to :
0
length
1
 [ start : stop : step]
>>> word = 'Computer'
>>> print(word[:4:2])
Cm
>>> print(word[::-1])
retupmoC
Using a step size
of -1, and not
providing start
and stop means
that Python will
step through the
word right to left,
from end to
beginning
word
C
o
m
p
u
t
e
r
Positive
index
0
1
2
3
4
5
6
7
Negative
index
-8
-7
-6
-5
-4
-3
-2
-1
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 Used to iterate through a sequence
numbers = [2, 3, 5, 7, 11]
for i in numbers:
print(i)
name = 'Andrew'
for c in name:
print(c)
2
3
5
7
11
A
n
d
r
e
w
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 A list can be created using instructions that appear within the
square brackets
my_list = [x for x in range(90, 100)]
print(my_list)
[90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
 The general format of a list comprehension is:
[expression for variable in sequence]
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 Examples
Converting a string into a list of characters:
my_list = [c for c in 'COMPSCI107']
print(my_list)
['C', 'O', 'M', 'P', 'S', 'C', 'I', '1', '0', ‘7']
Generating all even numbers between 1 and 20
my_list = [n * 2 for n in range(1, 10)]
print(my_list)
[2, 4, 6, 8, 10, 12, 14, 16, 18]
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 We can extend the syntax for a list comprehension to include a
condition:
my_list = [x for x in range(0, 10) if x % 2 == 0]
print(my_list)
[0, 2, 4, 6, 8]
 The general format of a list comprehension using a condition is:
[expression for variable in sequence if condition]
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 What is the output?
values = [x + x for x in range(5) if x + x < x * x]
print('Result =', values)
(a) Result = [2, 4]
(b) Result = [3, 4]
(c) Result = [0, 2, 4, 6, 8]
(d) Result = [6, 8]
(e) Result = [0, 1, 2, 3, 4]
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 What is the output?
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
primes = [2, 3, 5, 7]
my_list = [x+x for x in numbers if x not in primes]
print(my_list)
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 Example
Generate a list of the non-vowel letters that appear in a string:
phrase = 'The quick brown fox jumps over the lazy dog'
vowels = 'aeiou'
my_list = [c for c in phrase if c not in vowels]
for c in my_list:
print(c, end = '')
Th qck brwn fx jmps vr th lzy dg
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 Information in a list is stored contiguously in memory (i.e. in
consecutive memory locations)
 Location (memory address) of the information can be calculated as:
 location = address of start of list +
index * size of each element
 Efficiency issues
 It takes the same amount of time to access any of the elements
 It is slow to move elements around (i.e. add and delete elements from within the list)
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