Python Lecture 11

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BS GIS Instructor: Inzamam Baig
Lecture 11
Fundamentals of Programming

Tuples
A tuple is a sequence of values much like a list
The values stored in a tuple can be any type, and they are
indexed by integers
Tuples are immutable
Tuples are also comparable and hashable

Defining a tuple
>>> t = 'a', 'b', 'c', 'd', ‘e’
>>> t = ('a', 'b', 'c', 'd', 'e')

To create a tuple with a single element, you have to include the
final comma:
>>> t1 = ('a',)
>>> type(t1)
<type 'tuple'>

Without the comma Python treats ('a') as an expression with a
string in parentheses that evaluates to a string:
>>> t2 = ('a')
>>> type(t2)
<type 'str'>

>>> t = tuple()
>>> print(t)
()

>>> t = tuple('lupins’)
>>> print(t)
('l', 'u', 'p', 'i', 'n', 's')

The bracket operator indexes an element:
>>> t = ('a', 'b', 'c', 'd', 'e')
>>> print(t[0])
'a'
>>> print(t[1:3])
('b', 'c')

If you try to modify one of the elements of the tuple, you get an
error:
>>> t[0] = 'A'
TypeError: object doesn't support item assignment

You can’t modify the elements of a tuple, but you can replace one
tuple with another:
>>> t = ('A',) + t[1:]
>>> print(t)
('A', 'b', 'c', 'd', 'e')

Comparing tuples
Python starts by comparing the first element from each sequence
If they are equal, it goes on to the next element, and so on, until it
finds elements that differ

Comparing tuples
>>> (0, 1, 2) < (0, 3, 4)
True
>>> (0, 1, 2000000) < (0, 3, 4)
True

txt = 'but soft what light in yonder window breaks'
words = txt.split()
t = list()
for word in words:
t.append((len(word), word))
t.sort(reverse=True)
res = list()
for length, word in t:
res.append(word)
print(res)

Tuple assignment
>>> m = [ 'have', 'fun' ]
>>> x, y = m
>>> x
'have'
>>> y
'fun'
>>>

Tuple assignment
>>> m = [ 'have', 'fun' ]
>>> x = m[0]
>>> y = m[1]
>>> x
'have'
>>> y
'fun'

Tuple assignment
>>> m = [ 'have', 'fun' ]
>>> (x, y) = m
>>> x
'have'
>>> y
'fun'

Tuple unpacking
>>> a, b = b, a
Both sides of this statement are tuples, but the left side is a tuple
of variables; the right side is a tuple of expressions

>>> a, b = 1, 2, 3
ValueError: too many values to unpack

>>> addr = 'monty@python.org'
>>> uname, domain = addr.split('@’)
The return value from split is a list with two elements; the first
element is assigned to uname, the second to domain

DICTIONARIES AND TUPLES
Dictionaries have a method called items that returns a list of tuples,
where each tuple is a key-value pair
>>> d = {'a':10, 'b':1, 'c':22}
>>> t = list(d.items())
>>> print(t)
[('b', 1), ('a', 10), ('c', 22)]
the items are in no particular order

Converting a dictionary to a list of tuples is a way for us to
output the contents of a dictionary sorted by key
>>> d = {'a':10, 'b':1, 'c':22}
>>> t = list(d.items())
>>> t
[('b', 1), ('a', 10), ('c', 22)]
>>> t.sort()
>>> t
[('a', 10), ('b', 1), ('c', 22)]

Multiple assignment with dictionaries
for key, val in list(d.items()):
print(val, key)
This loop has two iteration variables because items returns a list
of tuples and key, val is a tuple assignment that successively
iterates through each of the key-value pairs in the dictionary

>>> d = {'a':10, 'b':1, 'c':22}
>>> l = list()
>>> for key, val in d.items() :
... l.append( (val, key) )
...
>>> l
[(10, 'a'), (22, 'c'), (1, 'b')]
>>> l.sort(reverse=True)
>>> l
[(22, 'c'), (10, 'a'), (1, 'b')]

import string
fhand = open('romeo-full.txt')
counts = dict()
for line in fhand:
line = line.translate(str.maketrans('', '', string.punctuation))
line = line.lower()
words = line.split()
for word in words:
if word not in counts:
counts[word] = 1
else:
counts[word] += 1
lst = list()
for key, val in list(counts.items()):
lst.append((val, key))
lst.sort(reverse=True)
for key, val in lst[:10]:
print(key, val)

Sets
A set is an unordered collection of items
Every set element is unique (no duplicates) and must be
immutable (cannot be changed)
Sets are mutable
Sets can also be used to perform mathematical set operations
like union, intersection, symmetric difference

Defining a Set
s1 = set()
my_set = {1, 2, 3}
print(my_set)
my_set = {1.0, "Hello", (1, 2, 3)}
print(my_set)

Duplicate Values
set cannot have duplicates
my_set = {1, 2, 3, 4, 3, 2}
print(my_set)
{1, 2, 3, 4}
my_set = set([1, 2, 3, 2])
print(my_set)
{1, 2, 3}
set cannot have mutable items
my_set = {1, 2, [3, 4]}

Distinguish set and dictionary while creating empty set
a = {}
print(type(a))
a = set()
print(type(a))

Sets are unordered therefore indexing has no meaning
my_set = {1, 3}
my_set[0]
TypeError: 'set' object is not subscriptable

Add
my_set.add(2)
print(my_set)
{1, 2, 3}

Update
my_set.update([2, 3, 4])
print(my_set)
{1, 2, 3, 4}
my_set.update([4, 5], {1, 6, 8})
print(my_set)
{1, 2, 3, 4, 5, 6, 8}

Removing Elements
my_set = {1, 3, 4, 5, 6}
my_set.discard(4)
print(my_set)
{1, 3, 5, 6}

Removing Elements
my_set.remove(6)
print(my_set)
{1, 3, 5}

discard an element
my_set.discard(2)
print(my_set)
{1, 3, 5}
my_set.remove(2)
KeyError: 2

discard vs remove
discard() function leaves a set unchanged if the element is not
present in the set
On the other hand, the remove() function will raise an error in
such a condition (if element is not present in the set)

Removing Elements
my_set = set("HelloWorld")
print(my_set)
pop random element
print(my_set.pop())
print(my_set)

Since set is an unordered data type, there is no way of
determining which item will be popped. It is completely arbitrary

IN Operator
my_set = set("apple")
print('a' in my_set)
True
print('p' not in my_set)
False

Iterating over a set
>>> for letter in set("apple"):
... print(letter)
...
a
p
e
l

Set Operations
>>> A = {1, 2, 3, 4, 5}
>>> B = {4, 5, 6, 7, 8}

Union is performed using | operator
A = {1, 2, 3, 4, 5}
B = {4, 5, 6, 7, 8}
print(A | B)
{1, 2, 3, 4, 5, 6, 7, 8}

>>> A.union(B)
{1, 2, 3, 4, 5, 6, 7, 8}
>>> B.union(A)
{1, 2, 3, 4, 5, 6, 7, 8}

A = {1, 2, 3, 4, 5}
B = {4, 5, 6, 7, 8}
print(A & B)
{4, 5}

>>> A.intersection(B)
{4, 5}
>>> B.intersection(A)
{4, 5}

A = {1, 2, 3, 4, 5}
B = {4, 5, 6, 7, 8}
print(A - B)
{1, 2, 3}

>>> A.difference(B)
{1, 2, 3}
>>> B.difference(A)
{8, 6, 7}

A = {1, 2, 3, 4, 5}
B = {4, 5, 6, 7, 8}
print(A ^ B)
{1, 2, 3, 6, 7, 8}

>>> A.symmetric_difference(B)
{1, 2, 3, 6, 7, 8}
>>> B.symmetric_difference(A)
{1, 2, 3, 6, 7, 8}

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