This document discusses dictionaries in Python. It defines dictionaries as mappings between keys and values, like lists but with the index as any immutable type rather than just integers. It demonstrates creating, accessing, updating, deleting items from dictionaries and various operations like getting keys/values, checking for keys, looping, and using dictionaries to count word frequencies in files.

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BS GIS Instructor: Inzamam Baig
Lecture 10
Fundamentals of Programming

2. Dictionaries
A dictionary is like a list, but more general
A dictionary has a mapping between a set of indices (which are
called keys) and a set of values
Each key maps to a value
The association of a key and a value is called a key-value pair or
sometimes an item

3. In a list, the index positions have to be integers; in a dictionary,
the indices can be (almost) any type

4. >>> student_info = dict()
>>> student_info = {}
>>> print(student_info)
{}
The curly brackets, {}, represent an empty dictionary

5. Adding Items to Dictionary
>>> student_info['name'] = 'Adil'
This line creates an item that maps from the key 'name' to the
value “Adil”
>>> print(student_info)
{'name': 'Adil'}

6. >>> student_info = {'name': 'Adil', 'age': '25', 'email': 'adil@kiu.edu.pk'}
>>> print(student_info)
{'name': 'Adil', 'email': 'adil@kiu.edu.pk', 'age': '25'}
The order of the key-value pairs is not the same
If you type the same example on your computer, you might get a different
result
the order of items in a dictionary is unpredictable

7. >>> print(student_info['age'])
'25'
The key 'age' always maps to the value “25” so the order of the
items doesn't matter

8. If the key isn't in the dictionary, you get an exception:
>>> print(student_info['gpa'])
KeyError: 'gpa'

9. >>> len(student_info)
3
The len function works on dictionaries; it returns the number of
key-value pairs

10. IN Operator
>>> 'name' in student_info
True
>>> 'Adil' in student_info
False
it tells whether something appears as a key in the dictionary

11. To see whether something appears as a value in a dictionary, you
can use the method values, which returns the values as a type
that can be converted to a list, and then use the in operator:
>>> vals = list(student_info.values())
>>> 'Adil' in vals
True

12. The in operator uses different algorithms for lists and dictionaries
For lists, it uses a linear search algorithm
As the list gets longer, the search time gets longer in direct proportion
to the length of the list
For dictionaries, Python uses an algorithm called a hash table that has
a remarkable property: the in operator takes about the same amount
of time no matter how many items there are in a dictionary

13. Updating a value in dictionary
student_info = {
'name': 'Zeeshan',
'age': 20,
'courses': ['C++','Python','C#']
}
student_info.update({'name': 'Fatima'})

14. Deleting a key
del student_info['name']
student_age = student_info.pop('age')

15. Getting Keys and Values
student_info.keys()
student_info.values()
student_info.items()

16. Looping over a dictionary
for key, values in student_info.items():
print(key, values)

17. Dictionary as a set of counters
word = 'Department of Computer Science Karakoram International University'
words = {}
for letter in word:
if letter not in words:
words[letter] = 1
else:
words[letter] = words[letter] + 1
print(words)

18. We are effectively computing a histogram, which is a statistical
term for a set of counters (or frequencies)

19. get method
Dictionaries have a method called get that takes a key and a
default value
If the key appears in the dictionary, get returns the corresponding
value; otherwise it returns the default value

20. >>> cs_department = {'cs' : 50, 'gis': 50}
>>> print(cs_department .get('emails', 0))
0
>>> print(cs_department .get('gis', 0))
50

21. department = 'Department of Computer Science Karakoram International University'
words = dict()
for word in department:
words[word] = words.get(word,0) + 1
print(words)

22. Dictionaries and files
name of the common uses of a dictionary is to count the
occurrence of words in a file with some written text

23. file_name = input('Enter the name of the file:')
count_words = {}
try:
with open(file_name) as file_handler:
for line in file_handler:
words_list = line.rstrip().split()
for word in words_list:
if word not in count_words:
count_words[word] = 1
else:
count_words[word] += 1
except:
print('File Not Found')
exit()

24. assignment_marks = {
'C++': 15,
'Java': 20
}
for key, value in assignment_marks.items():
if assignment_marks[key] > 10 :
print(key, value)

25. Sorting
assignment_marks = {
'C++': 15,
'Java': 20,
'Python': 18,
'Intro to GIS':19
}
sorted_lst = list(assignment_marks.keys())
print(sorted_lst)
sorted_lst.sort()
for data in sorted_lst:
print(data)

26. Advanced text parsing

27. file_name = input('Enter the name of the file:')
words_count = {}
try:
with open(file_name) as file_handler:
for line in file_handler:
line = line.rstrip()
table = line.maketrans('', '', string.punctuation)
line = line.translate(table)
line = line.lower()
words_list = line.split()
for word in words_list:
if word not in words_count:
words_count[word] = 1
else:
words_count[word] += 1
except Exception as e:
print(e)
exit()
print(words_count)