Lists and Dictionaries

Author

Ludovic Deneuville

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1 Containers

There are several data structures in Python:

Lists (list): ordered and mutable collection of elements
Dictionaries (dict): unordered collection of key-value pairs
Sets (set): unordered collection of unique elements
Tuples (tuple): ordered and immutable collection of elements

Here we will focus on the two most common structures: lists and dictionaries

2 Lists

Properties of Python lists:

Indexing: List elements are indexed, which means they can be accessed using their position in the list. The index of the first element is 0, and the index of the last element is len(list) - 1.
Mutable: Lists are mutable objects, which means you can modify their elements. You can add, remove, or modify elements.
Heterogeneity: Lists can contain elements of different data types: integers, strings, booleans, or even other lists.
Variable length: Lists can contain a variable number of elements. They can be empty (without elements) or contain an unlimited number of elements.

2.1 Creating a list

# Integer list
a = [22, 29, 35, 56]
a

type(a)

# Mixed list
b = ["bonjour", 20, True]
print(b)

# Concatenation
a + b

# Repetition
a * 2

There are many other possibilities for creating lists.

list(range(1, 7))

[x for x in range(1, 7)]

# Splitting a string
c = "bleu;blanc;rouge;vert;jaune"
c.split(";")

2.2 Useful methods

# Number of elements
len(a)

# First element
a[0]

# Last element
a[-1]

# All elements from position 1
a[1:]

# Test if a value belongs to a list
22 in a

# Find the position of an element
a.index(29)

# Reverse a list (without modifying the original list)
print("a[::-1] : " + str(a[::-1]))
print("a :       " + str(a))

# Reverse a list (here the reversal is saved in variable **a**)
a.reverse()
a

# Sort a list (without modifying the original list)
sorted(a)

# Descending order
sorted(a, reverse=True)

# Sort a list (saving the modification in variable **a**)
print("Before : " + str(a))
a.sort()
print("After : " + str(a))

2.3 Adding, modifying and deleting

# Add at the end
a.append(44)
a

# Insert at a specific position
a.insert(2, 88)
a

# Modify
a[0] = 99
a

# Delete by position
a.pop(3)
a

# Delete by value
a.remove(44)
a

2.4 Copying a list

Observe, then execute the code below. It contains the following instructions:

creation of the variable list
creation of the variable copy
deletion of the last element of copy
display of list

list = [1, 2, 3, 4]
list2 = list
list2.pop()
list

We might expect the variable list not to be modified and to display [1, 2, 3, 4]. However, variable list2 is not a copy of list, it is simply another way of referring to the same list.

# To create a real copy that will be distinct from the original
copy = list(list)
copy.append(9)
list, copy

# Another possibility to copy
cp = list.copy()
cp.append(8)
list, cp

3 Dictionaries

Important properties of a Python dictionary:

Indexing by key: Dictionary elements are indexed by keys rather than positions. Each key must be unique in the dictionary, and it is associated with a corresponding value. Searching for a value associated with a key is very fast.
Mutable: Dictionaries are mutable objects, which means you can add, remove, or modify dictionary elements after its creation.
Heterogeneity: Dictionaries can contain key-value pairs with different data types. Keys can be immutable types such as strings, integers… Values can be any valid Python data type.
Variable length: Dictionaries can contain a variable number of key-value pairs. They can be empty (without key-value pairs) or contain an unlimited number of pairs.
Unordered: Unlike lists, dictionary elements have no defined order. The order in which key-value pairs are stored is not guaranteed and may change during dictionary modifications.

3.1 Creating a dictionary

ingredients = {'sugar': '100g', 'pear': 2, 'milk': '1L', 'salt': True}
ingredients

type(ingredients)

# Number of elements
len(ingredients)

# Search by key
ingredients['milk']

# Search by key - another possibility
ingredients.get('milk')

3.2 Adding, modifying and deleting

# Add an element
ingredients['strawberry'] = '200g'
ingredients

# Modify
ingredients['sugar'] = '35g'
ingredients

# Delete
ingredients.pop('milk')
ingredients

3.3 Useful methods

# List of keys
list(ingredients.keys())      # same as: list(ingredients)

# List of values
list(ingredients.values())

# List of items (list of tuples)
list(ingredients.items())

4 “Empty” types

Now that we have seen lists and dictionaries, we can talk about empty types. For example, it is possible to create:

a variable that contains nothing: None
an empty list []
an empty dictionary {}

x = None
type(x)

y = []
type(y)

z = {}
type(z)

5 Exercises

5.1 Exercise 1

Starting from the list notes = ["do", "re", "re", "re", "fa", "sol", "solsi", "la"], add, remove and modify elements so that it contains the musical notes “do re mi fa sol la si” in the correct order.

# Test your answer in this cell

5.2 Exercise 2

Suggest two methods to reverse the list ["un", "deux", "trois", "quatre"]. What is the main difference between the two methods?

# Test your answer in this cell

5.3 Exercise 3

Test the behavior of the pop method on the list of integers from 1 to 9. For example, test pop() without parameters or pop(-1).

# Test your answer in this cell

5.4 Exercise 4

Test the behavior of the min and max methods on:

a list composed only of numeric objects (int and float);
a list composed only of strings;
a list composed of a mixture of numeric and text objects.

# Test your answer in this cell

5.5 Exercise 5

Try creating an empty list, then check its type. What could be the purpose of this?

# Test your answer in this cell

5.6 Exercise 6

Given the following dictionary: animals = {'cats': 5, 'dogs': 12}

What will the following membership tests return? Check your predictions.

'cats' in animals.keys()
'cats' in animals.values()
'cats' in animals

# Test your answer in this cell

5.7 Exercise 7

Given the dictionary defined in the cell below. Display using print operations:

the list of names of the different classes
Miranda’s history grade
the list of grades obtained by Hypolyte
the list of names of students in 6emeB
the list of subjects taught in 6eme A
the list of all taught subjects
the list of grades obtained by girls from both classes

resultats = {
    "6emeA": {"Miranda" : {"notes": {"physique": 16, "histoire": 12}},
              "Celestin": {"notes": {"physique": "absent", "histoire": 18}}
             },
    "6emeB": {"Hypolyte": {"notes": {"maths": 11, "anglais": 0}},
              "Josephine": {"notes": {"maths": 16, "anglais": 20}}
             }
}

# Test your answer in this cell

5.8 Exercise 8

Using a dictionary, count and display the number of occurrences of each character in the following sentence:

Je compte le nombre d'occurences de chaque caractère de la phrase courante.

Tip: first, test on a smaller string.

# Test your answer in this cell

5.9 Exercise 9

Clean this list to remove all duplicates and keep only one occurrence of each fruit:

fruits = ['pomme', 'banane', 'orange', 'fraise', 'citron', 'fraise', 'banane', 'orange', 'banane', 'pomme', 'fraise']

Hint: use sets

# Test your answer in this cell