Introduction to Data structures and algorithms using Python

Lists and dictionaries are one of the most often used data structures in Python for data storage and arrangement. This tutorial will explain how to carry out access, modify operations, etc., on lists and dictionaries.

Prerequisites

To follow along with this tutorial, you will require some knowledge of Python.

Creating a list

Lists enable you to store a set of items in place. Lists take the form of arrays in other languages like Java.

A list in Python is created using the following syntax:

listname = [item1, item2, item3, ...]

The following is an example of a list of cars:

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
print(cars) # ['Volvo', 'Subaru', 'Skyline', 'Ford']

Accessing elements in a list

You can access elements in a list using the square bracket notation. Indices begin at 0, not 1, hence the first item in a list takes index 0, not 1.

The syntax for accessing elements in a list is shown below.

listname = [item1,item2,item3]
print(listname[indexvalue])

The following code requests for the cars at index 0 and 3:

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
print(cars[0]) # Volvo
print(cars[3]) # Ford

Python has a unique way of getting the last item in a list. If you print an item at index -1, Python gives back the last item in the list. Indices -2,-3, and -4 gives the second last, third last, and fourth last item respectively.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
print(cars[-1])   # Ford

Changing elements in a list

To alter an item in the list, we can assign a new value using square bracket notation.

For instance, in our list of cars, the second item is Subaru. Let’s change the value to Suzuki.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
print(cars)      # ['Volvo', 'Subaru', 'Skyline', 'Ford']
cars[1]='suzuki' 
print(cars)      # ['Volvo', 'Suzuki', 'Skyline', 'Ford']

Adding elements to a list

To add new elements at the end of the list, use the .append() method.

For example, let’s add Audi to our list of cars.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford'] 
cars.append('Audi') # adds 'Audi' at the end of the list
print(cars)         # ['Volvo', 'Subaru', 'Skyline', 'Ford', 'Audi']

You can also use the .insert() method to add a new item at any position in your list, by specifying the position and value of the new item.

Let’s take a look at this example:

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
cars.insert(2,'Audi') # adds 'Audi' at the beginning of the list (at index 2)
print(cars)           # ['Volvo', 'Subaru', 'Audi', 'Skyline', 'Ford']

All the other values in the list are shifted to the right.

Removing elements from a list

If the location of an item that you want to do away with is well known, you can use the del statement.

Example:

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
del cars[0]
print(cars) # ['Subaru', 'Skyline', 'Ford']

The first car, i.e. Volvo is removed from the list.

If the value of the item you want to remove is known, use the .remove() method.

To illustrate this, let’s remove the item Subaru from our list of cars.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
cars.remove('Subaru') # removes Subaru from the list
print(cars)           # ['Volvo', 'Skyline', 'Ford']

NOTE: When a value appears more than once in a list, the .remove() method only deletes the first occurrence of the value.

Sorting a list

Sorting is organizing your list by some basis, e.g. reverse order, ascending order, or alphabetical order.

To sort a list, use the .sort() method. To understand this better, let's arrange our list of cars alphabetically.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
cars.sort()  # arranges the list alphabetically
print(cars)  # ['Ford', 'Skyline', 'Subaru', 'Volvo']

The .reverse() method is used to reverse the order of the list. For instance, let's print our list of cars in reverse.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford']
cars.reverse()
print(cars)    # ['Ford', 'Skyline', 'Subaru', 'Volvo']

NOTE: .reverse() doesn’t sort alphabetically backward. It just reverses the order of the list.

Slicing a list

To derive a segment, you need to specify the index of the first element and where to stop slicing.

In Python, we use a colon(:) as a slicing operator. Slicing follows the following syntax.

list_name[Index_start:Index_end]

For example, when you request [0:3], Python returns the items at index 0, 1, and 2.

To illustrate this, let's take a look at the following code.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford', 'Isuzu']
print(cars[0:3]) # ['Volvo', 'Subaru', 'Skyline']

When you exclude the starting index in a slice, Python automatically begins the slice at the start of the list. Similarly, if you omit the last index, Python returns all elements in the list from the first one.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford', 'Isuzu']
print(cars[:3]) # ['Volvo', 'Subaru', 'Skyline']
print(cars[3:]) # ['Ford', 'Isuzu']

Looping through a list

To perform an action to each element in a list, we can use the for loop.

For instance, let’s assume we have a list of cars and we want to pull out each name in the cars list.

cars = ['Volvo', 'Subaru', 'Skyline', 'Ford', 'Isuzu']  # initiate the list
for car in cars:
    print(car)
# Volvo
# Subaru
# Skyline
# Ford
# Isuzu

The line: for car in cars, tells Python to pull a car from the cars list and save it in the variable car. The line: print (car), then prints the name that was stored in the variable car. This line is re-executed for each item.

Dictionaries

In Python, a dictionary is a group of key:value pairs where each key is linked to a value. A dictionary is enclosed with curly brackets {}, with a series of key:value pairs inside.

A dictionary follows the following syntax:

dict_name={
    key:value,
    key:value}

Example:

stock_1={
    'type':'Volvo',
    'color':'Black'
}
print(stock_1) # {'type': 'Volvo', 'color': 'Black'}

Accessing values in a dictionaries

You can access values using the square brackets notation. To get a value, place its related key inside the square brackets as shown below.

stock_1={
    'type':'Volvo',
    'color':'Black'
}
print(stock_1['type'])  # Volvo

Adding items to a dictionary

To add a new item into a dictionary, give a value to a new key. Let’s add a new key: year to our stock_1 dictionary, and assign it the value 2014.

stock_1={
    'type':'Volvo',
    'color':'Black'
}
print(stock_1)        # prints the initial dictionary
stock_1['year']=2014  # Adding a new key:value
print(stock_1)        # returns the modified dictionary

Changing values in a dictionary

You can change a value in a dictionary by referring to its key name.

For example, let’s change the value of the year in our stock_1 dictionary from 2014 to 2021.

stock_1={
    'type':'Volvo',
    'color':'Black',
    'year':2014
}
stock_1['year']=2021  # change the value of year from 2014 to 2021
print(stock_1)        # {'type': 'Volvo', 'color': 'Black', 'year': 2021}

Removing key:value pairs from dictionaries

To remove a key:value pair you can use the del statement. Give the key you're trying to remove and the name of the dictionary to the del statement. For instance, let’s remove the key color in our stock_1 dictionary.

stock_1={
    'type':'Volvo',
    'color':'Black',
    'year':2014
}
del stock_1['color']   # removes the key `color` and its value `Black`
print(stock_1)         # {'type': 'Volvo', 'year': 2014}

Python removes the key color and its related value Black. The rest of the dictionary remains intact.

Looping through a dictionary

To access everything stored in a dictionary, you can loop through the dictionary using a for loop.

Assuming we have the names and models stored in a cars dictionary, we can access the name and model of each car using a for loop as follows:

cars={
    'Volvo':'V90',
    'Skyline':'R32',
    'Subaru':'2019WRX'
}
for name, model in cars.items():   # method `item()` returns a list of key value pairs
    print(name, ":", model)
# Volvo : V90
# Skyline : R32
# Subaru : 2019WRX

In the example above, the for loop stores the key in the variable name and its value in the variable model.

Conclusion

In this tutorial, we have gained an understanding of lists and dictionaries on how to:

• Access, alter and remove elements in both lists and dictionaries.
• Loop through dictionaries and lists.
• Sort and slice a list.

Happy coding!