Python Lists and Lists Operations

A list in python is an ordered sequence of items or data. Python assumes lists as sequences which are preceded and ending with a square bracket.

Note that you cannot use the word list as a variable to reference a list because it is a reserved keyword

The following code snippets will help you to understand more about what lists are:

# Declare a variable to reference an empty list
# Method 1
our_list = []

# Method 2
our_list = list()

A list with elements: A list can contain a large number of elements. These elements can be of any data type. For instance, it can contain integers, floats, boolean values, other lists, strings etc
The following code snippets clarify this.

# A list with only integer elements.
our_list = [1,3,7,4,7,6,9,3,6,9]

# A list with several data types
second_list  = [23, True, ["Joseph", 4.7], "Nairobi"]

"""
The second_list variable defines a list that has an integer, boolean, another list, and a string.
"""

Indexing in lists Indexes in lists are basically positions that values are in in a list. Python indexing start from 0. The following code snippets explain about indexes:

second_list  = [23, True, ["Joseph", 4.7], "Nairobi"]

# To get the value of the item in the first position in the list:
print(second_list[0]) # Expected result is 23

print(second_list[3]) # Expected result is "Nairobi"

print(second_list[5]) # An IndexError will occur because the index passed is out of range

print(second_list[2]) # Expected output is ["Joseph", 4.7]

# To get the value of the Item in the second list, we have to pass the index of the list and the index of the value as follows: 
print(second_list[2][0]) # Expected output is "Joseph"

List slicing:

Taking into consideration a list of numbers from 0 to 10:

numbers = [0,1,2,3,4,5,6,7,8,9,10]
# In our list, the index is equal to the value
numbers[5] == 5 # True

# To get the values from the fourth position(index 3) to the seventh position(index 6) in a separate list
print(numbers[3:7]) # [3,4,5,6]

# To get all the even numbers in the list
print(numbers[::2]) # [0,2,4,6,8,10]

# To get all the odd numbers in the list
print(numbers[1::2]) # [1,3,5,7,9]

# To reverse the list
print(numbers[::-1]) # [10,9,8,7,6,5,4,3,2,1,0]

# How do negative indices relate to positive indices?
numbers[-1] == numbers[10] # True
numbers[-4] == numbers[7] # True

# Negative indices are therefore counted from  the last element beginning with -1

List operations, methods

List are considered objects in python. They therefore have methods that can be used to alter them. The following code snippets show how:

# Append
numbers = [0,1,2,3,4,5,6,7,8,9,10]
# append(value)
numbers.append(11) # The append() method adds the item passed at the end of the list
# Printing the list after the append operation has been carried on it: 
print(numbers) # [0,1,2,3,4,5,6,7,8,9,10, 11]

# Insert
# insert(index, value)
numbers.insert(12, 12) # The first argument is the index and the second argument is the value
print(numbers) # [0,1,2,3,4,5,6,7,8,9,10, 11, 12]

# finding the length of a list using the len() function
# len(iterable)
print(len(numbers)) #13

# remove and item from the list using the remove keyword
# remove(item)
numbers.remove(4) # Removes the item 4 from the list
print(numbers) # [0,1,2,3,5,6,7,8,9,10,11,12]

# The pop() function
# Used to remove the last element from the list. In case you want to remove a specific element, the element's index is passed in the list

numbers.pop()
print(numbers) # [0,1,2,3,5,6,7,8,9,10,11]

numbers.pop(4)
print(numbers) # [0,1,2,3,6,7,8,9,10,11]

Sum of all integers in the above list:

print(sum(numbers)) # 57

Concatenating two lists

"""Lists can be concatenated using the operand +"""

universities_a = ["JKUAT", "TUK", "KU", "UON"]
universities_b = ["Egerton", "Moi"]

print(universities_a + universities_b)  # ["JKUAT", "TUK", "KU", "UON", "Egerton", "Moi"]

These are just some of the basic lists operations
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Lists and Lists Operations in Python