Lists

Code sample link: https://replit.com/@jjoco/python-lists

In Python, lists are used to keep a record of values of a specific data type in an ordered way, each of which can be processed. Lists can contain duplicates of the same value. These can be thought of as arrays in other languages, but more function similarly to ArrayLists in Java. Python lists start indexing at 0.

Creating a list

To create an empty list, one can use square brackets [], like so:

my_list = []

This empty list, as the name suggests, contains no elements within it. This is can be used to initalize a list that will be modified later on.

One can also type hint a list; to type hint the list of strings, one can write the following

from typing import List

.
.
.
my_list : List[str] = []

A list need not have to be empty every time when intialized. Lists can be initialized with elements inside, like so:

my_list = ["hello", "there", "my", "name", "is"]

Unlike strictly typed languages, Python allows lists to contain more than one type. For example, the following multi-type list is valid in Python:

my_list = ["hello", 0, 1.5 , "name", True]

The above example contains strings, an integer, a float, and a boolean; strictly-typed languages forbid multi-type arrays and will throw a compile time error (if an array is initalized like above) or during runtime (if an element to be added is not of the same type as that already in the array).

Even if a developer type hints, the multi-type list is still a valid list, like so:

my_list: List[str] = ["hello", 0, 1.5 , "name", True]

If it is critical that a list is not to contain multiple types, do not rely on the Python interpreter out-of-the-box to prevent multi-type lists.

A cool feature is that one can create a list of a certain size, with all values initialized. For example, to create a list of size 7 filled with None, you can write the following:

occupied_list = [None] * 7
# occupied_list == [None, None, None, None, None, None, None]

Iterating through a list

The Python way of iterating through a list is very similar to how other languages use iterator looping.

for elem in my_list:
    #Do stuff to elem

Let's say I wanted to print out the elements in a given list. You can do the following:

hello_list = ["hello", "world", "!"]
for elem in hello_list:
  print(elem)
'''
Prints out:
hello
world
!
'''

Iterating through a list via enumerate

Sample: https://replit.com/@jjoco/python-enumerate

Python has a special function called enumerate. enumerate's parameter is a list, and it returns an enumeration of the list that allows a dev to loop through a list's indicies and elements simultaneously.

my_list = [2, 21, 7, 8, 65 ,9 , 0]

for index, value in enumerate(my_list):
  print("Index: ", index)
  print("Value: ", value)
'''
Prints out:
Index:  0
Value:  2
Index:  1
Value:  21
Index:  2
Value:  7
Index:  3
Value:  8
Index:  4
Value:  65
Index:  5
Value:  9
Index:  6
Value:  0
'''

In case the value of an element does not matter, a dev can use an underscore _ in place of the value to denote a dont-care.

for jndex, _ in enumerate(my_list):
  print("jndex: ", jndex)
'''
Prints out:
jndex:  0
jndex:  1
jndex:  2
jndex:  3
jndex:  4
jndex:  5
jndex:  6
'''

Likewise if the index of an element does not matter, use an underscore in place of the index.

for _, value in enumerate(my_list):
  print("Value: ", value)
'''
Value:  2
Value:  21
Value:  7
Value:  8
Value:  65
Value:  9
Value:  0
'''

Setting a list element

A dev can set the value of an element as a given index like the following.

my_list = [2, 21, 7, 8, 65 ,9 , 0]
my_list[3] = 5

The above line sets the element at index 3 (whose value is 8) to 5. Keep in mind that if the input index is outside the scope of the list, then Python will throw an IndexOutOfRangeException.

Getting a list element

To get an element at a given index, use the following syntax:

elem = my_list[index]

Let's say we wanted to print the value of my_list = [2, 21, 7, 8, 65 ,9 , 0] at index 2, we can write:

print(my_list[2])
#Output = 7

Getting the length of a list

To get the number of elements in a list (ie. its length), use the len function:

list_length = len(my_list)

So, if my_list = [2, 21, 7, 8, 65 ,9 , 0], then len(my_list) would be 7.

Iterating through a list via range

Devs can also iterate through a list via indices, but they still need to use the iterator syntax mentioned earlier using a special function range(n). range(n) returns a sequence of numbers from 0 to n-1. To iterate through an entire list using range, input the length of the list as its parameter.

bigger_list = [1,5,8,3,8,0,3]

for i in range(len(bigger_list)):
  print(bigger_list[i])

Try not to iterate through lists like this too often unless using the array indicies are used meaningfully.

Appending list element to the end of a list

To append to the end of a list, use the list method append(elem) function to add elem to the end of a given list.

bigger_list = [1,5,8,3,8,0,3]

bigger_list.append(7)
# bigger_list == [1,5,8,3,8,0,3,7]

Slicing a list

A list can be created from a previously created list by using the following syntax.

Syntax

• example_list[:end] => gets a list of elements from beginning of exampleList until index end (excluding element at index end)
• example_list[start:] => gets a list of elements starting from index start (inclusive) until the end of exampleList
• example_list[start:end] => gets a list of elements from exampleList starting from index start (inclusive) until the end index end(exclusive)

Example:

int_list = [1, 2, 3, 4, 5, 6]

begin_int_list = int_list[:2]
# begin_int_list == [1,2]

middle_int_list = int_list[2:4]
# middle_int_list == [3,4]

end_int_list = int_list[4:]
# end_int_list == [5,6]

Popping a list element from end of a list

To remove the last element of a given list (or "pop" it), use the pop() function. Note that this function also returns the item popped.

bigger_list = [1,5,8,3,8,0,3]

popped_item = bigger_list.pop()
# bigger_list == [1,5,8,3,8,0]
# popped_item == 3

Extending a list with another list

To extend a list with another list (ie. concatenating the two lists), use the extend function:

first_list = [2, 3, 5]
# first_list == [2, 3, 5]

second_list = [1, 4, 6]
first_list.extend(second_list)
# first_list == [2, 3, 5, 1, 4, 6]