List Indexing and Slicing in Python Code
List indexing and slicing in Python are powerful tools for accessing and manipulating elements within a list. Here's a detailed overview of how they work:
List Indexing
Indexing refers to accessing individual elements of a list using their position (index). Python uses zero-based indexing, meaning the first element is at index 0, the second element at index 1, and so on.
Basic Indexing
Python Code
# Example list
my_list = [10, 20, 30, 40, 50]
# Accessing elements
first_element = my_list[0] # 10
second_element = my_list[1] # 20
last_element = my_list[-1] # 50 (negative index for last element)
second_last_element = my_list[-2] # 40
Modifying Elements
Python Code
# Modifying elements
my_list[2] = 35 # Now my_list is [10, 20, 35, 40, 50]
IndexError
Python Code
# Accessing an index out of range raises an IndexError
out_of_range_element = my_list[10] # Raises IndexError
List Slicing
Slicing allows you to access a subset of the list by specifying a range of indices. The syntax for slicing is list[start:end:step].
start: The starting index (inclusive).
end: The ending index (exclusive).
step: The step size (optional, default is 1).
Basic Slicing
Python Code
# Example list
my_list = [10, 20, 30, 40, 50]
# Slicing elements
sub_list1 = my_list[1:4] # [20, 30, 40] (from index 1 to 3)
sub_list2 = my_list[:3] # [10, 20, 30] (from start to index 2)
sub_list3 = my_list[2:] # [30, 40, 50] (from index 2 to end)
sub_list4 = my_list[:] # [10, 20, 30, 40, 50] (whole list)
Using Step in Slicing
Python Code
# Example list
my_list = [10, 20, 30, 40, 50, 60, 70]
# Slicing with step
sub_list5 = my_list[::2] # [10, 30, 50, 70] (every second element)
sub_list6 = my_list[1:5:2] # [20, 40] (from index 1 to 4, every second element)
sub_list7 = my_list[::-1] # [70, 60, 50, 40, 30, 20, 10] (reverse list)
Modifying with Slicing
Python Code
# Example list
my_list = [10, 20, 30, 40, 50]
# Modifying multiple elements
my_list[1:3] = [25, 35] # Now my_list is [10, 25, 35, 40, 50]
my_list[:2] = [5, 15] # Now my_list is [5, 15, 35, 40, 50]
Slicing with Negative Indices
Python Code
# Example list
my_list = [10, 20, 30, 40, 50]
# Slicing with negative indices
sub_list8 = my_list[-4:-1] # [20, 30, 40] (from index -4 to -2)
sub_list9 = my_list[-5:-1:2] # [10, 30] (from index -5 to -2, every second element)
Key Points to Remember
Indices start at 0 and go up to the length of the list minus one.
Negative indices can be used to access elements from the end of the list.
The slicing end index is exclusive.
The step size can be used to skip elements and can also be negative to reverse the list.
By mastering list indexing and slicing, you can efficiently access and manipulate parts of a list in Python.
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