3_5_section.md

3.5 Section Notes

Sorting and Reversing Lists

This section explains how to change the order of items in a list using list methods.

Lists are ordered, so the sequence of items matters. Python gives you two common ways to rearrange that order:

• sort() arranges items in ascending order by default.
• reverse() flips the current order of the list.

Both methods change the original list directly.

1. Sorting a list with sort()

Use sort() when you want the items in a list ordered from smallest to largest, or alphabetically for text.

numbers = [5, 3, 1, 2, 4]
print(numbers)

numbers.sort()
print(numbers)

Output:

[5, 3, 1, 2, 4]
[1, 2, 3, 4, 5]

Key idea

sort() does not create a new list. It updates the list you already have.

letters = ["D", "F", "A", "Z"]
letters.sort()
print(letters)

Output:

['A', 'D', 'F', 'Z']

2. Reversing a list with reverse()

Use reverse() when you want to keep the same items but read them in the opposite order.

numbers = [5, 3, 1, 2, 4]
print(numbers)

numbers.reverse()
print(numbers)

Output:

[5, 3, 1, 2, 4]
[4, 2, 1, 3, 5]

Key idea

reverse() does not sort the list. It only flips the current order.

If a list is already sorted, reversing it gives you descending order. If it is not sorted, reversing it only changes the direction, not the ranking.

3. Working with numbers and strings

Lists can contain values stored in variables, not just literal values.

a = 3
b = 1
c = 2

numbers = [a, c, b]
numbers.sort()
print(numbers)

Output:

[1, 2, 3]

For strings, sorting follows alphabetical order.

a = "A"
b = "B"
c = "C"
d = " "

letters = [a, b, c, d]
letters.reverse()
print(letters)

Output:

[' ', 'C', 'B', 'A']

4. What to remember

• sort() changes the list order into sorted order.
• reverse() changes the list order into the opposite order.
• Both methods modify the original list.
• sort() is for ordering by value.
• reverse() is for flipping whatever order already exists.

5. Common mistake

A common mistake is expecting reverse() to sort the list backward.

values = [5, 1, 4, 2, 3]
values.reverse()
print(values)

This does not produce sorted descending order. It only produces:

[3, 2, 4, 1, 5]

If you want descending order, first sort the list, then reverse it.

values = [5, 1, 4, 2, 3]
values.sort()
values.reverse()
print(values)

Output:

[5, 4, 3, 2, 1]

6. Quick recap

Sorting changes the order based on value. Reversing changes the order based on position.

If you want to practice, try predicting the output before running each snippet:

lst = ["D", "F", "A", "Z"]
lst.sort()
print(lst)

a = 3
b = 1
c = 2

lst = [a, c, b]
lst.sort()
print(lst)

a = "A"
b = "B"
c = "C"
d = " "

lst = [a, b, c, d]
lst.reverse()
print(lst)