📝 Data Structures

Lists

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⏱ 3 hr3 topicsInteractive
🎯 By the end: You can create and modify lists, use the common list methods, and build nested lists, and you understand how lists differ from strings.

A list stores many values in one ordered collection, written in square brackets: marks = [85, 92, 78]. Lists are the workhorse data structure of Python — and unlike strings, they're mutable, meaning you can change them after creation. That one difference unlocks a lot.

1Creating, indexing and changing lists

A list holds an ordered sequence of items, which can even be of different types. You index and slice it exactly like a string (starting at 0, negative indices, [start:stop]) — but because lists are mutable, you can also assign to a position to change it:

fruits = ["apple", "mango", "kiwi"]
print(fruits[0])      # apple
fruits[1] = "banana"  # allowed! lists are mutable
print(fruits)         # ['apple', 'banana', 'kiwi']
String vs List — the key difference: a string is immutable (you can't change a character in place); a list is mutable (you can change, add and remove items). Both are ordered and both support indexing and slicing.
Lists are mutable▶ runs real Python
main.py
Output
Click Run to execute.
Key points
  • A list is an ordered collection in square brackets; items can be different types.
  • Index and slice like strings (0-based, negative indices, [start:stop]).
  • Lists are mutable: you can change an item with list[i] = value (strings can't).

2List methods

Lists have a rich set of methods for adding, removing and rearranging items:

MethodDoes
append(x)Add x to the end
insert(i, x)Insert x at index i
extend(list2)Add all items of another list
remove(x)Remove the first x
pop(i)Remove and return the item at i (last if no i)
sort() / reverse()Sort / reverse the list in place
index(x) / count(x)Find position of x / count how many x

Useful built-in functions also work on lists: len(), min(), max(), sum(), and sorted() (which returns a new sorted list without changing the original).

List methods▶ runs real Python
main.py
Output
Click Run to execute.
Key points
  • Add: append (end), insert (at index), extend (another list). Remove: remove (by value), pop (by index).
  • sort() and reverse() change the list in place; sorted() returns a new sorted list.
  • len(), min(), max(), sum() also work on lists.

3Nested lists and matrices

A list can contain other lists. This nested list is how you represent a grid or a matrix (a table of rows and columns):

matrix = [[1, 2, 3],
          [4, 5, 6],
          [7, 8, 9]]

You reach an inner item with two indices: matrix[0] is the first row [1, 2, 3], and matrix[0][2] is the item in row 0, column 2 → 3. Nested loops (a loop inside a loop) are the natural way to walk through every cell.

A matrix (nested list)▶ runs real Python
main.py
Output
Click Run to execute.
Key points
  • A nested list (list of lists) represents a grid or matrix of rows and columns.
  • Use two indices: matrix[row][col].
  • Nested loops walk through every cell of a matrix.

★ Project: a marks manager

In the playground, build a small marks program:

  1. Make a list of 5 test marks.
  2. Add a new mark with append(), then remove the lowest using remove(min(marks)).
  3. Print the highest, lowest and average (use max, min, sum and len).
  4. Sort the list and print it from highest to lowest (sort then reverse).

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Strings
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Tuples