Construct programs that apply arrays and lists

One-Dimensional Lists (1D)

A 1D list is a sequence of elements stored in a single row.

# Declaration
numbers: list[int] = [1, 2, 3, 4, 5]

# Indexing (0-based)
print(numbers[0])   # 1 — first element
print(numbers[-1])  # 5 — last element

# Slicing
print(numbers[1:4])  # [2, 3, 4]

# Common operations
numbers.append(6)       # Add to end
numbers.insert(0, 0)    # Insert at index
numbers.remove(3)       # Remove first occurrence of 3
popped = numbers.pop()  # Remove and return last element
numbers.sort()          # Sort in place
numbers.reverse()       # Reverse in place
len(numbers)            # Length of list

---

Two-Dimensional Lists (2D)

A 2D list is a list of lists — rows and columns, like a table or matrix.

# Declaration — 3 rows, 4 columns
matrix: list[list[int]] = [
    [1, 2,  3,  4],
    [5, 6,  7,  8],
    [9, 10, 11, 12]
]

# Accessing elements
print(matrix[0][0])   # 1 — row 0, col 0
print(matrix[2][3])   # 12 — row 2, col 3

# Iterating — row by row
for row in matrix:
    for item in row:
        print(item, end=" ")
    print()   # Newline after each row
# Output:
# 1 2 3 4
# 5 6 7 8
# 9 10 11 12

# Iterating with indices
for i in range(len(matrix)):
    for j in range(len(matrix[i])):
        print(f"matrix[{i}][{j}] = {matrix[i][j]}")

Accessing a Specific Row or Column

# Get entire first row
first_row = matrix[0]          # [1, 2, 3, 4]

# Get first element of each row (column 0)
col_0 = [row[0] for row in matrix]  # [1, 5, 9]

# Get diagonal (square matrix only)
diagonal = [matrix[i][i] for i in range(len(matrix))]  # [1, 6, 11]

Shallow and deep copies

A shallow copy creates a new outer list while keeping references to the same nested objects. Changing a nested mutable value through either list therefore affects both lists.

original = [[1, 2], [3, 4]]
shallow = original[:]
shallow[0][0] = 99
print(original)  # [[99, 2], [3, 4]]

A deep copy recursively copies nested objects, so later changes do not affect the original structure.

import copy

original = [[1, 2], [3, 4]]
deep = copy.deepcopy(original)
deep[0][0] = 99
print(original)  # [[1, 2], [3, 4]]