list_comprehensions.py

# List comprehensions — concise syntax for building lists.
#
# Also demonstrates a common pitfall with 2D list initialisation
# using multiplication vs comprehension.

from random import seed, random

seed(42)


# --- 1. List comprehension with nested loops ---
# Build a 10x10 grid, randomly setting cells to 1 with 30% probability.

size = 10
density = 0.3

grid = [[0 for _ in range(size)] for _ in range(size)]
for i in range(size):
    for j in range(size):
        if random() < density:
            grid[i][j] = 1

print('Random grid (30% density):')
for row in grid:
    print(row)


# --- 2. The shared-reference pitfall ---
# [[0] * 3] * 2 does NOT create two independent rows.
# Both rows are the same list object in memory.
# Modifying one modifies both.

print('\nShared reference pitfall:')
L = [[0] * 3] * 2
print(L)           # [[0, 0, 0], [0, 0, 0]]
L[0][1] = 10
print(L)           # [[0, 10, 0], [0, 10, 0]] — both rows changed!


# --- 3. Correct 2D list with comprehension ---
# Each row is a distinct list object.

print('\nCorrect 2D list using comprehension:')
L = [[0] * 3 for _ in range(2)]
print(L)           # [[0, 0, 0], [0, 0, 0]]
L[0][1] = 10
print(L)           # [[0, 10, 0], [0, 0, 0]] — only first row changed


# --- 4. Simple 1D list creation ---

N = 5
ar = [0] * N
print(f'\n[0] * {N}:  {ar}')

arr = [0 for _ in range(N)]
print(f'Comprehension: {arr}')