Seminar 2: Python Basics I — Control Structures

# --- Basic if/elif/else ---

def classify_grade(score):
    """Return a letter grade for a score in [0, 100]."""
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    elif score >= 60:
        return "D"
    else:
        return "F"


scores = [95, 83, 71, 64, 50, 100, 0]
for s in scores:
    print(f"Score {s:>3} → Grade {classify_grade(s)}")

print()

# --- Chained comparisons (Python-specific, very readable) ---
x = 15
if 10 <= x <= 20:   # equivalent to: 10 <= x and x <= 20
    print(f"{x} is between 10 and 20 (inclusive)")

# --- Ternary / conditional expression (one-liner if/else) ---
temperature = 22
weather = "warm" if temperature >= 20 else "cold"
print(f"Temperature {temperature}°C is {weather}")

# --- Interactive demo: which branch executes? ---
import ipywidgets as widgets
from IPython.display import display, HTML


def show_branch(number):
    """Show which if/elif/else branch is taken for a given number."""
    output_lines = [f"<b>Input number: {number}</b><br>"]
    output_lines.append("<pre style='font-family:monospace; font-size:13px;'>")

    # We manually trace through each condition
    if number > 0:
        branch = "POSITIVE"
        colour = "#2ecc71"
    elif number < 0:
        branch = "NEGATIVE"
        colour = "#e74c3c"
    else:
        branch = "ZERO"
        colour = "#3498db"

    # Build a visual "code trace"
    conditions = [
        (f"if {number} > 0:",     number > 0),
        (f"elif {number} < 0:",   number < 0),
        ("else:",                 not (number > 0 or number < 0)),
    ]
    for code_line, taken in conditions:
        marker = " ← TAKEN" if taken else ""
        style = f"color:{colour}; font-weight:bold;" if taken else "color:#888;"
        output_lines.append(
            f"<span style='{style}'>{code_line}{marker}</span><br>"
        )

    output_lines.append("</pre>")
    output_lines.append(
        f"<p style='color:{colour}; font-size:15px;'>Result: <b>{branch}</b></p>"
    )
    display(HTML("".join(output_lines)))


slider = widgets.IntSlider(
    value=0, min=-20, max=20, step=1,
    description="Number:",
    style={"description_width": "initial"},
    layout=widgets.Layout(width="55%")
)

out = widgets.interactive_output(show_branch, {"number": slider})
display(widgets.Label("Move the slider and watch which branch is taken:"), slider, out)

# --- range() examples ---
print("range(5):",        list(range(5)))
print("range(2, 7):",     list(range(2, 7)))
print("range(0,10,2):",   list(range(0, 10, 2)))
print("range(5,0,-1):",   list(range(5, 0, -1)))

print()

# --- Iterating over a list ---
fruits = ["apple", "banana", "cherry", "date"]

print("Direct iteration:")
for fruit in fruits:
    print(f"  {fruit}")

print()

# --- enumerate() ---
print("With enumerate():")
for index, fruit in enumerate(fruits):
    print(f"  [{index}] {fruit}")

# enumerate() with a start index
print("\nStarting from index 1:")
for rank, fruit in enumerate(fruits, start=1):
    print(f"  #{rank}: {fruit}")

print()

# --- zip() ---
names = ["Alice", "Bob", "Carol"]
scores = [95, 87, 72]
grades = ["A", "B", "C"]

print("With zip():")
for name, score, grade in zip(names, scores, grades):
    print(f"  {name:>6}: {score} ({grade})")

print()

# --- Iterating over a string (strings are iterables!) ---
word = "Python"
print(f"Letters in '{word}': ", end="")
for ch in word:
    print(ch, end=" ")
print()

# --- Iterating over a dict ---
capitals = {"France": "Paris", "Japan": "Tokyo", "Brazil": "Brasília"}
print("\nCountry capitals:")
for country, capital in capitals.items():
    print(f"  {country}: {capital}")

import math

# --- CORE: Basic while loop ---
counter = 0
while counter < 5:
    print(f"  counter = {counter}")
    counter += 1   # IMPORTANT: increment to avoid infinite loop
print("Loop finished.")

print()

# --- BONUS: Collatz conjecture (enrichment) ---
# Start with any positive integer n.
# If n is even: divide by 2. If odd: multiply by 3 and add 1.
# Conjecture: it always reaches 1 (unproven for all numbers!).

def collatz(n):
    """Return the Collatz sequence starting at n."""
    sequence = [n]
    while n != 1:
        if n % 2 == 0:
            n = n // 2
        else:
            n = 3 * n + 1
        sequence.append(n)
    return sequence


for start in [6, 11, 27]:
    seq = collatz(start)
    print(f"Collatz({start}): {seq}")
    print(f"  Steps to reach 1: {len(seq) - 1}")

print()

# --- BONUS: Newton's method (enrichment) ---
# Uses while loop because convergence time is not known in advance.

def sqrt_newton(n, tolerance=1e-10):
    """Approximate sqrt(n) using Newton-Raphson iteration."""
    if n < 0:
        raise ValueError("Cannot take square root of a negative number")
    guess = n / 2.0       # initial guess
    iterations = 0
    while abs(guess * guess - n) > tolerance:
        guess = (guess + n / guess) / 2  # Newton step
        iterations += 1
    return guess, iterations



for num in [2, 9, 100, 12345]:
    approx, iters = sqrt_newton(num)
    print(f"sqrt({num:>5}): approx={approx:.8f}, math.sqrt={math.sqrt(num):.8f}, iters={iters}")

# --- break: stop the loop early ---
print("=== break ===")
for i in range(10):
    if i == 5:
        print(f"  Found 5! Breaking out of loop.")
        break
    print(f"  i = {i}")

print()

# --- continue: skip even numbers ---
print("=== continue (printing only odd numbers) ===")
for i in range(10):
    if i % 2 == 0:
        continue    # skip even
    print(f"  {i}")

print()

# --- pass: placeholder in an empty branch ---
print("=== pass ===")
for i in range(5):
    if i == 2:
        pass   # placeholder — do nothing for 2 (no error without a body)
    else:
        print(f"  Processing {i}")

print()

# --- for/else: prime checking ---
print("=== for...else: prime checking ===")

def is_prime(n):
    """Return True if n is prime, False otherwise."""
    if n < 2:
        return False
    for divisor in range(2, int(n ** 0.5) + 1):
        if n % divisor == 0:
            return False  # found a divisor — not prime
    return True            # loop completed without break → prime


primes = [n for n in range(2, 30) if is_prime(n)]
print(f"  Primes below 30: {primes}")

print()

# Version explicitly using for/else:
def is_prime_explicit(n):
    """Same as above but uses for...else explicitly to illustrate the pattern."""
    if n < 2:
        return False
    for divisor in range(2, int(n ** 0.5) + 1):
        if n % divisor == 0:
            break       # composite — break skips the else
    else:
        return True     # else only runs if no break occurred
    return False


test_nums = [2, 7, 9, 13, 25, 29]
for num in test_nums:
    print(f"  is_prime({num}) = {is_prime_explicit(num)}")

# --- CORE: Nested loop: 5×5 multiplication table ---
print("5 × 5 Multiplication Table:")
print("   ", end="")
for j in range(1, 6):
    print(f"{j:>4}", end="")
print()
print("   " + "-" * 20)

for i in range(1, 6):          # outer loop: rows
    print(f"{i:>2} |", end="")
    for j in range(1, 6):      # inner loop: columns
        print(f"{i * j:>4}", end="")
    print()   # newline after each row

print()

# --- BONUS: Count operations to visualise O(n²) growth ---
import matplotlib.pyplot as plt

ns = list(range(1, 51))
ops_linear = ns                          # O(n)
ops_quadratic = [n ** 2 for n in ns]     # O(n²)
ops_nlogn = [n * (n.bit_length()) for n in ns]  # rough O(n log n)

fig, ax = plt.subplots(figsize=(8, 4))
ax.plot(ns, ops_linear,    label="O(n)      — linear",    linewidth=2)
ax.plot(ns, ops_nlogn,     label="O(n log n) — merge sort", linewidth=2, linestyle="--")
ax.plot(ns, ops_quadratic, label="O(n²)     — nested loop", linewidth=2, linestyle=":")
ax.set_xlabel("Input size n")
ax.set_ylabel("Number of operations (approx.)")
ax.set_title("Growth of Algorithm Complexity", fontweight="bold")
ax.legend()
ax.set_xlim(1, 50)
ax.set_ylim(0)
ax.spines["top"].set_visible(False)
ax.spines["right"].set_visible(False)
plt.tight_layout()
plt.show()

# --- CORE: Basic list comprehension ---
squares = [x ** 2 for x in range(1, 11)]
print(f"Squares 1..10: {squares}")

# --- CORE: With condition (filter) ---
even_squares = [x ** 2 for x in range(1, 11) if x % 2 == 0]
print(f"Even squares:  {even_squares}")

# --- BONUS: Transforming strings ---
words = ["hello", "world", "python", "algorithmics"]
upper_long = [w.upper() for w in words if len(w) > 5]
print(f"Uppercased long words: {upper_long}")

# --- BONUS: Flattening a 2D list ---
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [elem for row in matrix for elem in row]
print(f"Flattened matrix: {flat}")

# --- BONUS: Dictionary comprehension ---
word_lengths = {word: len(word) for word in words}
print(f"Word lengths: {word_lengths}")

# --- BONUS: Set comprehension ---
unique_lengths = {len(word) for word in words}
print(f"Unique lengths: {unique_lengths}")

# --- BONUS: Performance note ---
# Comprehensions are generally faster than equivalent loops because
# the iteration is optimised at the C level in CPython.
import timeit

loop_time = timeit.timeit(
    "result = []\nfor x in range(1000):\n    result.append(x**2)",
    number=10000
)
comp_time = timeit.timeit(
    "result = [x**2 for x in range(1000)]",
    number=10000
)

print(f"\nPerformance (10000 runs of squaring 1000 elements):")
print(f"  for loop:          {loop_time:.4f}s")
print(f"  list comprehension:{comp_time:.4f}s")
print(f"  Speedup: {loop_time / comp_time:.2f}x")

# Exercise 1 — Maximum of three numbers

def max_of_three(a, b, c):
    # TODO: implement with if/elif/else
    pass


# Quick checks
print(max_of_three(3, 7, 5))
print(max_of_three(-2, -9, -1))

# Exercise 2 — FizzBuzz

def fizzbuzz(n):
    # TODO: implement
    pass


# Quick check
fizzbuzz(16)

# Exercise 3 — Leap year checker

def is_leap_year(year):
    # TODO: implement
    pass


# Quick checks
for y in [2000, 1900, 2024, 2023]:
    print(y, is_leap_year(y))

# Exercise 4 — Interactive calculator with while loop

while True:
    op = input("Operator (+, -, *, /) or 'quit': ").strip()
    if op == "quit":
        print("Goodbye!")
        break

    # TODO: read two numbers and apply operation
    # a = float(input("First number: "))
    # b = float(input("Second number: "))
    # ...

# Exercise 5 — Multiplication table

def multiplication_table(n):
    # TODO: implement
    pass


# Quick check
multiplication_table(4)