Lab 06: Control Flow — Conditionals & Loops

🎯 Objective

Master Python's decision-making structures (if/elif/else) and loops (for, while) — the logic that drives every real program.

📚 Background

Control flow determines the order in which code executes. Without it, programs would just run top-to-bottom and stop. Conditionals let code make decisions; loops let code repeat. Python's syntax is cleaner than most languages — no curly braces, just indentation. Python also has unique features like for/else, while/else, and the ternary expression.

⏱️ Estimated Time

35 minutes

📋 Prerequisites

• Lab 5: Dictionaries

🛠️ Tools Used

• Python 3.12

🔬 Lab Instructions

Step 1: Basic if/elif/else

temperature = 22  # Celsius

if temperature < 0:
    status = "Freezing"
elif temperature < 10:
    status = "Cold"
elif temperature < 20:
    status = "Cool"
elif temperature < 30:
    status = "Comfortable"
else:
    status = "Hot"

print(f"{temperature}°C is: {status}")

# Multiple conditions with and/or
age = 25
has_id = True

if age >= 18 and has_id:
    print("Access granted")
elif age >= 18 and not has_id:
    print("Show ID please")
else:
    print("Access denied — underage")

📸 Verified Output:

22°C is: Comfortable
Access granted

💡 Python evaluates elif chains top-to-bottom and stops at the first True condition. Always put more specific conditions first, general conditions last.

Step 2: Ternary Expression (One-Line If)

# Traditional
x = 10
if x > 0:
    label = "positive"
else:
    label = "non-positive"
print(label)

# Ternary (Pythonic one-liner)
label = "positive" if x > 0 else "non-positive"
print(label)

# Nested ternary (use sparingly — readability first)
score = 75
grade = "A" if score >= 90 else "B" if score >= 80 else "C" if score >= 70 else "F"
print(f"Score {score} → Grade {grade}")

# Common use: default value
user_input = ""
name = user_input if user_input else "Anonymous"
print(f"Hello, {name}!")

📸 Verified Output:

positive
positive
Score 75 → Grade C
Hello, Anonymous!

Step 3: for Loop Fundamentals

# Loop over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"  - {fruit}")

# Loop with range()
print("\nCounting:")
for i in range(5):        # 0,1,2,3,4
    print(i, end=" ")
print()

for i in range(2, 10, 2): # Start, stop, step (even numbers)
    print(i, end=" ")
print()

# Countdown
for i in range(5, 0, -1):
    print(i, end=" ")
print("Go!")

📸 Verified Output:

  - apple
  - banana
  - cherry

Counting:
0 1 2 3 4 
2 4 6 8 
5 4 3 2 1 Go!

Step 4: enumerate() and zip()

# enumerate() — get index AND value
fruits = ["apple", "banana", "cherry"]
for i, fruit in enumerate(fruits, start=1):
    print(f"{i}. {fruit}")

print()

# zip() — loop over multiple lists together
names = ["Alice", "Bob", "Charlie"]
scores = [95, 87, 92]
grades = ["A", "B+", "A-"]

for name, score, grade in zip(names, scores, grades):
    print(f"{name:10}: {score} ({grade})")

📸 Verified Output:

1. apple
2. banana
3. cherry

Alice     : 95 (A)
Bob       : 87 (B+)
Charlie   : 92 (A-)

💡 enumerate() is preferred over for i in range(len(list)) — it's more Pythonic and less error-prone. zip() stops at the shortest sequence.

Step 5: while Loop

# Basic while
count = 0
while count < 5:
    print(f"count = {count}")
    count += 1

print("Done!")
print()

# While with user validation simulation
attempts = 0
max_attempts = 3
correct_pin = "1234"
test_inputs = ["9999", "0000", "1234"]  # Simulated user inputs

for test_input in test_inputs:
    attempts += 1
    if test_input == correct_pin:
        print(f"✅ PIN correct on attempt {attempts}!")
        break
    elif attempts >= max_attempts:
        print(f"❌ Too many failed attempts. Locked.")
        break
    else:
        print(f"❌ Wrong PIN ({attempts}/{max_attempts})")

📸 Verified Output:

count = 0
count = 1
count = 2
count = 3
count = 4
Done!

❌ Wrong PIN (1/3)
❌ Wrong PIN (2/3)
✅ PIN correct on attempt 3!

Step 6: break, continue, pass

# break — exit loop immediately
print("break demo:")
for i in range(10):
    if i == 5:
        break
    print(i, end=" ")
print(f"\nStopped at {i}")

print()

# continue — skip current iteration
print("continue demo (skip evens):")
for i in range(10):
    if i % 2 == 0:
        continue
    print(i, end=" ")
print()

print()

# pass — do nothing placeholder
for i in range(5):
    if i == 3:
        pass  # TODO: handle this case
    else:
        print(i, end=" ")
print()

📸 Verified Output:

break demo:
0 1 2 3 4 
Stopped at 5

continue demo (skip evens):
1 3 5 7 9 

0 1 2 4 

Step 7: Nested Loops

# Multiplication table (3x3)
print("Multiplication Table:")
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i*j:4}", end="")
    print()

print()

# Nested loop with break (searching 2D)
grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9],
]
target = 5
found = False

for row_idx, row in enumerate(grid):
    for col_idx, val in enumerate(row):
        if val == target:
            print(f"Found {target} at row={row_idx}, col={col_idx}")
            found = True
            break
    if found:
        break

📸 Verified Output:

Multiplication Table:
   1   2   3
   4   5   6
   7   8   9

Found 5 at row=1, col=1

Step 8: Comprehension with Conditions

# List comprehension with if
numbers = range(1, 21)
evens = [n for n in numbers if n % 2 == 0]
print(f"Evens: {evens}")

# FizzBuzz (classic interview question)
fizzbuzz = []
for n in range(1, 16):
    if n % 15 == 0:
        fizzbuzz.append("FizzBuzz")
    elif n % 3 == 0:
        fizzbuzz.append("Fizz")
    elif n % 5 == 0:
        fizzbuzz.append("Buzz")
    else:
        fizzbuzz.append(str(n))

print(", ".join(fizzbuzz))

# Comprehension version
result = ["FizzBuzz" if n%15==0 else "Fizz" if n%3==0 else "Buzz" if n%5==0 else str(n)
          for n in range(1, 16)]
print(", ".join(result))

📸 Verified Output:

Evens: [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
1, 2, Fizz, 4, Buzz, Fizz, 7, 8, Fizz, Buzz, 11, Fizz, 13, 14, FizzBuzz
1, 2, Fizz, 4, Buzz, Fizz, 7, 8, Fizz, Buzz, 11, Fizz, 13, 14, FizzBuzz

✅ Verification

# Prime number finder using nested loops
def is_prime(n):
    if n < 2:
        return False
    for i in range(2, int(n**0.5) + 1):
        if n % i == 0:
            return False
    return True

primes = [n for n in range(2, 50) if is_prime(n)]
print(f"Primes < 50: {primes}")
print(f"Count: {len(primes)}")
print("Lab 6 verified ✅")

Expected output:

Primes < 50: [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47]
Count: 15
Lab 6 verified ✅

🚨 Common Mistakes

1. Infinite while loop: Always ensure the loop condition eventually becomes False or has a break.

2. Off-by-one in range(): range(n) gives 0..n-1; range(1, n+1) gives 1..n.

3. Modifying a list while iterating: Causes skipped elements — iterate a copy: for item in list(my_list).

4. Using = instead of == in conditions: if x = 5 is a SyntaxError in Python (assignment, not comparison).

5. Deep nesting: More than 3 levels of nesting is hard to read — use functions to break it up.

📝 Summary

• if/elif/else chains: evaluated top-to-bottom, first match wins

• Ternary: value_if_true if condition else value_if_false

• for item in iterable — Python's primary loop; works on any iterable

• enumerate(iterable, start=0) gives index+value pairs

• zip(a, b) loops two iterables together

• break exits the loop; continue skips to next iteration; pass does nothing

• Prefer comprehensions for simple transformations; use regular loops for complex logic

🔗 Further Reading

• Python Docs: Control Flow

• Real Python: Python for Loops