Weeks 4–6 — Conditionals · Loops · Lists & Tuples

What you'll learn

• 1Use if / elif / else to branch on conditions
• 2Write nested if statements
• 3Use the ternary (conditional) expression
• 4Combine multiple conditions with and/or
• 5Understand truthiness of non-boolean values
• 6Iterate over a range with for/range()
• 7Use while for condition-driven loops
• 8Control loop flow with break and continue
• 9Explain the loop else clause
• 10Nest loops to produce 2D output
• 11Create and modify lists
• 12Access elements by index and slice
• 13Use append, insert, remove, pop, sort, reverse
• 14Understand the difference between list and tuple
• 15Unpack sequences into variables

Week 4 — Conditionals

Control program flow with if/elif/else blocks. Learn nested conditions, the ternary expression, and how to structure complex decision logic.

1. if / elif / else

score = int(input("Score: "))
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"
print(f"Grade: {grade}")

2. Ternary Expression

A concise single-line conditional: value_if_true if condition else value_if_false.

age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

3. Truthiness

In Python, many non-boolean values evaluate as True or False in a boolean context.

4. Common Mistakes

1. Using = instead of == in a condition — this is a SyntaxError in Python (unlike C).
2. Forgetting the colon after if/elif/else — SyntaxError.
3. Overlapping conditions — put the most specific (highest/lowest) check first.

Week 5 — Loops

Automate repetitive tasks with for and while loops. Master range(), break, continue, and the rarely-known loop else clause.

1. for Loop & range()

for i in range(5):        # 0 1 2 3 4
    print(i, end=' ')

for i in range(1, 6):     # 1 2 3 4 5
    print(i, end=' ')

for i in range(0, 10, 2): # 0 2 4 6 8
    print(i, end=' ')

2. while Loop

n = 1
while n <= 5:
    print(n)
    n += 1

# Infinite loop with break
while True:
    cmd = input("Command (quit to exit): ")
    if cmd == "quit":
        break
    print(f"You entered: {cmd}")

3. break, continue, else

for i in range(1, 11):
    if i == 5:
        continue   # skip 5
    if i == 8:
        break      # stop at 8
    print(i, end=' ')  # 1 2 3 4 6 7

for i in range(2, 10):
    for j in range(2, i):
        if i % j == 0:
            break
    else:           # only runs if no break
        print(f"{i} is prime")

4. Nested Loops

for row in range(1, 4):
    for col in range(1, 4):
        print(f"{row}×{col}={row*col}", end='  ')
    print()

Output: 1×1=1 1×2=2 1×3=3 / 2×1=2 2×2=4 2×3=6 / 3×1=3 3×2=6 3×3=9

5. Common Mistakes

1. Forgetting to increment the counter in a while loop → infinite loop. Always update the condition variable.
2. range(1, 10) ends at 9, not 10. Off-by-one errors are very common.
3. Modifying a list while iterating over it — iterate over a copy instead: for x in lst[:].

Week 6 — Lists & Tuples

Work with Python's most-used sequence types. Master list CRUD operations, slicing syntax, common list methods, and tuple unpacking.

1. Lists

Lists are ordered, mutable sequences. They can hold values of any type.

fruits = ["apple", "banana", "cherry"]
print(fruits[0])    # apple
print(fruits[-1])   # cherry
fruits.append("date")
fruits.insert(1, "blueberry")
fruits.remove("banana")
print(len(fruits))  # 4

2. Slicing

nums = [0, 1, 2, 3, 4, 5]
print(nums[1:4])    # [1, 2, 3]
print(nums[:3])     # [0, 1, 2]
print(nums[3:])     # [3, 4, 5]
print(nums[::2])    # [0, 2, 4]  every other
print(nums[::-1])   # [5, 4, 3, 2, 1, 0]  reversed

3. Common List Methods

4. Tuples

Tuples are like lists but immutable (cannot be changed after creation). Use them for fixed data.

point = (3, 4)
x, y = point          # unpacking
print(f"x={x}, y={y}")

rgb = (255, 128, 0)
r, g, b = rgb

# swap without temp variable
a, b = 10, 20
a, b = b, a
print(a, b)  # 20 10

5. List vs Tuple — and the copy trap

Lists and tuples look similar, but picking the right one prevents whole classes of bugs — and understanding how Python copies them prevents the rest.

The copy trap (aliasing)

Assignment never copies a list — it just makes a second name for the SAME list. Mutating through one name shows up in the other. Copy explicitly when you need independence.

a = [1, 2, 3]
b = a            # NOT a copy — same list object
b.append(4)
print(a)         # [1, 2, 3, 4]  <- surprised?

c = a[:]         # shallow copy (also list(a) or a.copy())
c.append(5)
print(a)         # unchanged

Common Mistakes (FAQ)

Q. Why does sorted_lst = lst.sort() give me None?

list.sort() sorts in place and returns None. Call it for the side effect (lst.sort()), or use sorted(lst) when you want a brand-new sorted list back.

Q. list[10] on a 5-item list crashes — how do I guard it?

That's an IndexError. Check len(lst) first, use a slice (lst[10:11] returns [] safely), or wrap the access in try/except IndexError.

Q. I did new = old, but editing new changed old too.

new = old doesn't copy — both names point to the same list. Use new = old[:], list(old), or old.copy() for a shallow copy (and copy.deepcopy for nested lists).

Q. Why isn't (5) a tuple?

Parentheses alone don't make a tuple — the comma does. Write (5,) for a one-element tuple; for multiple items like (1, 2) the comma is already there.

Q. When should I pick a tuple over a list?

Use a tuple for a fixed, heterogeneous record (a coordinate, an RGB color, multiple return values) or when you need a hashable dict key. Use a list when the collection grows, shrinks, or gets reordered.

💻 Examples

Run these examples and check the output yourself.

score = int(input("Score (0-100): "))
if score >= 90:
    print("A — Excellent")
elif score >= 80:
    print("B — Good")
elif score >= 70:
    print("C — Average")
elif score >= 60:
    print("D — Below average")
else:
    print("F — Fail")

month = int(input("Month (1-12): "))
if month in [3, 4, 5]:
    season = "Spring"
elif month in [6, 7, 8]:
    season = "Summer"
elif month in [9, 10, 11]:
    season = "Autumn"
else:
    season = "Winter"
print(f"Month {month} is {season}.")

USER, PASS = "admin", "1234"
username = input("Username: ")
password = input("Password: ")
if username == USER and password == PASS:
    print("Login successful!")
else:
    print("Invalid credentials.")

for i in range(1, 10):
    for j in range(1, 10):
        print(f"{i*j:3d}", end='')
    print()

n = int(input("N: "))
# for version
total = sum(range(1, n+1))
print(f"for: {total}")
# while version
total, i = 0, 1
while i <= n:
    total += i
    i += 1
print(f"while: {total}")

N: 100
for: 5050
while: 5050

import random
secret = random.randint(1, 100)
attempts = 0
while True:
    guess = int(input("Guess (1-100): "))
    attempts += 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
    else:
        print(f"Correct! It took {attempts} attempts.")
        break

names = ["Alice", "Bob", "Charlie"]
names.append("Diana")
names.insert(1, "Eve")
names.remove("Bob")
names.sort()
for i, name in enumerate(names, 1):
    print(f"{i}. {name}")

1. Alice
2. Charlie
3. Diana
4. Eve

data = list(range(10))   # [0..9]
print("First 5:  ", data[:5])
print("Last 5:   ", data[5:])
print("Middle:   ", data[3:7])
print("Reversed: ", data[::-1])
print("Every 3rd:", data[::3])

First 5:   [0, 1, 2, 3, 4]
Last 5:    [5, 6, 7, 8, 9]
Middle:    [3, 4, 5, 6]
Reversed:  [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
Every 3rd: [0, 3, 6, 9]

# Coordinates
point = (10, 20)
x, y = point
print(f"Point: ({x}, {y})")

# Swap two variables
a, b = 100, 200
a, b = b, a
print(f"a={a}, b={b}")

# Return multiple values from function
def min_max(lst):
    return min(lst), max(lst)
lo, hi = min_max([3, 1, 4, 1, 5, 9])
print(f"min={lo}, max={hi}")

Point: (10, 20)
a=200, b=100
min=1, max=9

📝 Exercises

Try them yourself first, then open the solution to compare.

a: 5
b: 5
c: 8
Isosceles triangle

a = float(input("a: "))
b = float(input("b: "))
c = float(input("c: "))
if a+b > c and b+c > a and a+c > b:
    if a == b == c:
        print("Equilateral triangle")
    elif a == b or b == c or a == c:
        print("Isosceles triangle")
    else:
        print("Scalene triangle")
else:
    print("Not a valid triangle")

computer = "rock"
user = input("rock / paper / scissors: ").lower()
if user == computer:
    print("Draw!")
elif (user == "paper" and computer == "rock") or \
     (user == "scissors" and computer == "paper") or \
     (user == "rock" and computer == "scissors"):
    print("You win!")
else:
    print("You lose!")

for i in range(1, 101):
    if i % 15 == 0:
        print('FizzBuzz')
    elif i % 3 == 0:
        print('Fizz')
    elif i % 5 == 0:
        print('Buzz')
    else:
        print(i)

N: 5
    *
   **
  ***
 **** 
*****

n = int(input('N: '))
for i in range(1, n+1):
    print(' ' * (n-i) + '*' * i)

Enter 5 scores:
85 90 78 92 88
Sum: 433  Avg: 86.6  Max: 92  Min: 78

scores = [int(input(f'Score {i+1}: ')) for i in range(5)]
print(f'Sum: {sum(scores)}  Avg: {sum(scores)/len(scores):.1f}  Max: {max(scores)}  Min: {min(scores)}')

List: 1 2 3 4 5
k: 2
[4, 5, 1, 2, 3]

lst = list(map(int, input('List: ').split()))
k = int(input('k: ')) % len(lst)
print(lst[-k:] + lst[:-k])