Loading......

Password

Section 1. Control Flow

Conditional Statements

Like traffic lights control vehicles:

light_color = input("Traffic light color: ")

if light_color == "red":
    print("STOP")
elif light_color == "yellow":
    print("SLOW DOWN")
elif light_color == "green":
    print("GO")
else:
    print("INVALID COLOR")

Flowchart - Decision Making

graph TD
    A[Start] --> B{Light Color?}
    B -->|Red| C[Stop]
    B -->|Yellow| D[Slow Down]
    B -->|Green| E[Go]
    C --> F[End]
    D --> F
    E --> F

While Loops

Like repeating morning runs:

import time
runs_remaining = 3
while runs_remaining > 0:
    time.sleep(1) # suppose you are a superman, 400m in 1 second 
    print("Morning run #"+str(4 - runs_remaining)+" completed!")
    runs_remaining -= 1
print("All runs finished!")

Flowchart - Loop Execution

graph TD
    A[Start] --> B{Runs Left?}
    B -->|Yes| C[Run]
    C --> D[Reduce Count]
    D --> B
    B -->|No| E[End]

Why Control Flow Matters

Programs need to make decisions
Handle different situations
Repeat tasks efficiently
Makes code adaptable

Example: Automatic door control

# since we don't have a sensor, we just pretend to be a sensor.

import time

def object_detected():
    # get signal from the sensor
    signal = input('detected(y/n):')
    if signal=='y':
        result=True
        print('detected!')
    else:
        result=False
        print('no object!')
    time.sleep(1)
    return(result)

def keep_open():
    print('open door!')
    # send signal to the door
    time.sleep(1)

def close_door():
    print('close door!')
    # send signal to the door
    time.sleep(1)

sensor_active = True

while sensor_active:
    if object_detected():
        keep_open()
    else:
        close_door()

Vending machine

Let’s code a drink machine:

coins = 0
while coins < 5:
    insert_coins = int(input("Insert coin (1-3): "))
    coins = coins + insert_coins
    print("Total inserted: "+str(coins))
print("Here's your drink!")

Question: how can we limit the number of inserted coins each time

Question: how can we get the extra coins

Common Patterns

Flag-controlled loops

running = True
while running:
 user_input = input("Continue? (y/n): ")
 if user_input == "n":
     running = False

Counter-controlled loops ```python attempts = 3 while attempts > 0: # code here attempts=attempts-1

attempts = 0 while attempts < 10: # code here attempts=attempts+1

---

# Take-home message
1. `if-elif-else` for decisions
2. `while` for repeating tasks
3. Condition evaluation (True/False)
4. Loop control variables (i, counter)
5. Avoid infinite loops!
```python
# Always update condition!
i = 0
while i < 10:
    print(i)
    i = i+1  # Don't forget this!

Section 2. Conditionals in Python

1. `if` Statement Basics

# Basic structure
age = 18
if age >= 18:
    print("You can vote!")

if = Program’s “what if” question
Colon : is mandatory (show error without colon)
Indentation = 4 spaces (never mix with Tab)

2. Indentation Demo

# Correct vs Wrong
if True:
    print("Right")  # ← 4 spaces

if True:
print("Wrong")     # ← No indentation

3. Comparison Operators

height = 175
print(height > 160)  # True
print(height == 170) # False

# Real-life example
bus_height_limit = 200
luggage_height = 210
if luggage_height > bus_height_limit:
    print("Cannot take bus")

4. Logical Operators

# Weather check
is_raining = True
has_umbrella = False

if is_raining and has_umbrella:
    print("Go outside")
else:
    print("Stay home")

and: Both must be True
or: At least one True
not: Reverse boolean

5. Common Mistake Fix

# Wrong
if temperature = 30:
    print("Hot")

# Correct
if temperature == 30:
    print("Hot")

Single = is assignment
Double == is comparison

6. Multi-Condition Flow

age = 15

if age < 13:
    print("Child ticket")
elif age < 18:
    print("Teen ticket")
else:
    print("Adult ticket")

7. Nested Conditions

age = 20
is_student = True

if age < 18:
    print("50% discount")
else:
    if is_student:
        print("30% discount")
    else:
        print("No discount")

Each level needs 4-space indent
Max 3 nesting levels recommended

8. Ticket System Flowchart

# Full example
age = 25
is_student = False
has_membership = True

if age < 5:
    print("Free ticket")
elif age < 18:
    print("Child ticket")
elif is_student:
    print("Student discount")
elif has_membership:
    print("Membership discount")
else:
    print("Regular ticket")

Question: what if you are a 19 years old student without membership.

Question: what if you are a 19 years old student with membership.

Question: how can we get both the Student discount and the Membership discount

9. Truth Value Testing

# Common False values:
- False
- None
- 0
- "" (empty string)
- [] (empty list)

name = ""
if not name:
    print("Name is required!")

10. Short-form Conditional Expression (not recommended)

# Short form
temperature = 28
message = "Hot" if temperature > 25 else "OK"
print(message)

# preferred
temperature = 28
if temperature >25:
    message='Hot'
else:
    message='OK'
print(message)

Use for simple two-way choices
Not recommended for complex logic

Section 3. Loop Structures in Python

1. For Loop Basics

# Like checking a shopping list
groceries = ["apple", "milk", "bread"]
for item in groceries:
    print("Don't forget to buy:", item)

flowchart TD
    A[Start] --> B{Items left?}
    B -->|Yes| C[Process item]
    C --> B
    B -->|No| D[End]

2. The range() Function

Three usages:

# Single argument
for num in range(5):
    print(num)  # 0-4

# Two arguments
for num in range(2, 5):
    print(num)  # 2-4

# Three arguments
for num in range(0, 10, 2):
    print(num)  # 0,2,4,6,8

3. String Traversal with Index

word = "hello"
for index, letter in enumerate(word):
    print("Character at position "+str(index)+": "+letter)

Output:

Character at position 0: h
Character at position 1: e
...

4. While Loop Mechanics

# Like a persistent alarm clock
countdown = 3
while countdown > 0:
    print(countdown)
    countdown = countdown-1
print("Go!")

5. Termination Condition Design

# Dangerous version (for demonstration)
# temperature = 25
# while temperature > 20:
#     print("AC running")
# Forgot to decrease temperature!

6. Safe Loop Writing Technique

Always write termination first:

# 1. Set initial value
counter = 0
# 2. Define termination
MAX_ATTEMPTS = 3

while counter < MAX_ATTEMPTS:
    print("Attempt:", counter+1)
    counter += 1

7. Break Statement

# Emergency exit example
passwords = ["123", "admin", "letmein"]
for pwd in passwords:
    if pwd == "admin":
        print("Security alert!")
        break
    print("Checking:", pwd)

8. Continue Statement

# Skip odd numbers
for num in range(10):
    if num % 2 != 0:
        continue
    print(num, "is even")

9. Else Clause in Loops

# Only executes if loop completes normally
numbers = [2, 4, 6]
for n in numbers:
    if n % 2 != 0:
        break
else:
    print("All numbers are even!")

10. Nested Loops: Multiplication Table

for i in range(1, 4):  # Rows
    for j in range(1, 4):  # Columns
        print(str(i)+'x'+str(j)+'='+str(i*j), end="\t")
    print()  # New line after each row