STEM Neo-Nalanda

Chapter 2: Control Flow

Making decisions and repetition in code

Control Flow in Python

Control flow refers to the order in which statements are executed in a program. Python provides various control structures that allow you to alter the flow of execution.

Conditional Statements

Conditional statements allow you to execute code based on whether certain conditions are true or false.

If Statement

age = 18

if age >= 18:
    print("You are an adult.")

If-Else Statement

temperature = 15

if temperature > 25:
    print("It's warm outside.")
else:
    print("It's cool outside.")

If-Elif-Else Statement

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Your grade is {grade}")

Loops

Loops are used to repeat a block of code multiple times.

For Loop

The for loop is used to iterate over a sequence (like a list, tuple, dictionary, string, or range).

# Iterating over a range
for i in range(5):  # 0, 1, 2, 3, 4
    print(i)

# Iterating over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

While Loop

The while loop executes a block of code as long as a condition is true.

count = 0
while count < 5:
    print(count)
    count += 1

Break and Continue

These statements can be used to control the flow of loops:

  • break: Exits the loop prematurely
  • continue: Skips the current iteration and moves to the next one
# Using break
for i in range(10):
    if i == 5:
        break
    print(i)  # Prints 0, 1, 2, 3, 4

# Using continue
for i in range(5):
    if i == 2:
        continue
    print(i)  # Prints 0, 1, 3, 4

Combining Conditions

You can combine conditions using logical operators:

  • and: True if both conditions are true
  • or: True if at least one condition is true
  • not: Inverts the condition
age = 25
has_license = True

if age >= 18 and has_license:
    print("You can drive.")

temperature = 30
is_raining = True

if temperature > 25 or is_raining:
    print("Weather is notable.")

is_weekend = False
if not is_weekend:
    print("It's a weekday.")

Solutions to Chapter 1 Practice Exercises

  1. Area of rectangle:
width = 7
height = 5
area = width * height
print(f"The area of the rectangle is {area} square units.")
  1. Variables for personal information:
name = "Alice"
age = 25
is_student = True

print(name)
print(age)
print(is_student)
  1. Fahrenheit to Celsius converter:
fahrenheit = 98.6
celsius = (fahrenheit - 32) * 5/9
print(f"{fahrenheit}°F is equal to {celsius:.2f}°C")

Practice Exercises

  1. Write a program that checks if a number is positive, negative, or zero.
  2. Create a program that prints all even numbers between 1 and 20 using a loop.
  3. Write a program that asks the user for a password and keeps asking until they enter “python123”.