Control Flow in Python

1. Conditional Statements: if, else, elif

These statements allow your program to execute different blocks of code based on whether a condition is True or False.

The if Statement

• Purpose: "If this condition is true, then do this."

• Syntax: The code under the if statement must be indented (usually 4 spaces). This is how Python defines code blocks.

python

age = 18

if age >= 18:
    print("You are an adult.") # This line is inside the if-block
    print("You can vote!")     # This line is also inside the if-block

print("This line runs regardless") # This is outside the if-block

The else Statement

• Purpose: "If the if condition is false, then do this instead."

• Syntax: Must follow an if statement.

python

temperature = 30

if temperature > 25:
    print("It's a hot day!")
else:
    print("It's not too hot today.")

The elif Statement

• Purpose: Short for "else if". It allows you to check multiple conditions in sequence.

• Syntax: Comes after an if and before an else (if there is one).

python

x

temperature = 30

if temperature > 25:
    print("It's a hot day!")
else:
    print("It's not too hot today.")

Key Point: Python checks these conditions in order. As soon as one condition is True, it executes that block and skips the rest.

2. Loops

Loops allow you to execute a block of code multiple times.

for Loops

• Purpose: Used for iterating over a sequence (like a list, string, or range) a known number of times.

• Syntax: for item in sequence:

python

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

# Using range() to loop a specific number of times
for i in range(5):  # range(5) generates numbers 0 to 4
    print("Iteration number:", i)

# Iterating over a string
for letter in "Hello":
    print(letter)

while Loops

• Purpose: Repeats a block of code as long as a condition remains true. Use when you don't know how many iterations you'll need in advance.

• Syntax: while condition:

python

# Simple counter
count = 1
while count <= 5:
    print("Count is:", count)
    count += 1  # This is crucial! Without it, the loop would run forever.
# Output: Count is: 1, 2, 3, 4, 5

# User input until a condition is met
password = ""
while password != "secret123":
    password = input("Enter the password: ")
print("Access granted!")

Warning: Be careful with while loops! If the condition never becomes false, you'll create an infinite loop that will crash your program.

3. Loop Control Statements: break and continue

These statements give you more precise control over how loops execute.

break

• Purpose: Exit the loop immediately, before it has finished all its iterations.

python

x

# Simple counter
count = 1
while count <= 5:
    print("Count is:", count)
    count += 1  # This is crucial! Without it, the loop would run forever.
# Output: Count is: 1, 2, 3, 4, 5

# User input until a condition is met
password = ""
while password != "secret123":
    password = input("Enter the password: ")
print("Access granted!")

continue

• Purpose: Skip the rest of the current iteration and jump to the next one.

python

# Print only odd numbers
for i in range(10):
    if i % 2 == 0:  # If the number is even
        continue     # Skip the rest of this iteration
    print(i)
# Output: 1, 3, 5, 7, 9

Simple way to remember:

• break = STOP the entire loop

• continue = SKIP this one, go to the next

Putting It All Together: A Complete Example

Let's create a simple number guessing game that uses all these control flow concepts.

python

import random

# Generate a random number between 1 and 10
secret_number = random.randint(1, 10)
attempts = 0
max_attempts = 3

print("I'm thinking of a number between 1 and 10.")
print(f"You have {max_attempts} attempts to guess it!")

while attempts < max_attempts:
    # Get user guess
    guess = int(input("Enter your guess: "))
    attempts += 1
    
    # Check if guess is valid
    if guess < 1 or guess > 10:
        print("Please enter a number between 1 and 10.")
        continue  # Skip the rest of this iteration
    
    # Check the guess
    if guess == secret_number:
        print(f"Congratulations! You guessed it in {attempts} attempts!")
        break  # Exit the loop - game over, player won
    elif guess < secret_number:
        print("Too low! Try again.")
    else:
        print("Too high! Try again.")
    
    # Tell player how many attempts remain
    remaining = max_attempts - attempts
    if remaining > 0:
        print(f"You have {remaining} attempt(s) left.")
    else:
        print(f"Game over! The secret number was {secret_number}.")

This program demonstrates:

• while loop for the main game cycle

• if/elif/else for checking the guess

• break to exit when the player wins

• continue to skip invalid input

Control flow is what makes programs interactive and dynamic. Mastering these concepts is a huge step toward becoming a proficient programmer!