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Python Control Flow

Control flow is the order in which statements are executed in a program. In this tutorial, we'll learn how Python provides structures that let you control the flow of your code, allowing you to make decisions, repeat actions, and skip certain code blocks when needed.

Introduction to Control Flow​

Control flow refers to the order in which individual statements, instructions, or function calls are executed in a program. By default, Python executes code sequentially from top to bottom. However, with control flow structures, you can:

  • Execute code only if certain conditions are met
  • Skip sections of code
  • Repeat sections of code
  • Jump to different parts of your program

These capabilities are fundamental to creating dynamic and responsive programs.

Conditional Statements​

The if Statement​

The if statement allows you to execute a block of code only if a certain condition is true.

python
temperature = 25

if temperature > 30:
    print("It's hot outside!")
    
print("Program continues...")

Output:

Program continues...

In this example, since the temperature is not greater than 30, the message "It's hot outside!" is not printed.

if-else Statement​

The if-else statement provides an alternative execution path when the condition is false.

python
temperature = 25

if temperature > 30:
    print("It's hot outside!")
else:
    print("The temperature is moderate.")
    
print("Program continues...")

Output:

The temperature is moderate.
Program continues...

if-elif-else Statement​

For multiple conditions, use the elif (short for "else if") statement.

python
temperature = 25

if temperature > 30:
    print("It's hot outside!")
elif temperature > 20:
    print("The weather is pleasant.")
elif temperature > 10:
    print("It's a bit cool.")
else:
    print("It's cold outside!")

Output:

The weather is pleasant.

Nested Conditionals​

You can place conditional statements inside other conditional statements.

python
temperature = 25
is_raining = True

if temperature > 20:
    print("It's warm outside.")
    if is_raining:
        print("But bring an umbrella!")
    else:
        print("Enjoy the sunshine!")
else:
    print("It's cold outside.")

Output:

It's warm outside.
But bring an umbrella!

Loops​

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

for Loops​

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

python
fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(f"I like {fruit}s")

Output:

I like apples
I like bananas
I like cherrys

The range() Function​

The range() function generates a sequence of numbers that you can iterate over.

python
# Print numbers from 0 to 4
for i in range(5):
    print(i)

Output:

0
1
2
3
4
python
# Print numbers from 1 to 5
for i in range(1, 6):
    print(i)

Output:

1
2
3
4
5
python
# Print even numbers from 2 to 10
for i in range(2, 11, 2):
    print(i)

Output:

2
4
6
8
10

while Loops​

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

python
count = 0

while count < 5:
    print(f"Count is: {count}")
    count += 1

Output:

Count is: 0
Count is: 1
Count is: 2
Count is: 3
Count is: 4

Loop Control Statements​

Loop control statements change the execution from its normal sequence.

break Statement​

The break statement terminates the loop containing it.

python
for i in range(10):
    if i == 5:
        print("Breaking the loop")
        break
    print(i)

Output:

0
1
2
3
4
Breaking the loop

continue Statement​

The continue statement skips the current iteration and continues with the next one.

python
for i in range(10):
    if i % 2 == 0:  # Skip even numbers
        continue
    print(i)

Output:

1
3
5
7
9

else Clause with Loops​

Python allows you to use an else clause with both for and while loops. The else block is executed when the loop condition becomes false.

python
for i in range(5):
    print(i)
else:
    print("Loop completed successfully!")

Output:

0
1
2
3
4
Loop completed successfully!

The else clause is not executed if the loop is terminated by a break statement.

python
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("Loop completed successfully!")  # This won't be executed

Output:

0
1
2

Practical Examples​

Example 1: Finding Elements in a List​

Let's use control flow to find specific elements in a list:

python
numbers = [10, 25, 33, 42, 57, 68, 79, 84, 95]
even_numbers = []
odd_numbers = []

for num in numbers:
    if num % 2 == 0:
        even_numbers.append(num)
    else:
        odd_numbers.append(num)
        
print(f"Even numbers: {even_numbers}")
print(f"Odd numbers: {odd_numbers}")

Output:

Even numbers: [10, 42, 68, 84]
Odd numbers: [25, 33, 57, 79, 95]

Example 2: Simple Number Guessing Game​

python
import random

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

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

while attempts < max_attempts:
    try:
        guess = int(input("Enter your guess: "))
        attempts += 1
        
        if guess < secret_number:
            print("Too low!")
        elif guess > secret_number:
            print("Too high!")
        else:
            print(f"Congratulations! You guessed the number {secret_number} correctly!")
            break
            
    except ValueError:
        print("Please enter a valid number.")
        
    remaining = max_attempts - attempts
    if remaining > 0:
        print(f"You have {remaining} attempts left.")
else:
    print(f"Sorry, you've used all your attempts. The secret number was {secret_number}.")

Example 3: Data Processing for Pandas​

This example shows how control flow can help prepare data for analysis with Pandas:

python
# Sample data (imagine this is from a CSV file)
sales_data = [
    {"date": "2023-01-01", "product": "A", "units": 50, "price": 10},
    {"date": "2023-01-02", "product": "B", "units": 30, "price": 15},
    {"date": "2023-01-03", "product": "A", "units": 25, "price": 10},
    {"date": "2023-01-04", "product": "C", "units": 10, "price": 25},
    {"date": "2023-01-05", "product": "B", "units": 35, "price": 15}
]

# Calculate total revenue for each product
product_revenue = {}

for sale in sales_data:
    product = sale["product"]
    revenue = sale["units"] * sale["price"]
    
    if product in product_revenue:
        product_revenue[product] += revenue
    else:
        product_revenue[product] = revenue

# Find the best-selling product
best_seller = None
max_revenue = 0

for product, revenue in product_revenue.items():
    print(f"Product {product} generated ${revenue} in revenue")
    
    if revenue > max_revenue:
        max_revenue = revenue
        best_seller = product

print(f"\nThe best-selling product is {best_seller} with ${max_revenue} in revenue")

Output:

Product A generated $750 in revenue
Product B generated $975 in revenue
Product C generated $250 in revenue

The best-selling product is B with $975 in revenue

Summary​

Control flow is a fundamental concept in Python programming. In this tutorial, you learned:

  • How to use if, elif, and else statements to make decisions in your code
  • How to use for loops to iterate over sequences
  • How to use while loops for condition-based iteration
  • How to control loops with break and continue statements
  • How to use the else clause with loops

Understanding control flow is essential for working with Pandas, as you'll often need to:

  • Filter data based on conditions
  • Iterate through data frames
  • Perform different operations based on data values
  • Process and transform data

Exercises​

  1. Write a program that prints all numbers from 1 to 100, but for multiples of 3, print "Fizz" instead of the number, for multiples of 5, print "Buzz", and for multiples of both 3 and 5, print "FizzBuzz".

  2. Create a simple calculator program that takes two numbers and an operator (+, -, *, /) as input and performs the corresponding operation.

  3. Write a program that takes a list of temperatures in Celsius and converts them to Fahrenheit using a for loop.

  4. Create a program that checks if a given year is a leap year.

  5. Write a program that removes all duplicate items from a list while preserving the original order.

Additional Resources​

Understanding control flow is crucial for your journey with Python and Pandas. These concepts will form the foundation of how you'll structure your data analysis code.


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