Control Flow: Conditional Statements and Loops

When writing a Python program, one of the essential aspects to master is controlling the flow of your code. This guide covers two fundamental constructs: conditional statements and loops. We'll explore how to use if statements to make decisions in your code, and how to utilize for and while loops to repeat actions. Prepare to dive into some practical examples and exercises to cement your understanding!

Conditional Statements: The Power of `if`

Conditional statements are used to execute certain pieces of code only when a specific condition is met. In Python, the primary construct for this is the if statement. You can also extend conditions using elif (else if) and else.

Basic Structure of an `if` Statement

The basic syntax for an if statement looks like this:

if condition:
    # code to execute if the condition is true

Here’s a simple example:

temperature = 30

if temperature > 25:
    print("It's a hot day!")

If temperature is greater than 25, Python prints "It's a hot day!" Otherwise, the code block underneath the if statement is skipped.

Using `elif` and `else`

To handle multiple conditions, you can use elif and else. This allows for more complex decision-making. Here’s an example:

temperature = 20

if temperature > 25:
    print("It's a hot day!")
elif temperature > 15:
    print("It's a pleasant day!")
else:
    print("It's a cold day!")

In this example, if temperature is 20, Python evaluates the first condition (temperature > 25), which is false, so it moves to the elif, which is true, and prints "It's a pleasant day!".

Logical Operators

You can combine multiple conditions using logical operators like and, or, and not. Here’s how that works:

temperature = 18
is_raining = True

if temperature > 15 and not is_raining:
    print("It's a good day for a walk!")
else:
    print("Better stay inside!")

In this case, the output would be "Better stay inside!" since it's raining.

Short-Circuit Evaluation

Python uses short-circuit evaluation, meaning it will stop evaluating as soon as the result is determined. For instance:

def is_valid_age(age):
    return age >= 0 and (age < 130)

print(is_valid_age(25))   # True
print(is_valid_age(-5))   # False
print(is_valid_age(150))  # False

In the is_valid_age function, if age is -5, Python won’t check the second condition because the first is already false.

Loops: `for` and `while`

Loops are used to repeat a block of code multiple times. In Python, there are two main types of loops: for loops and while loops.

`for` Loops

for loops are often used to iterate over a collection of items, such as a list, tuple, or string. Here's the basic syntax:

for item in iterable:
    # code to execute for each item

Example of a `for` Loop

fruits = ['apple', 'banana', 'cherry']

for fruit in fruits:
    print(fruit)

This loop will output each fruit in the list. You can also iterate over a range of numbers:

for i in range(5):
    print(i)

This will print numbers from 0 to 4.

Nesting Loops

You can even nest loops within each other. For instance:

colors = ['red', 'green', 'blue']
objects = ['car', 'bike']

for color in colors:
    for obj in objects:
        print(f"A {color} {obj}")

`while` Loops

while loops continue to execute a block of code as long as a specified condition is true. This is useful when the number of iterations isn't known in advance. Here’s the basic syntax:

while condition:
    # code to execute

Example of a `while` Loop

count = 0

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

This will print the numbers 0 through 4, incrementing the count variable with each iteration.

Infinite Loops

Be cautious with while loops, as it's easy to create an infinite loop:

while True:
    print("I'm stuck!")

The loop will run forever unless you manually stop it or include a break condition.

Using `break` and `continue`

Within loops, Python lets you control the flow using break to exit a loop prematurely and continue to skip the current iteration and proceed to the next one.

Example of `break`

for i in range(10):
    if i == 5:
        break
    print(i)

This will print numbers 0 through 4 and stop when i reaches 5.

Example of `continue`

for i in range(5):
    if i == 2:
        continue
    print(i)

This will print 0, 1, 3, and 4, skipping the number 2.

Exercises

To practice what you've learned, try these exercises:

Conditional Statement Exercise: Write a program that checks if a number entered by the user is even or odd.
Loop Exercise 1: Create a for loop that prints the first ten square numbers (0, 1, 4, 9, 16, ...).
Loop Exercise 2: Write a while loop that counts down from 10 to 0 and prints each number.
Combine Both: Create a program that asks the user for a number and prints whether it is prime, along with all prime numbers up to that number using loops.

Conclusion

In this article, we explored how to control the flow of your Python programs with conditional statements and loops. We discussed the if, elif, and else constructs for decision-making, and how to use for and while loops to repeat actions.

By understanding how to adequately control flow, you can write more efficient and powerful scripts that can adapt to different conditions and perform tasks repeatedly. Now it's time to practice and apply these concepts to your own projects! Happy coding!

Programming - Python