If Statement in Python

An if statement in Python is used to execute a block of code only if a specified condition is true. It allows you to control the flow of your program by checking conditions and performing different actions based on the results.

Syntax of the if Statement

Here’s the basic syntax of an if statement in Python:

if condition:
    # Block of code to execute if condition is True

The keyword if is followed by a condition (or expression).
If the condition evaluates to True, the block of code underneath the if statement runs.
If the condition evaluates to False, the block is skipped, and the program continues to the next - statement after the if block.

Indentation in Python

Python relies on indentation (typically 4 spaces) to define the blocks of code associated with control structures. This is different from many other programming languages that use curly braces {}. So, proper indentation is crucial in Python.

Checking a Simple Condition

Let’s start with a simple example that checks if a number is positive.

number = 10
if number > 0:
    print("The number is positive.")

Checking a Boolean Variable

The condition in an if statement can also be a boolean value. Let’s explore how it works.

is_logged_in = True

if is_logged_in:
    print("Welcome back, user!")

More Complex Conditions

You can use comparison operators (<, >, ==, etc.) or logical operators (and, or, not) in the if condition.

temperature = 30

if temperature > 20 and temperature < 35:
    print("It's a warm day.")

Common Pitfalls with if Statements

Forgetting Indentation In Python, the code block after the if statement must be indented. Forgetting to indent will raise an error.
Forgetting Indentation
```
# Incorrect: Will raise an IndentationError

if number > 0:
print("The number is positive.") # This line should be indented
```
Using Assignment (=) Instead of Comparison (==)
Another common mistake is using a single equals sign (=) instead of a double equals sign (==). Remember, = is for assignment, while == is for comparison.
```
# Incorrect: Will not behave as expected

if number = 10: # Should be if number == 10
print("The number is 10.")
```

Nested If Statements

You can place an if statement inside another if statement to check multiple conditions in sequence.

age = 20

if age >= 18:
    if age < 21:
        print("You are an adult but not old enough to drink alcohol.")

Explanation:

The first if checks if the person is 18 or older. The second if inside the first one checks if the person is younger than 21. If both conditions are satisfied, the message will be printed.

Exercise: if Statements

Write a program that checks if a number entered by the user is even or odd.
Hint:
Use the modulus operator % to find the remainder of a division.
If the remainder of the number divided by 2 is 0, it’s even.
Create a program that takes the temperature as input and prints “It’s freezing!” if the temperature is below 0.
Hint:
check if the temperature is less than 0.