If...Else Statements in Python

The if...else statement is a fundamental part of control flow in Python, allowing you to execute code based on certain conditions. It helps you make decisions in your code by evaluating whether an expression is True or False.

Syntax of if...else Statements

1. If Statement:
   The if statement checks whether a condition is True. If it is, the block of code inside the if statement runs.

if condition:    # Code to execute if condition is True

2. Else Statement:
   The else statement is optional and runs when the if condition is False.

if condition:    # Code to execute if condition is Trueelse:    # Code to execute if condition is False

3. Elif Statement:
   The elif (else if) statement is also optional and checks another condition if the if condition is False.

if condition1:    # Code to execute if condition1 is Trueelif condition2:    # Code to execute if condition1 is False and condition2 is Trueelse:    # Code to execute if both conditions are False

Examples

1. Basic If-Else

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

Output:

You are an adult.

In this example, the condition age >= 18 is True, so the first block of code is executed.

2. If-Elif-Else

x = 15if x < 10:    print("x is less than 10.")elif x == 10:    print("x is equal to 10.")else:    print("x is greater than 10.")

Output:

x is greater than 10.

In this case, the condition x < 10 is False, so it moves to the elif statement and checks if x == 10. Since both conditions are False, it finally executes the else block.

3. Nested If-Else

You can also nest if statements within other if statements.

x = 20y = 10if x > 15:    if y < 15:        print("x is greater than 15 and y is less than 15.")    else:        print("x is greater than 15 but y is not less than 15.")else:    print("x is not greater than 15.")

Output:

x is greater than 15 and y is less than 15.

In this example, since x > 15 is True, the program enters the nested if block and checks the condition y < 15. Because y is 10, the condition is True, so it executes the corresponding block.

4. Multiple Conditions with and and or

You can combine multiple conditions using logical operators like and, or, and not.

• and: Both conditions must be True.

• or: At least one condition must be True.

x = 10y = 20if x > 5 and y < 25:    print("Both conditions are True.")else:    print("At least one condition is False.")

Output:

Both conditions are True.

In this case, both x > 5 and y < 25 are True, so the if block is executed.

Comparison Operators in If Statements

Commonly used comparison operators in if statements include:

• ==: Equal to

• !=: Not equal to

• >: Greater than

• <: Less than

• >=: Greater than or equal to

• <=: Less than or equal to

Example:

a = 10b = 20if a != b:    print("a and b are not equal.")else:    print("a and b are equal.")

Output:

a and b are not equal.

Conclusion

The if...else structure is essential for controlling the flow of a program based on conditions. It allows your code to make decisions and execute different blocks of code based on whether certain conditions are true or false. You can combine multiple conditions with elif and logical operators to handle more complex scenarios.

Let me know if you have any more questions or need further examples!