If-Else Statements

In programming, control flow statements allow you to execute different blocks of code depending on certain conditions. In Python, one of the most commonly used control flow statements is the if-else statement.

The if-else statement works by evaluating a given condition, and executing a block of code if the condition is true. If the condition is false, another block of code can be executed instead. The general syntax of an if-else statement in Python is as follows:

if condition:
    # code to execute if condition is true
else:
    # code to execute if condition is false

For example, consider the following code:

x = 5
if x > 0:
    print("x is positive")
else:
    print("x is negative or zero")

In this example, the if statement checks whether x is greater than 0. Since x is equal to 5, which is greater than 0, the code inside the if block is executed and the output is "x is positive".

It is also possible to use multiple conditions in an if-else statement by using the elif keyword. The elif keyword allows you to specify additional conditions to check, in case the first condition is false. The general syntax of an if-else statement with multiple conditions is as follows:

if condition1:
    # code to execute if condition1 is true
elif condition2:
    # code to execute if condition1 is false and condition2 is true
else:
    # code to execute if both condition1 and condition2 are false

For example, consider the following code:

x = 5
if x > 10:
    print("x is greater than 10")
elif x > 0:
    print("x is positive but not greater than 10")
else:
    print("x is negative or zero")

In this example, the if statement checks whether x is greater than 10. Since x is equal to 5, which is not greater than 10, the code inside the if block is not executed. The elif statement then checks whether x is greater than 0. Since x is equal to 5, which is greater than 0, the code inside the elif block is executed and the output is "x is positive but not greater than 10".

It’s important to note that in Python, whitespace is significant, and each code block must be indented by the same amount of spaces. This is what separates one code block from another, and allows you to clearly define which code belongs to which if or elif statement.

In conclusion, the if-else statement is a powerful tool in Python that allows you to control the flow of your program based on conditions. By using if, elif, and else in combination, you can specify multiple conditions to check and execute different blocks of code depending on the outcome of each condition. This allows you to write code that can respond to different scenarios and make decisions based on the values of your variables.

In Python, you can use the re module to perform operations with regular expressions. The re module provides various functions for matching, searching, and replacing strings that match a specific pattern. In combination with the if-else statement, you can use regular expressions to perform more complex pattern-matching tasks.

Here is an example that demonstrates how to use if-else statements with regular expressions in Python:

import re

email = "example@gmail.com"

# Check if the email address is valid using a regular expression
match = re.match(r"[^@]+@[^@]+\.[^@]+", email)

if match:
    print("The email address is valid")
else:
    print("The email address is invalid")

In this example, the re.match() function is used to check if the string email matches a specific pattern. The pattern is defined using a regular expression, which specifies that the string must contain one or more characters ([^@]+) followed by an @ symbol, followed by one or more characters ([^@]+), followed by a ., followed by one or more characters ([^@]+).

If the re.match() function returns a match object, the if statement will evaluate to True, and the code inside the if block will be executed, printing "The email address is valid". If the re.match() function returns None, the if statement will evaluate to False, and the code inside the else block will be executed, printing "The email address is invalid".

In this example, the regular expression is used in combination with the if-else statement to check if the string email is a valid email address. However, this is just one of many ways that you can use regular expressions and if-else statements together in Python. With a combination of the re module and control flow statements, you can perform a wide range of pattern matching and conditional operations in your Python programs.