Conditional or Selection Statement in Python

In your everyday lives, you make choices based on different situations, like deciding whether to watch a movie or a web series. Similarly, in programming, you need to make decisions and take different actions based on specific conditions. In Python, you have a useful tool for this called conditional or selection statements.

Conditional statements help you control the flow of your programs by checking conditions and executing specific actions based on those conditions. For example, you can instruct your program to display an umbrella icon if it is raining, and a sun icon if it’s not. This capability makes your programs more dynamic and efficient.

What is conditional statement

A conditional statement allows a program to make decisions based on certain conditions. It execute a block of code based on a certain condition. It is used to control the flow of a program.

Types of Conditional Statements:

if statement

The if statement is used to specify a condition and executes the block of code only if the condition is True.

Syntax:

if condition:
    statement 1
    statement 2

• The condition is an expression that evaluates to either True or False.
• The colon : after the if statement is important because it indicates the start of the code block.
• The block of code inside if must be indented (typically 4 spaces).

for example:

num = 124
if num > 0: 
    print("The number is positive.")  # output: The number is positive.

if-else statement

Python also provide you the if-else statement, which allows you to execute different blocks of code based on a condition. Basically, if-else statement is used to execute one block of code if the condition is true which is if block, and another block of code if the condition is false that is else block.

Syntax:

if condition:
    # Code block executed if the condition is True
    statements
else:
    # Code block executed if the condition is False
    statements

• The if-else statement follows the same indentation rules as the if statement.
• The colon : after the else statement used to indicate the beginning of the else code block.
• If the condition is True, the if block runs and if the condition is False, the else block runs.

for example:

age = 16
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")

# Output: 
# You are not eligible to vote

Now, as the condition age >= 18 is False, the else block is executed.

elif statement

The if-elif statement allows you to check multiple conditions and execute different blocks of code based on the True condition. This is also called if-else ladder.

Syntax:

1. Single elif

if condition1:
    # code block for condition1
elif condition2:
    # code block for condition2

2. Multiple elif

if condition1:
    # code block for condition1
elif condition2:
    # code block for condition2
elif condition3:
    # code block for condition3

3. If-elif-else

if condition1:
    # code block for condition1
elif condition2:
    # code block for condition2
# ... more elif statements if needed
else:
    # code block if none of the conditions are true

• The program first checks the initial if statement. If it’s True, that block runs, and the rest are skipped. If the initial condition is False, the program will move on to the next condition, which is written as elif.
• There is no strict limit to the number of elif statements. You can have as many elif statements as needed.
• The program will check each elif condition, one by one in order, until it finds a condition that is True.
• Once it finds a True condition, the code inside that particular elif block will be executed, and python will skip the remaining blocks.
• The else block executes only if all previous conditions are False. The else statement is not required after every elif statement. It is optional.

for example

marks = 75
if marks >= 90:
    print("Grade: A+")
elif marks >= 80:
    print("Grade: A")
elif marks >= 70:
    print("Grade: B")
else:
    print("Grade: C")

# Output:
# Grade: B

Nested if-else statement

A nested if-else or nested if or nested elif statement is an if-elif-else (each optional other than if) statement inside another if or else or elif block. This structure allows for checking multiple conditions in a more organized way.

Here are four different syntax examples for nested if statements but you can create it according to your need

Syntax 1

if condition1:
    # Outer if block
    if condition2:
        # Inner if block
    else:
        # Inner else block
else:
    # Outer else block

Syntax 2

if condition1:
    # Outer if block
    if condition2:
        # Inner if block
    elif condition3:
        # Inner elif block
    else:
        # Inner else block
else:
    # Outer else block

Syntax 3

if condition1:
    # Outer if block
else:
    if condition2:
        # Inner if block
    elif condition3:
        # Inner elif block
    # Outer else block

Syntax 4

if condition1:
    # Outer if block
    if condition2:
        # Inner if block
    else:
        # Inner else block
elif condition3:
    # Outer elif block
    if condition4:
        # Inner if block
    else:
        # Inner else block
else:
    # Outer else block

• First, the program evaluates the outer (level 1) if or elif or else statement to check if it is true or false.
• If the outer if or elif or else condition is true, the program proceeds to the inner (level 2) if’s statements and evaluates its condition.
• If the inner if condition is also true, the code inside the inner if block is executed.
• If either the outer or inner if condition is false, the code inside the inner if block is skipped, and the program continue.

for example:

age = int(input("Enter your age: "))

if age >= 18:  # First condition check
    print("You are an adult.")

    if age >= 60:  # Second condition inside first if
        print("You are a senior citizen.")
    else:
        print("You are not a senior citizen yet.")

else:
    print("You are a minor.")

# Output:
# Enter your age: 25
# You are an adult.
# You are not a senior citizen yet.

Common Mistakes

• Forgetting the colon : at the end of if, elif, or else lines.

age = 18
if age >= 18  # Missing colon
    print("You are an adult.")

Error:

IndentationError: expected an indented block

• using assignment operator instead of comparision operator.

x = 5
if x = 5:  # Single '=' is for assignment, not comparison
    print("x is 5")

Error:

SyntaxError: invalid syntax

• Not Using else When Necessary.

score = 40
if score >= 50:
    print("You passed!")  # No output if score < 50

Issue:
If score = 40, nothing happens

• Using elif Instead of else When No Further Condition Is Needed

marks = 30
if marks >= 50:
    print("Pass")
elif marks < 50:  # Unnecessary `elif`
    print("Fail")

Correct code

marks = 30
if marks >= 50:
    print("Pass")
else:  # Use `else` since it's the only alternative
    print("Fail")

• Forgetting to Handle All Possible Cases

number = 0
if number > 0:
    print("Positive")
elif number < 0:
    print("Negative")

Corrected code

f number > 0:
    print("Positive")
elif number < 0:
    print("Negative")
else:
    print("Zero")  # Handle all cases

Uses of conditional statement

Conditional statements serve several important purposes:

• Decision Making: They enable programs to make choices based on different situations, making the code more flexible and intelligent.
• User Interaction: They allow programs to respond to user actions, such as clicking a button, by deciding what to do next.
• Error Handling: Conditional statements can check if inputs are valid. For instance, if a user is asked to enter a number, the program can verify whether the input is correct.
• Simplifying Logic: They help break down complex problems into simpler, manageable parts.

Conclusion

Understanding if-else statements is crucial for writing logical and efficient Pythonprograms. By avoiding common mistakes, you can improve the readability, maintainability, and correctness of your code.

Always ensure that all possible conditions are handled, use elif where appropriate, and write concise, structured logic to enhance clarity. This will not only make your code more effective but also easier for others to understand and work with.

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Conditional or Selection Statement in Python

Checks extra conditions

1 / 4

What is the purpose of the “elif” keyword?

To check additional conditions if previous ones are false

To end the conditional statement

To repeat the if block

To define a function

Wrong Answer

“elif” (short for “else if”) allows you to test multiple conditions in a sequential manner after an initial if statement.

Begins condition check

2 / 4

Which keyword is used to start a conditional block?

if

def

for

while

Wrong Answer

The “if” keyword is used to begin a conditional statement that checks if a condition is true.

Controls program flow

3 / 4

What is a conditional statement in Python used for?

Storing data permanently

Iterating over a collection

Declaring variables

Controlling program flow

Wrong Answer

Conditional statements let you execute different blocks of code based on whether a condition is true or false, thereby controlling the flow of the program.

Runs if false

4 / 4

What does the “else” block do in an if-else statement?

Executes regardless of the condition

Executes when the condition is true

Repeats the if block

Executes when the condition is false

Wrong Answer

The “else” block is executed only when the condition in the “if” statement is false.

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