Handling Alternatives: elif and else

The if statement allows a program to execute a block of code only when a specific condition evaluates to True. However, programs often need to check multiple possible conditions or perform a default action when an initial condition is not met. The elif and else clauses extend the if statement to handle these more complex decision-making scenarios.

Checking Multiple Conditions with elif

Imagine you want to check more than just one condition. For example, determining if a number is positive, negative, or zero requires checking multiple possibilities. The elif statement, short for "else if," lets you test additional conditions only if the preceding if (or elif) conditions were False.

You can chain multiple elif statements after an initial if. Python evaluates the conditions one by one from top to bottom. As soon as it finds a condition that is True, it executes the code block associated with that condition and then skips the rest of the entire if-elif-else structure.

Here's the general structure:

if condition_1:
    # Block 1: Executes if condition_1 is True
    statement(s)
elif condition_2:
    # Block 2: Executes if condition_1 is False AND condition_2 is True
    statement(s)
elif condition_3:
    # Block 3: Executes if condition_1 AND condition_2 are False,
    # AND condition_3 is True
    statement(s)
# ... you can have more elif blocks

Let's modify our number example:

number = -10

if number > 0:
    print("The number is positive.")
elif number < 0:
    print("The number is negative.")

# If number is 0, neither condition above is True, so nothing happens yet.

In this case, if number is -10:

1. number > 0 is False, so the first block is skipped.
2. number < 0 is True, so the message "The number is negative." is printed. The program then skips any subsequent elif or else clauses in this structure.

What if the number is 0? Neither number > 0 nor number < 0 is true. In the code above, nothing would be printed. To handle cases where none of the if or elif conditions are met, we use the else clause.

The Default Case: else

The else clause provides a default block of code that executes only when all preceding if and elif conditions in the structure have evaluated to False. It acts as a catch-all. An else clause, if used, must always come last, after all if and elif clauses, and it doesn't have a condition associated with it.

The complete structure looks like this:

if condition_1:
    # Block 1: Executes if condition_1 is True
    statement(s)
elif condition_2:
    # Block 2: Executes if condition_1 is False AND condition_2 is True
    statement(s)
# ... more elif blocks (optional) ...
else:
    # Block N: Executes if ALL preceding conditions (condition_1, condition_2, ...) are False
    statement(s)

Let's complete our number sign example using else:

number = 0

print(f"Checking the number: {number}")

if number > 0:
    print("The number is positive.")
elif number < 0:
    print("The number is negative.")
else:
    # This block runs only if number is not > 0 AND number is not < 0
    print("The number is zero.")

print("Finished checking.")

If you run this with number = 0:

1. number > 0 is False.
2. number < 0 is False.
3. Since all preceding conditions were false, the else block executes, printing "The number is zero."

The following diagram illustrates the flow of control for this example:

Flowchart showing the decision process for classifying a number as positive, negative, or zero using if-elif-else.

Order and Exclusivity

It's important to remember two things about if-elif-else structures:

1. Order Matters: The conditions are checked in the order they appear. If multiple conditions could potentially be true, only the block associated with the first true condition encountered will execute. Consider a grading example:

   score = 85
   
   if score >= 70: # This is True first
       grade = 'C'
   elif score >= 80: # This is also True, but checked later
       grade = 'B'
   else:
       grade = 'Not C or B'
   
   print(grade) # Output: C
   

   Because score >= 70 is checked first and is true for 85, the grade is set to 'C', and the elif score >= 80 check is never reached. To get the correct grade, you should order the checks from most specific (highest score) to least specific (lowest score).

   score = 85
   
   if score >= 90:
       grade = 'A'
   elif score >= 80: # Checked second, True for 85
       grade = 'B'
   elif score >= 70: # Checked third
       grade = 'C'
   # ... other grades
   else:
       grade = 'F'
   
   print(grade) # Output: B
   

2. Only One Block Executes: Within a single if-elif-...-else chain, at most one block of code will be executed. Even if later elif conditions might also be true, they are skipped once an earlier if or elif condition evaluates to True. The else block executes only if none of the conditions above it are true.

By combining if, elif, and else, you can construct clear and effective logic to handle various alternatives within your programs, making them react appropriately to different inputs and situations.