Conditional statements

Imagine writing a story where the next part depends on the reader’s choice. Conditional statements in Python work similarly. They let you tell the computer, “If this happens, do that. Otherwise, do something else.” This lets your program respond dynamically to different situations.

What are conditional statements?

Conditional statements are used to control the flow of your program by setting rules for what should happen under different circumstances. Python uses if, elif (short for else if), and else to implement this logic.

The if statement: Asking "What If?"

The if statement checks a condition. If it’s true, the code inside the if block runs; if not, Python skips it.

temperature = 30

if temperature > 25:
    print("It's a warm day!")

Explanation: If the temperature is greater than 25, the program prints "It's a warm day!" Otherwise, it skips this block.

The else statement: The backup plan

The else statement provides a fallback. If the if condition isn’t true, Python runs the code inside the else block.

temperature = 15

if temperature > 25:
    print("It's a warm day!")
else:
    print("It's a cool day.")

Explanation: If the temperature is not greater than 25, Python prints "It's a cool day." The else block handles everything the if doesn’t.

The elif statement: Checking additional conditions

The elif statement (short for else if) allows you to check multiple conditions sequentially.

temperature = 20

if temperature > 25:
    print("It's a warm day!")
elif temperature > 15:
    print("It's a mild day.")
else:
    print("It's a cool day.")

Explanation: Python checks the conditions in order:

1. If the temperature is greater than 25, it prints "It's a warm day!"

2. If not, it checks if the temperature is greater than 15. If true, it prints "It's a mild day."

3. If none of these conditions are true, it prints "It's a cool day."

How if, elif, and else work together

1. Python checks the if condition first.

2. If the if condition is true, it executes that block and skips the rest.

3. If the if condition is false, Python checks the next elif condition (if present).

4. If no conditions are true, the else block (if present) runs.

Examples of conditional statements

Example 1: Even or Odd numbers

number = 7

if number % 2 == 0:
    print("It's an even number!")
else:
    print("It's an odd number!")

Explanation: The program checks if the number is divisible by 2 with no remainder (number % 2 == 0). If true, it’s even; otherwise, it’s odd.

Example 2: A grading system

grade = 85

if grade >= 90:
    print("A")
elif grade >= 80:
    print("B")
elif grade >= 70:
    print("C")
else:
    print("D")

Explanation: The program checks the score range and assigns a grade:

• If the score is 90 or above, it prints "A".
• If the score is between 80 and 89, it prints "B".
• If the score is between 70 and 79, it prints "C".
• If the score is below 70, it prints "D".

Best practices for writing conditional statements

• Keep conditions clear: Write conditions that are easy to understand.

  # Good
  if age >= 18:
      print("Adult")
  
  # Confusing
  if not (age < 18):
      print("Adult")
  

• Avoid redundant checks: Use elif instead of multiple if statements.

  # Good
  if score >= 90:
      print("A")
  elif score >= 80:
      print("B")
  
  # Inefficient
  if score >= 90:
      print("A")
  if score >= 80 and score < 90:
      print("B")
  

• Use descriptive variables: Make your code readable by choosing meaningful variable names.

  temperature = 30  # Clear
  t = 30            # Vague
  

Conclusion

Conditional statements (if, elif, and else) are powerful tools for controlling program flow. They allow your code to make decisions and respond to different inputs or situations. With these, you can build dynamic, interactive programs that adapt based on the conditions you define.