Conditional Statements and Loops

Key Points:

1. The condition temperature > 30 is evaluated as True or False.

2. The colon : marks the end of the condition.

3. Indentation (4 spaces) defines which code belongs to the if block.

4. If the condition is False, the indented code is skipped entirely.

The if-else Statement

Often you need one action when a condition is true and a different action when it's false:

The else block provides an alternative path—exactly one of the two blocks will execute.

The if-elif-else Statement

When you have multiple conditions to check, use elif (short for "else if"):

python

score = 85

if score >= 90:

    grade = "A"

    print("Excellent work!")

elif score >= 80:

    grade = "B"

    print("Good job!")

elif score >= 70:

    grade = "C"

    print("Fair performance.")

elif score >= 60:

    grade = "D"

    print("Needs improvement.")

else:

    grade = "F"

    print("Failed. Please retry.")

print(f"Your grade is: {grade}")

How It Works:

1. Python checks conditions from top to bottom.

2. The first True condition executes its block.

3. All subsequent conditions are skipped.

4. If no condition is True, the else block runs (if present).

Nested Conditionals

You can place conditional statements inside other conditionals for complex logic:

While nesting works, excessive nesting reduces readability. Try to keep logic as flat as possible using logical operators when appropriate:

Conditional Expressions (Ternary Operator)

For simple conditions, Python offers a one-line syntax:

This is equivalent to:

Use this for simple assignments, but avoid it for complex logic where readability suffers.

Loops: Repeating Actions

Loops execute the same block of code multiple times, essential for processing datasets, performing calculations, and automating repetitive tasks.

The for Loop

The for loop iterates over a sequence (list, range, string, etc.), executing code for each item:

Looping Through a Range

The range() function generates numbers:

1. range(5) produces 0, 1, 2, 3, 4

2. range(1, 6) produces 1, 2, 3, 4, 5

3. range(0, 10, 2) produces 0, 2, 4, 6, 8 (step of 2)

Looping Through a List

Practical Example: Calculating Total Sales

The while Loop

The while loop continues executing as long as a condition remains true:

Critical Warning: Always ensure the condition eventually becomes false, or you'll create an infinite loop:

Practical Use Case: User Input Validation

Loop Control Statements

Python provides special keywords to modify loop behavior mid-execution.

break: Exit the Loop

The break statement immediately terminates the loop:

continue: Skip to Next Iteration

The continue statement skips the remaining code in the current iteration and moves to the next one:

Practical Example: Data Cleaning

else with Loops

Python allows an else clause with loops—it executes when the loop completes normally (not via break):

This feature is rarely used but can make certain algorithms cleaner.

Nested Loops

Loops can contain other loops, useful for working with multi-dimensional data:

Performance Consideration: Nested loops multiply execution time. A loop running 100 times containing another loop running 100 times executes 10,000 times total. Use caution with large datasets.

Combining Conditionals and Loops

The real power emerges when combining these constructs:

Example: Categorizing Data

python

temperatures = [22, 35, 18, 40, 28, 15, 33]

hot_days = 0

warm_days = 0

cool_days = 0

for temp in temperatures:

    if temp >= 35:

        hot_days += 1

    elif temp >= 25:

        warm_days += 1

    else:

        cool_days += 1

print(f"Hot days (≥35°C): {hot_days}")

print(f"Warm days (25-34°C): {warm_days}")

print(f"Cool days (<25°C): {cool_days}")

Example: Processing Sales Data

python

sales = [1200, 1500, 800, 2000, 1100, 1800, 950]

target = 1000

days_met = 0

total_above_target = 0

for day, amount in enumerate(sales, start=1):

    if amount >= target:

        days_met += 1

        excess = amount - target

        total_above_target += excess

        print(f"Day {day}: Target met! (${amount})")

    else:

        shortfall = target - amount

        print(f"Day {day}: Below target by ${shortfall}")

print(f"\nDays meeting target: {days_met}/{len(sales)}")

print(f"Total above target: ${total_above_target}")

Best Practices

1. Keep Conditions Simple

2. Avoid Deep Nesting: If you find yourself nesting more than 2-3 levels, consider restructuring your logic or creating functions.

3. Use Meaningful Variable Names

4. Choose the Right Loop

Use for when you know how many iterations you need.

Use while when iterations depend on a condition that might change unpredictably.