Table of Contents

1. Introduction to Conditional Logic
2. If Statement Basics
3. If-Else Statements
4. Else-If Ladders
5. Nested If Statements
6. Comparison Operators
7. Logical Operators
8. Ternary Operator
9. Switch Statements
10. Pattern Matching
11. Common Patterns
12. Best Practices
13. Real-World Examples

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Introduction to Conditional Logic

Conditional logic is the foundation of decision-making in programming. It allows programs to execute different code paths based on different conditions, making programs dynamic and responsive.

What Are Conditional Statements?

Conditional statements allow a program to make decisions and execute different code based on whether a condition is true or false.

# Real-world analogy """ Weather App Decision: IF it's raining: Take an umbrella ELSE IF it's sunny: Wear sunglasses ELSE: Go out normally """

Why Conditionals Matter

Importance of Conditionals: ├── Decision Making - Choose between different actions ├── Validation - Check if input is valid ├── Error Handling - Handle exceptional cases ├── Flow Control - Direct program execution ├── User Interaction - Respond to user choices └── Business Logic - Implement business rules

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If Statement Basics

Simple If Statement

The most basic conditional statement executes code only when a condition is true.

# Python age = 18 if age >= 18: print("You can vote!") print("You can drive!") # Code continues here regardless print("Program continues")

// JavaScript let temperature = 25; if (temperature > 30) { console.log("It's hot outside!"); console.log("Stay hydrated!"); }

// Java int score = 85; if (score >= 60) { System.out.println("You passed!"); System.out.println("Congratulations!"); }

// C int x = 10; if (x > 0) { printf("x is positive\n"); }

Indentation and Code Blocks

# Python - Indentation is critical if condition: # These lines are indented print("Inside if block") print("Still inside if") # This line is outside print("Outside if block") # Multiple statements require consistent indentation if True: print("First line") print("Second line") # All indented lines execute together

// JavaScript/C/Java - Curly braces define blocks if (condition) { console.log("Inside if block"); console.log("Still inside if"); } console.log("Outside if block"); // Without braces, only one statement executes if (condition) console.log("This executes if true"); console.log("This ALWAYS executes!"); // Not part of if!

Common Pitfall: Single Statement vs Block

// JavaScript - Without braces if (x > 5) console.log("x is greater than 5"); console.log("This always runs!"); // Bug! Not part of if // Always use braces for clarity if (x > 5) { console.log("x is greater than 5"); console.log("This only runs when condition is true"); }

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If-Else Statements

Basic If-Else

If-Else provides an alternative path when the condition is false.

# Python password = "secret123" if password == "secret123": print("Access granted!") print("Welcome, user!") else: print("Access denied!") print("Incorrect password.")

// JavaScript let age = 16; if (age >= 18) { console.log("You can vote!"); console.log("You're eligible for driver's license"); } else { console.log("You're too young to vote!"); console.log("Wait until you turn 18"); }

If-Else with Multiple Statements

# Python score = 75 if score >= 60: print("Congratulations!") print("You passed the exam!") print(f"Your score: {score}") else: print("Sorry, you failed.") print("Better luck next time!") print(f"Your score: {score} (needed 60)")

Visual Representation

If-Else Flowchart: ┌─────────────┐ │ Condition │ └──────┬──────┘ │ ┌───────┴───────┐ │ │ True False │ │ ▼ ▼ ┌─────────────┐ ┌─────────────┐ │ Execute │ │ Execute │ │ If block │ │ Else block │ └─────────────┘ └─────────────┘ │ │ └───────┬───────┘ ▼ Continue program

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Else-If Ladders

Multiple Conditions

When you need to check multiple conditions in sequence, use else-if (elif in Python).

# Python - elif score = 85 if score >= 90: grade = "A" message = "Excellent!" elif score >= 80: grade = "B" message = "Good job!" elif score >= 70: grade = "C" message = "Fair" elif score >= 60: grade = "D" message = "Passing" else: grade = "F" message = "Needs improvement" print(f"Grade: {grade} - {message}")

// JavaScript - else if let temperature = 25; if (temperature > 30) { console.log("It's hot!"); console.log("Stay hydrated!"); } else if (temperature > 20) { console.log("It's warm"); console.log("Perfect weather!"); } else if (temperature > 10) { console.log("It's cool"); console.log("Bring a jacket"); } else { console.log("It's cold!"); console.log("Wear warm clothes"); }

// Java - else if int dayOfWeek = 3; if (dayOfWeek == 1) { System.out.println("Monday"); } else if (dayOfWeek == 2) { System.out.println("Tuesday"); } else if (dayOfWeek == 3) { System.out.println("Wednesday"); } else if (dayOfWeek == 4) { System.out.println("Thursday"); } else if (dayOfWeek == 5) { System.out.println("Friday"); } else if (dayOfWeek == 6) { System.out.println("Saturday"); } else if (dayOfWeek == 7) { System.out.println("Sunday"); } else { System.out.println("Invalid day"); }

Important: Order Matters

# Wrong order - The first true condition stops checking score = 95 # Bad - Will always hit first condition if score >= 60: print("Passing") elif score >= 90: print("Excellent") # This never executes! # Output: "Passing" # Good - Check most specific first if score >= 90: print("Excellent") elif score >= 80: print("Good") elif score >= 70: print("Fair") elif score >= 60: print("Passing") else: print("Failing")

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Nested If Statements

If Inside If

You can place if statements inside other if statements for more complex logic.

# Python age = 25 has_license = True if age >= 18: print("You're old enough to drive") if has_license: print("And you have a license!") print("You can drive legally") else: print("But you need to get a license first") else: print("You're too young to drive")

// JavaScript let isLoggedIn = true; let isAdmin = true; let hasPermission = true; if (isLoggedIn) { console.log("User is logged in"); if (isAdmin) { console.log("Admin user detected"); if (hasPermission) { console.log("Admin has full access"); console.log("Can delete users, view logs"); } else { console.log("Admin has limited access"); } } else { console.log("Regular user"); } } else { console.log("Please log in"); }

Deep Nesting - Handle with Care

# Deep nesting can become hard to read if condition1: if condition2: if condition3: if condition4: print("Deeply nested") else: print("Level 4 false") else: print("Level 3 false") else: print("Level 2 false") else: print("Level 1 false") # Better approach - combine conditions if condition1 and condition2 and condition3 and condition4: print("All conditions true") elif condition1 and condition2 and condition3: print("First three true") elif condition1 and condition2: print("First two true") elif condition1: print("First condition true") else: print("First condition false")

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Comparison Operators

Numerical Comparisons

# Python - Numerical operators a = 10 b = 20 if a == b: # Equal to print("Equal") if a != b: # Not equal to print("Not equal") if a > b: # Greater than print("a > b") if a < b: # Less than print("a < b") if a >= b: # Greater than or equal print("a >= b") if a <= b: # Less than or equal print("a <= b")

// JavaScript - Similar operators let x = 5; let y = "5"; // Value equality if (x == y) { console.log("Values equal"); // True (type coercion) } // Strict equality (type + value) if (x === y) { console.log("Strictly equal"); // False (different types) }

String Comparisons

# Python name = "Alice" if name == "Alice": print("Hello Alice!") if name != "Bob": print("You're not Bob") # Case-sensitive if name.lower() == "alice": print("Case-insensitive match")

// JavaScript let username = "admin"; if (username === "admin") { console.log("Welcome admin!"); } // String comparison (lexicographic) if ("apple" < "banana") { console.log("apple comes before banana"); }

Boolean Comparisons

# Python is_logged_in = True is_premium = False # Direct boolean check (most Pythonic) if is_logged_in: print("User is logged in") # Explicit comparison (also valid) if is_logged_in == True: print("Also works, but redundant") # Check for False if not is_premium: print("Not a premium user")

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Logical Operators

AND Operator (&&)

All conditions must be true for the overall condition to be true.

# Python age = 25 has_license = True if age >= 18 and has_license: print("You can drive") else: print("Cannot drive") # Multiple conditions score = 85 attendance = 90 if score >= 70 and attendance >= 80: print("Student passes")

// JavaScript let username = "admin"; let password = "secret123"; if (username === "admin" && password === "secret123") { console.log("Login successful!"); }

OR Operator (||)

At least one condition must be true.

# Python day = "Saturday" if day == "Saturday" or day == "Sunday": print("It's the weekend!") else: print("It's a weekday")

// JavaScript let userRole = "editor"; let hasPermission = true; if (userRole === "admin" || hasPermission) { console.log("Access granted"); }

NOT Operator (!)

Reverses the truth value of a condition.

# Python is_raining = False if not is_raining: print("It's not raining, go outside!") # Common pattern if not user_logged_in: print("Please log in")

// JavaScript let isComplete = false; if (!isComplete) { console.log("Task not yet complete"); }

Truth Tables

AND (&&) Truth Table: ┌─────────┬─────────┬─────────┐ │ A │ B │ A && B │ ├─────────┼─────────┼─────────┤ │ true │ true │ true │ │ true │ false │ false │ │ false │ true │ false │ │ false │ false │ false │ └─────────┴─────────┴─────────┘ OR (||) Truth Table: ┌─────────┬─────────┬─────────┐ │ A │ B │ A || B │ ├─────────┼─────────┼─────────┤ │ true │ true │ true │ │ true │ false │ true │ │ false │ true │ true │ │ false │ false │ false │ └─────────┴─────────┴─────────┘ NOT (!) Truth Table: ┌─────────┬─────────┐ │ A │ !A │ ├─────────┼─────────┤ │ true │ false │ │ false │ true │ └─────────┴─────────┘

Short-Circuit Evaluation

# Python - Stops at first false in AND def expensive_check(): print("Expensive operation...") return True x = False # expensive_check() never called because x is False if x and expensive_check(): print("Both true") # Stops at first true in OR y = True # expensive_check() never called because y is True if y or expensive_check(): print("At least one true")

Combining Multiple Conditions

# Python - Complex conditions age = 25 is_student = True income = 30000 has_scholarship = False # Complex eligibility check if (age >= 18 and age <= 65) and (is_student or income < 50000) and not has_scholarship: print("Eligible for financial aid") else: print("Not eligible") # Use parentheses for clarity if (age >= 18 and age <= 65) and \ (is_student or income < 50000) and \ not has_scholarship: print("Eligible")

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Ternary Operator

Basic Ternary (Conditional Expression)

A shorthand for simple if-else assignments.

# Python age = 18 status = "Adult" if age >= 18 else "Minor" print(status) # "Adult" # Multiple assignments is_raining = True weather = "Stay inside" if is_raining else "Go outside"

// JavaScript let score = 85; let grade = score >= 60 ? "Pass" : "Fail"; console.log(grade); // "Pass" // With multiple statements (use sparingly) let result = score >= 90 ? "Excellent" : score >= 80 ? "Good" : score >= 70 ? "Fair" : "Poor";

// Java int number = 10; String parity = (number % 2 == 0) ? "Even" : "Odd"; System.out.println(parity); // "Even"

When to Use Ternary

# Good - Simple assignments max_value = a if a > b else b discount = 0.1 if is_member else 0.05 # Bad - Complex logic (use if-else instead) result = (a if a > b else b) if (a > 0 and b > 0) else 0 # Too complex # Better with if-else if a > 0 and b > 0: result = a if a > b else b else: result = 0

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Switch Statements

Traditional Switch (C, Java, JavaScript)

// Java switch statement int day = 3; String dayName; switch (day) { case 1: dayName = "Monday"; break; case 2: dayName = "Tuesday"; break; case 3: dayName = "Wednesday"; break; case 4: dayName = "Thursday"; break; case 5: dayName = "Friday"; break; case 6: dayName = "Saturday"; break; case 7: dayName = "Sunday"; break; default: dayName = "Invalid day"; } System.out.println(dayName); // "Wednesday"

// JavaScript switch let fruit = "apple"; switch (fruit) { case "apple": console.log("Red or green fruit"); break; case "banana": console.log("Yellow fruit"); break; case "orange": console.log("Orange citrus fruit"); break; default: console.log("Unknown fruit"); }

Python Match Statement (Python 3.10+)

# Python match statement (structural pattern matching) def describe_number(num): match num: case 0: return "Zero" case 1: return "One" case 2: return "Two" case _: return "Something else" # Matching patterns def process_command(command): match command.split(): case ["quit"]: print("Exiting program") case ["hello", name]: print(f"Hello, {name}!") case ["add", x, y]: print(f"Sum: {int(x) + int(y)}") case _: print("Unknown command")

Switch vs If-Else

Switch vs If-Else: Switch is better for: ├── Single variable with many possible values ├── When comparing to constants ├── When performance matters (jump table) └── Code readability with many cases If-Else is better for: ├── Complex conditions ├── Range checks ├── Boolean expressions └── When conditions vary

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Pattern Matching

Advanced Pattern Matching (Python 3.10+)

# Python - Complex pattern matching def analyze_data(data): match data: case []: return "Empty list" case [x]: return f"Single element: {x}" case [x, y]: return f"Two elements: {x}, {y}" case [x, *rest]: return f"First: {x}, Rest length: {len(rest)}" case {"name": name, "age": age}: return f"Person: {name} is {age} years old" case _: return "Something else" # Using match with classes class Point: def __init__(self, x, y): self.x = x self.y = y def describe_point(point): match point: case Point(0, 0): return "Origin" case Point(0, y): return f"On Y-axis at y={y}" case Point(x, 0): return f"On X-axis at x={x}" case Point(x, y): return f"Point at ({x}, {y})"

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Common Patterns

Guard Clauses (Early Returns)

# Without guard clauses - nested ifs def process_user(user): if user: if user.is_active: if user.is_authenticated: if user.has_permission: return "Process user" else: return "No permission" else: return "Not authenticated" else: return "Account inactive" else: return "No user" # With guard clauses - cleaner def process_user(user): if not user: return "No user" if not user.is_active: return "Account inactive" if not user.is_authenticated: return "Not authenticated" if not user.has_permission: return "No permission" return "Process user"

Validation Patterns

# Input validation def validate_form(data): errors = [] if not data.get("name"): errors.append("Name is required") elif len(data["name"]) < 2: errors.append("Name must be at least 2 characters") if not data.get("email"): errors.append("Email is required") elif "@" not in data["email"]: errors.append("Invalid email format") if not data.get("age"): errors.append("Age is required") elif data["age"] < 18: errors.append("Must be 18 or older") if errors: return {"valid": False, "errors": errors} return {"valid": True, "data": data}

State Machines

# Simple state machine with if statements def traffic_light(current_state, car_present): if current_state == "red": if car_present: next_state = "green" else: next_state = "red" elif current_state == "green": next_state = "yellow" elif current_state == "yellow": next_state = "red" else: next_state = "red" return next_state

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Best Practices

1. Keep Conditions Simple

# Bad if x > 0 and y > 0 and z > 0 and x < 100 and y < 100 and z < 100: pass # Good - Extract to named variables is_positive = x > 0 and y > 0 and z > 0 is_within_bounds = x < 100 and y < 100 and z < 100 if is_positive and is_within_bounds: pass # Or create a function def is_valid_coordinate(x, y, z): return (x > 0 and x < 100 and y > 0 and y < 100 and z > 0 and z < 100) if is_valid_coordinate(x, y, z): pass

2. Avoid Deep Nesting

# Bad - Too deep if condition1: if condition2: if condition3: if condition4: do_something() # Good - Flatten with early returns if not condition1: return if not condition2: return if not condition3: return if not condition4: return do_something()

3. Use Meaningful Variable Names

# Bad if x > 0: pass # Good if user_age >= MINIMUM_VOTING_AGE: pass

4. Consider All Cases

def get_discount(customer_type): # Always include default/else case if customer_type == "premium": return 0.20 elif customer_type == "regular": return 0.10 elif customer_type == "new": return 0.05 else: return 0.00 # Default case

5. Use Enums for Multiple Options

from enum import Enum class UserType(Enum): ADMIN = "admin" EDITOR = "editor" VIEWER = "viewer" def get_permissions(user_type): if user_type == UserType.ADMIN: return ["read", "write", "delete", "manage_users"] elif user_type == UserType.EDITOR: return ["read", "write"] elif user_type == UserType.VIEWER: return ["read"] else: return []

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Real-World Examples

Example 1: Login System

class LoginSystem: def __init__(self): self.users = { "alice": {"password": "secret123", "is_active": True, "role": "admin"}, "bob": {"password": "pass456", "is_active": True, "role": "user"}, "charlie": {"password": "123", "is_active": False, "role": "user"} } self.max_attempts = 3 def login(self, username, password): # Guard clauses for validation if not username or not password: return {"success": False, "message": "Username and password required"} if username not in self.users: return {"success": False, "message": "User not found"} user = self.users[username] if not user["is_active"]: return {"success": False, "message": "Account is deactivated"} if user["password"] != password: return {"success": False, "message": "Invalid password"} # Success - check role if user["role"] == "admin": return {"success": True, "message": "Welcome admin!", "role": "admin"} else: return {"success": True, "message": f"Welcome {username}!", "role": "user"} def change_password(self, username, old_password, new_password): # Validate user exists if username not in self.users: return False user = self.users[username] # Check current password if user["password"] != old_password: return False # Validate new password if len(new_password) < 6: return False if new_password == old_password: return False # Update password user["password"] = new_password return True

Example 2: E-commerce Checkout

class CheckoutSystem: def calculate_total(self, cart, user, coupon=None): """Calculate order total with discounts and shipping""" subtotal = sum(item["price"] * item["quantity"] for item in cart) # Apply coupon discount discount = 0 if coupon: if coupon["type"] == "percentage": discount = subtotal * coupon["value"] / 100 elif coupon["type"] == "fixed": discount = min(coupon["value"], subtotal) # Apply user tier discount if user.get("tier") == "gold": discount += subtotal * 0.10 # 10% gold discount elif user.get("tier") == "silver": discount += subtotal * 0.05 # 5% silver discount after_discount = subtotal - discount # Calculate shipping shipping = 0 if after_discount > 0: if after_discount > 100: shipping = 0 # Free shipping elif after_discount > 50: shipping = 5.99 else: shipping = 9.99 # Add tax tax_rate = 0.08 # 8% tax tax = after_discount * tax_rate total = after_discount + shipping + tax return { "subtotal": round(subtotal, 2), "discount": round(discount, 2), "shipping": round(shipping, 2), "tax": round(tax, 2), "total": round(total, 2) }

Example 3: Game Logic

class GameCharacter: def __init__(self, name, health, mana): self.name = name self.health = health self.mana = mana self.level = 1 self.experience = 0 self.is_alive = True def take_damage(self, damage): # Apply damage with armor reduction if self.health <= 0: return "Already dead" # Critical hit chance import random if random.random() < 0.1: # 10% crit chance damage *= 2 print("Critical hit!") self.health -= damage if self.health <= 0: self.health = 0 self.is_alive = False return f"{self.name} has been defeated!" else: return f"{self.name} took {damage} damage. Health: {self.health}" def heal(self, amount): if not self.is_alive: return "Cannot heal a dead character" self.health += amount if self.health > 100: self.health = 100 return f"{self.name} healed for {amount}. Health: {self.health}" def cast_spell(self, spell_name, cost): if not self.is_alive: return "Dead character cannot cast spells" if self.mana < cost: return f"Not enough mana! Need {cost}, have {self.mana}" self.mana -= cost return f"{self.name} casts {spell_name}! Mana left: {self.mana}" def level_up(self): if self.experience >= 100 * self.level: self.level += 1 self.experience -= 100 * (self.level - 1) self.health = 100 # Full heal on level up self.mana = 100 return f"{self.name} reached level {self.level}!" return f"Need {100 * self.level - self.experience} more XP to level up"

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Conclusion

Key Takeaways

1. If statements are fundamental to decision-making in programs
2. Always consider all paths - think about what happens when conditions are true AND false
3. Keep conditions simple - complex conditions are hard to understand and debug
4. Use appropriate patterns - guard clauses, early returns, validation
5. Test edge cases - always test boundary conditions

When to Use What

Construct	Best Use Case
Simple if	Single decision, one branch
if-else	Two mutually exclusive paths
else-if	Multiple mutually exclusive conditions
Nested if	Complex hierarchical decisions
Switch/match	Single variable with many specific values
Ternary	Simple conditional assignment

Common Mistakes to Avoid

❌ Forgetting else cases ❌ Deep nesting without reason ❌ Complex conditions without parentheses ❌ Using == with floating point numbers ❌ Forgetting break in switch statements ❌ Not considering all possible inputs

Testing Conditions

Always test your conditions with:

• Normal cases
• Edge cases (minimum, maximum)
• Boundary values
• Invalid inputs
• Empty/null values

Conditional logic is the foundation of dynamic programs. Master it, and you'll be able to create responsive, intelligent applications!