if / else — Decision Making

Every useful program needs to make decisions. Should we show a welcome message or an error? Is the user old enough? Did the login succeed? These are all conditional situations — and in Dart, we handle them with if, else if, and else.

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🔹 The Basic if Statement

The simplest form: run a block of code only if a condition is true.

dart
void main() {
  int age = 20;

  if (age >= 18) {
    print('You are an adult.');
  }
}

• The condition inside () must evaluate to a bool (true or false).
• The block inside {} runs only when the condition is true.
• If the condition is false, the block is simply skipped.

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🔹 if-else

Use else to handle the case when the condition is false.

dart
void main() {
  int age = 15;

  if (age >= 18) {
    print('Access granted.');
  } else {
    print('Access denied. You must be 18+.');
  }
}

Output: Access denied. You must be 18+.

---

🔹 if-else if-else Chains

When you have more than two possibilities, chain multiple conditions:

dart
void main() {
  int score = 75;

  if (score >= 90) {
    print('Grade: A');
  } else if (score >= 80) {
    print('Grade: B');
  } else if (score >= 70) {
    print('Grade: C');
  } else if (score >= 60) {
    print('Grade: D');
  } else {
    print('Grade: F');
  }
}

Output: Grade: C

Dart checks each condition top to bottom and executes only the first block whose condition is true. The rest are skipped.

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🔹 Boolean Conditions

Conditions must be bool in Dart. Unlike JavaScript or C, Dart does not treat 0, null, or empty strings as false.

dart
void main() {
  bool isLoggedIn = true;

  // Correct — using a bool directly
  if (isLoggedIn) {
    print('Welcome back!');
  }

  // ERROR in Dart — non-boolean used as condition
  // int x = 0;
  // if (x) { ... }  // This does NOT compile!
}

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🔹 Relational Operators in Conditions

Operator	Meaning
==	Equal to
!=	Not equal to
>	Greater than
<	Less than
>=	Greater than or equal
<=	Less than or equal

dart
void main() {
  int a = 10, b = 20;

  if (a != b) {
    print('a and b are different');
  }

  if (a <= b) {
    print('a is less than or equal to b');
  }
}

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🔹 Logical Operators in Conditions

Combine multiple conditions using logical operators:

Operator	Meaning
&&	AND — both must be true
||	OR — at least one must be true
!	NOT — flips true/false

dart
void main() {
  int age = 25;
  bool hasTicket = true;

  // AND — both conditions must be true
  if (age >= 18 && hasTicket) {
    print('You may enter the event.');
  }

  // OR — at least one must be true
  bool isAdmin = false;
  bool isModerator = true;

  if (isAdmin || isModerator) {
    print('You have elevated access.');
  }

  // NOT — flip a condition
  bool isBanned = false;
  if (!isBanned) {
    print('Account is active.');
  }
}

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🔹 Nested if Statements

You can place if statements inside other if blocks:

dart
void main() {
  bool hasAccount = true;
  String password = 'secret123';

  if (hasAccount) {
    if (password == 'secret123') {
      print('Login successful!');
    } else {
      print('Wrong password.');
    }
  } else {
    print('Please create an account first.');
  }
}

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🔹 Common Patterns

Range Check

dart
void main() {
  int temperature = 22;

  if (temperature >= 20 && temperature <= 30) {
    print('Comfortable temperature.');
  } else if (temperature < 20) {
    print("It's cold. Grab a jacket!");
  } else {
    print("It's hot. Stay hydrated!");
  }
}

Null Check

dart
void main() {
  String? username; // nullable String

  if (username != null) {
    print('Hello, $username!');
  } else {
    print('Hello, Guest!');
  }
}

Type Check with is

Use the is keyword to check the runtime type of a variable:

dart
void main() {
  dynamic value = 42;

  if (value is int) {
    print('It is an integer: $value');
  } else if (value is String) {
    print('It is a string: $value');
  } else {
    print('Unknown type');
  }
}

After if (value is int), Dart smart-casts value to int automatically inside that block — no explicit casting needed!

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🔹 Real-World Examples

Example 1: Age Verification

dart
void checkAge(int age) {
  if (age < 0) {
    print('Invalid age.');
  } else if (age < 13) {
    print('Child');
  } else if (age < 18) {
    print('Teenager');
  } else if (age < 65) {
    print('Adult');
  } else {
    print('Senior');
  }
}

void main() {
  checkAge(10);  // Child
  checkAge(17);  // Teenager
  checkAge(30);  // Adult
  checkAge(70);  // Senior
}

Example 2: Grade System

dart
String getGrade(int score) {
  if (score < 0 || score > 100) {
    return 'Invalid score';
  } else if (score >= 90) {
    return 'A — Excellent';
  } else if (score >= 80) {
    return 'B — Good';
  } else if (score >= 70) {
    return 'C — Average';
  } else if (score >= 60) {
    return 'D — Below Average';
  } else {
    return 'F — Fail';
  }
}

void main() {
  print(getGrade(95));  // A — Excellent
  print(getGrade(73));  // C — Average
  print(getGrade(45));  // F — Fail
}

Example 3: Login Validation

dart
void login(String username, String password) {
  if (username.isEmpty || password.isEmpty) {
    print('Username and password are required.');
    return;
  }

  if (username.length < 3) {
    print('Username must be at least 3 characters.');
    return;
  }

  if (password.length < 6) {
    print('Password must be at least 6 characters.');
    return;
  }

  // If all checks pass:
  print('Credentials look valid. Logging in...');
}

void main() {
  login('', 'pass');          // Username and password are required.
  login('ab', 'password');    // Username must be at least 3 characters.
  login('alice', '123');      // Password must be at least 6 characters.
  login('alice', 'secret');   // Credentials look valid. Logging in...
}

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Summary

Concept	Purpose
if	Run code only when condition is true
else	Handle the false case
else if	Check multiple conditions in sequence
&&, ||, !	Combine or negate boolean conditions
is	Check the runtime type of a variable
Nested if	Conditional logic inside another condition
Guard clauses	Early returns to avoid deep nesting

Mastering if/else is the foundation of all program logic. In the next lesson, we'll explore switch statements and Dart 3's powerful pattern matching for cleaner multi-branch logic!