Control Flow Statements

Control flow statements determine the order in which code executes. Dart provides various control structures for decision-making and iteration.

Conditional Statements

if-else

int age = 18;

if (age >= 18) {
  print('Adult');
} else {
  print('Minor');
}

// if-else if-else
int score = 85;

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

Conditional Expressions

Ternary Operator

int age = 20;
String status = age >= 18 ? 'Adult' : 'Minor';
print(status);  // Adult

Null-aware Operator

String? name;
String displayName = name ?? 'Guest';
print(displayName);  // Guest

// Assign if null
name ??= 'Default Name';

switch-case

String grade = 'B';

switch (grade) {
  case 'A':
    print('Excellent!');
    break;
  case 'B':
    print('Good!');
    break;
  case 'C':
    print('Fair');
    break;
  default:
    print('Invalid grade');
}

// Multiple cases
String day = 'Saturday';

switch (day) {
  case 'Saturday':
  case 'Sunday':
    print('Weekend');
    break;
  default:
    print('Weekday');
}

Loops

for Loop

// Standard for loop
for (int i = 0; i < 5; i++) {
  print('Count: $i');
}

// For-in loop
List<String> fruits = ['apple', 'banana', 'orange'];
for (var fruit in fruits) {
  print(fruit);
}

// forEach method
fruits.forEach((fruit) {
  print(fruit);
});

while Loop

int count = 0;

while (count < 5) {
  print('Count: $count');
  count++;
}

do-while Loop

int num = 0;

do {
  print('Number: $num');
  num++;
} while (num < 5);

Loop Control

break

Exits the loop immediately:

for (int i = 0; i < 10; i++) {
  if (i == 5) {
    break;  // Exit loop when i is 5
  }
  print(i);
}
// Output: 0, 1, 2, 3, 4

continue

Skips the current iteration:

for (int i = 0; i < 5; i++) {
  if (i == 2) {
    continue;  // Skip when i is 2
  }
  print(i);
}
// Output: 0, 1, 3, 4

Assert

Used for debugging to ensure conditions are true:

int age = 15;
assert(age >= 18, 'Age must be 18 or older');
// Throws error in debug mode if condition is false

Pattern Matching (Dart 3.0+)

Switch Expressions

String grade = 'B';

String message = switch (grade) {
  'A' => 'Excellent!',
  'B' => 'Good!',
  'C' => 'Fair',
  _ => 'Invalid grade',
};

print(message);  // Good!

if-case

Object value = 42;

if (value case int n when n > 0) {
  print('Positive integer: $n');
}

Collection Control Flow

Collection if

bool includeZero = true;
List<int> numbers = [
  1,
  2,
  if (includeZero) 0,
  3,
];
print(numbers);  // [1, 2, 0, 3]

Collection for

List<int> original = [1, 2, 3];
List<int> doubled = [
  for (var num in original) num * 2,
];
print(doubled);  // [2, 4, 6]

Spread Operator

List<int> list1 = [1, 2, 3];
List<int> list2 = [4, 5, 6];
List<int> combined = [...list1, ...list2];
print(combined);  // [1, 2, 3, 4, 5, 6]

// Null-aware spread
List<int>? nullableList;
List<int> safe = [...?nullableList, 1, 2];
print(safe);  // [1, 2]

Best Practices

1. Use switch expressions for cleaner code (Dart 3.0+)
2. Prefer for-in over traditional for loops when iterating collections
3. Use collection if/for for conditional list building
4. Avoid deep nesting of if-else statements
5. Use early returns to reduce nesting

Complete Example

void main() {
  // Grade calculator
  List<int> scores = [85, 92, 78, 95, 88];
  
  print('=== Grade Report ===');
  
  for (int i = 0; i < scores.length; i++) {
    int score = scores[i];
    
    String grade = switch (score) {
      >= 90 => 'A',
      >= 80 => 'B',
      >= 70 => 'C',
      >= 60 => 'D',
      _ => 'F',
    };
    
    print('Test ${i + 1}: $score - Grade $grade');
  }
  
  // Calculate average
  int total = 0;
  for (var score in scores) {
    total += score;
  }
  double average = total / scores.length;
  
  print('\nAverage: ${average.toStringAsFixed(2)}');
  
  // Pass/Fail status
  String status = average >= 70 ? 'PASS' : 'FAIL';
  print('Status: $status');
  
  // Filter passing scores
  List<int> passingScores = [
    for (var score in scores)
      if (score >= 70) score,
  ];
  
  print('\nPassing scores: $passingScores');
}

Next Steps

• Functions
• Operators
• Null Safety