Functions and arrow syntax in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the output of a simple arrow function

What is the output of this Flutter Dart code when the button is pressed?

Flutter

void main() {
  final greet = () => 'Hello Flutter!';
  print(greet());
}

A Hello

B null

C Hello Flutter!

D Error: Missing return statement

💡 Hint

Arrow functions return the expression after the arrow automatically.

The arrow syntax (=>) returns the expression directly. So greet() returns 'Hello Flutter!'.

❓ ui_behavior

intermediate

Arrow function in Flutter onPressed callback

Which option correctly uses an arrow function to update the text when a button is pressed in Flutter?

Flutter

class MyWidget extends StatefulWidget {
  @override
  State<MyWidget> createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  String text = 'Before';

  void updateText() => setState(() => text = 'After');

  @override
  Widget build(BuildContext context) {
    return ElevatedButton(
      onPressed: updateText,
      child: Text(text),
    );
  }
}

A void updateText() { text = 'After'; }

B void updateText() { setState(() { text = 'After'; }); }

C void updateText() => text = 'After';

D void updateText() => setState(() => text = 'After');

💡 Hint

setState must be called to update UI; arrow syntax can be nested.

Option D uses arrow syntax for updateText and inside setState to update text and refresh UI correctly.

❓ lifecycle

advanced

Arrow function in Flutter widget lifecycle method

What happens if you replace this initState method with arrow syntax in a StatefulWidget?

Flutter

class _MyState extends State<MyWidget> {
  @override
  void initState() {
    super.initState();
    print('Init');
  }
}

A Prints nothing because arrow functions do not run in lifecycle

B Syntax error because initState cannot be an arrow function with multiple statements

C Prints 'Init' correctly when widget initializes

D Does not call super.initState(), causing runtime error

💡 Hint

Arrow functions can only have one expression, but initState has two statements.

initState has two statements: super.initState() and print(). Arrow syntax only supports one expression, so this causes a syntax error.

❓ navigation

advanced

Arrow function in Navigator callback

Which option correctly uses arrow syntax to navigate to a new screen on button press?

Flutter

ElevatedButton(
  onPressed: () {
    Navigator.push(context, MaterialPageRoute(builder: (context) => NewScreen()));
  },
  child: Text('Go'),
)

A onPressed: () => Navigator.push(context, MaterialPageRoute(builder: (context) => NewScreen())),

B onPressed: () => { Navigator.push(context, MaterialPageRoute(builder: (context) => NewScreen())); },

C onPressed: () { Navigator.push(context, MaterialPageRoute(builder: (context) => NewScreen())); },

D onPressed: Navigator.push(context, MaterialPageRoute(builder: (context) => NewScreen())),

💡 Hint

Arrow functions can omit braces if only one expression is executed.

Option A uses arrow syntax correctly for a single expression. Option A uses braces with arrow syntax which is invalid. Option A calls Navigator.push immediately, not on button press.

🔧 Debug

expert

Debugging arrow function with async in Flutter

What error occurs with this arrow function in Flutter and why?

Flutter

Future<void> fetchData() async {
  await Future.delayed(Duration(seconds: 1));
  print('Data loaded');
};

A Syntax error: Arrow function body cannot be a block with multiple statements

B No error; prints 'Data loaded' after 1 second

C Runtime error: await used outside async function

D Prints immediately without delay

💡 Hint

Arrow functions can only have one expression, not multiple statements in braces.

The arrow function body cannot be a block with multiple statements inside braces. This causes a syntax error.

Practice

1. What does the arrow syntax => in a Flutter function do?

easy

A. It marks a function as private.

B. It declares a function without any return value.

C. It creates a function that returns the expression after the arrow.

D. It defines a function that runs asynchronously.

Solution

1. Step 1: Understand arrow syntax meaning

   The arrow => is a shortcut for functions that return a single expression.

2. Step 2: Compare with normal function

   Instead of writing curly braces and return, arrow syntax returns the expression directly.

3. Final Answer:

   It creates a function that returns the expression after the arrow. -> Option C

4. Quick Check:

   Arrow syntax = single expression return [OK]

Hint: Arrow syntax means single expression return [OK]

Common Mistakes:

• Thinking arrow means no return value
• Confusing arrow with async keyword
• Assuming arrow marks function private

2. Which of the following is the correct arrow function syntax in Flutter to return the sum of two integers a and b?

easy

A. int sum(int a, int b) => a + b;

B. int sum(int a, int b) { a + b; }

C. int sum(int a, int b) => { return a + b; };

D. int sum(int a, int b) => return a + b;

Solution

1. Step 1: Check arrow function syntax

   Arrow functions use => followed by an expression without braces or return keyword.

2. Step 2: Validate each option

   int sum(int a, int b) => a + b; correctly uses arrow syntax with expression a + b. The other options misuse braces or return keyword.

3. Final Answer:

   int sum(int a, int b) => a + b; -> Option A

4. Quick Check:

   Arrow function = expression after => without braces [OK]

Hint: Arrow functions omit braces and return keyword [OK]

Common Mistakes:

• Adding braces {} with arrow syntax
• Using return keyword after arrow
• Missing semicolon at end

3. What is the output of this Flutter Dart code?

int multiply(int x, int y) => x * y;
void main() {
  print(multiply(3, 4));
}

medium

A. Error

B. 7

C. 34

D. 12

Solution

1. Step 1: Understand the multiply function

   The function returns the product of x and y using arrow syntax.

2. Step 2: Calculate multiply(3, 4)

   3 * 4 equals 12, so the print statement outputs 12.

3. Final Answer:

   12 -> Option D

4. Quick Check:

   3 * 4 = 12 [OK]

Hint: Multiply returns product directly with arrow syntax [OK]

Common Mistakes:

• Adding numbers instead of multiplying
• Confusing output as string concatenation
• Thinking arrow syntax causes error

4. Identify the error in this Flutter Dart function using arrow syntax:

int add(int a, int b) => {
  return a + b;
};

medium

A. Missing semicolon after return statement.

B. Arrow functions cannot have braces and return statements.

C. Function must be async to use arrow syntax.

D. Function parameters must be optional.

Solution

1. Step 1: Check arrow function syntax rules

   Arrow syntax functions must have a single expression without braces or return keyword.

2. Step 2: Analyze the given code

   The code uses braces and return inside an arrow function, which is invalid syntax.

3. Final Answer:

   Arrow functions cannot have braces and return statements. -> Option B

4. Quick Check:

   Arrow syntax = single expression, no braces [OK]

Hint: Arrow syntax disallows braces and return keyword [OK]

Common Mistakes:

• Adding braces {} in arrow functions
• Using return keyword after arrow
• Thinking arrow requires async keyword

5. You want to write a Flutter function using arrow syntax that returns the length of a string only if it is not empty; otherwise, it returns zero. Which is the correct function?

hard

A. int lengthOrZero(String s) => s.isNotEmpty ? s.length : 0;

B. int lengthOrZero(String s) => if (s.isNotEmpty) s.length else 0;

C. int lengthOrZero(String s) => { return s.length; };

D. int lengthOrZero(String s) => s.length > 0;

Solution

1. Step 1: Use arrow syntax with conditional expression

   Arrow functions can return a single expression, here a ternary operator checks if string is not empty.

2. Step 2: Validate each option

   int lengthOrZero(String s) => s.isNotEmpty ? s.length : 0; correctly uses ternary with s.isNotEmpty. int lengthOrZero(String s) => if (s.isNotEmpty) s.length else 0; uses invalid if-else syntax. int lengthOrZero(String s) => { return s.length; }; uses braces and return which is invalid for arrow. int lengthOrZero(String s) => s.length > 0; returns a boolean instead of int.

3. Final Answer:

   int lengthOrZero(String s) => s.isNotEmpty ? s.length : 0; -> Option A

4. Quick Check:

   Arrow + ternary = conditional return [OK]

Hint: Use ternary inside arrow for conditional return [OK]

Common Mistakes:

• Using if-else instead of ternary in arrow
• Adding braces and return in arrow function
• Returning boolean (length > 0) instead of int