Functions and arrow syntax in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a function that returns the sum of two numbers using arrow syntax.

Flutter

int add(int a, int b) => [1];

Drag options to blanks, or click blank then click option'

Aa - b

Breturn a + b

Ca + b

Dprint(a + b)

Attempts:

3 left

💡 Hint

Common Mistakes

Including the 'return' keyword after the arrow.

Using subtraction instead of addition.

Using print instead of returning a value.

The arrow syntax allows a concise function that returns the expression after the arrow. Here, a + b is returned.

2fill in blank

medium

Complete the code to define a function that returns true if a number is even using arrow syntax.

Flutter

bool isEven(int number) => number [1] 2 == 0;

Drag options to blanks, or click blank then click option'

A%

B+

C*

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using division instead of modulo.

Using multiplication or addition which do not check evenness.

The modulo operator % returns the remainder. If number % 2 == 0, the number is even.

3fill in blank

hard

Fix the error in the arrow function that should return the length of a string.

Flutter

int getLength(String text) => [1];

Drag options to blanks, or click blank then click option'

Atext.length

Blength(text)

Ctext.length()

Dtext.size

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses like a method call.

Using non-existent properties like size.

Using a function call syntax.

In Dart, the length of a string is accessed as a property text.length, not as a method.

4fill in blank

hard

Fill the blank to define an arrow function that returns the square of a number.

Flutter

int square(int n) => n [1] n;

Drag options to blanks, or click blank then click option'

A/

B*

C-

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using addition or subtraction instead of multiplication.

Using division which does not square the number.

To square a number, multiply it by itself: n * n.

5fill in blank

hard

Fill all three blanks to create a function that returns a greeting message with a name.

Flutter

String greet(String name) => "Hello" [1] ", " [2] name [3] "!";

Drag options to blanks, or click blank then click option'

A+

B-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction or multiplication operators for strings.

Forgetting to concatenate all parts.

String concatenation in Dart uses the + operator. The greeting combines "Hello, ", the name, and "!".

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