Variables and type inference in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Understanding Dart variable type inference

What is the type of variable x after this code runs?

var x = 42;

Flutter

var x = 42;

A dynamic

B double

C int

D String

💡 Hint

Dart infers the type from the assigned value.

The variable x is assigned the integer 42, so Dart infers its type as int.

❓ ui_behavior

intermediate

Effect of variable type on Flutter widget display

Given this Flutter code snippet, what will be displayed on the screen?

var message = 123;
return Text(message.toString());

Flutter

var message = 123;
return Text(message.toString());

A The number 123 as text

B An error because Text expects a String variable

C Nothing is displayed

D The word 'message' is displayed

💡 Hint

Check how the integer is converted to text.

The integer 123 is converted to a string using toString(), so the Text widget displays '123'.

❓ lifecycle

advanced

Variable type and widget rebuild behavior

In a Flutter StatefulWidget, if you declare a variable with var count = 0; inside the State class and update it inside setState(), what happens to the variable type during rebuilds?

Flutter

class _MyWidgetState extends State<MyWidget> {
  var count = 0;

  void increment() {
    setState(() {
      count++;
    });
  }
}

A The variable type changes to dynamic after first rebuild

B The variable type remains int across rebuilds

C The variable is reset to 0 on every rebuild

D The variable becomes null after rebuild

💡 Hint

Consider where the variable is declared and how Flutter rebuilds widgets.

The variable count is a field of the State class and keeps its type int and value across rebuilds.

🔧 Debug

advanced

Identifying type inference error in Dart

What error will this Dart code produce?

var name = null;
name = "Flutter";

Flutter

var name = null;
name = "Flutter";

A No error, name becomes a String

B Syntax error due to missing type declaration

C Runtime error when assigning String to null

D A compile-time error because type is inferred as Null

💡 Hint

Think about what type Dart infers when initialized with null.

Dart infers the type as Null when initialized with null, so assigning a String later causes a compile-time error.

🧠 Conceptual

expert

Why use explicit types instead of var in Flutter?

Which reason best explains why you might prefer explicit types over var in Flutter app variables?

A Explicit types improve code readability and catch type errors early

B Explicit types make the app run faster at runtime

C Using <code>var</code> disables null safety features

D Explicit types reduce the app size significantly

💡 Hint

Think about how explicit types help developers understand code.

Explicit types make code clearer and help the compiler catch mistakes before running the app.

Practice

1. What does the var keyword do in Dart when declaring a variable?

easy

A. It lets Dart guess the variable type from the assigned value.

B. It declares a variable without assigning any value.

C. It forces the variable to be of type String.

D. It creates a constant variable that cannot change.

Solution

1. Step 1: Understand var usage

   The var keyword tells Dart to infer the type based on the value assigned.

2. Step 2: Check other options

   Options B, C, and D describe different behaviors not related to var.

3. Final Answer:

   It lets Dart guess the variable type from the assigned value. -> Option A

4. Quick Check:

   Type inference = It lets Dart guess the variable type from the assigned value. [OK]

Hint: Remember: var means Dart guesses the type from the value [OK]

Common Mistakes:

• Thinking var declares a variable without a value
• Confusing var with const or final
• Assuming var always means String type

2. Which of the following is the correct way to declare an integer variable with explicit type in Dart?

easy

A. integer count = 10;

B. int count = 10;

C. var int count = 10;

D. count int = 10;

Solution

1. Step 1: Recall Dart syntax for typed variables

   In Dart, the type comes first, then the variable name, then the value.

2. Step 2: Check each option

   int count = 10; follows correct syntax: int count = 10;. Others are invalid syntax.

3. Final Answer:

   int count = 10; -> Option B

4. Quick Check:

   Explicit type declaration = int count = 10; [OK]

Hint: Type then name then value: int count = 10; [OK]

Common Mistakes:

• Putting var and type together
• Using 'integer' instead of 'int'
• Swapping variable name and type

3. What will be the output of this Dart code?

var name = 'Alice';
var age = 30;
print('$name is $age years old');

medium

A. Alice is 30 years old

B. name is age years old

C. 30 is Alice years old

D. Error: Cannot print variables like this

Solution

1. Step 1: Understand string interpolation

   The \$ syntax inserts variable values inside strings in Dart.

2. Step 2: Substitute variables in the print statement

   Variables name and age are replaced by 'Alice' and 30 respectively.

3. Final Answer:

   Alice is 30 years old -> Option A

4. Quick Check:

   String interpolation = Alice is 30 years old [OK]

Hint: Use \$variable to insert values in strings [OK]

Common Mistakes:

• Printing variable names instead of values
• Forgetting to escape $ in strings
• Assuming print shows variable types

4. Identify the error in this Dart code:

var number;
number = 5;
number = 'five';

medium

A. Error: Variable must be declared with final

B. Error: Cannot assign a String to a variable inferred as int

C. No error, Dart allows changing types with var

D. Error: Missing semicolon after first line

Solution

1. Step 1: Understand type inference with var and no initial value

   When declared without initial value, var number; defaults to dynamic type.

2. Step 2: Check assignments

   Assigning 5 (int) then 'five' (String) is allowed only if type is dynamic. But Dart infers type on first assignment if declared with var and initialized immediately. Here, no initial value means dynamic, so no error.

3. Final Answer:

   No error, Dart allows changing types with var -> Option C

4. Quick Check:

   Uninitialized var = dynamic type = No error, Dart allows changing types with var [OK]

Hint: Uninitialized var is dynamic, so type can change [OK]

Common Mistakes:

• Assuming var always fixes type on declaration
• Thinking missing semicolon causes error here
• Confusing final with var

5. Given this Dart code, what is the type of result?

var a = 10;
var b = 3.5;
var result = a + b;

hard

A. int

B. String

C. num

D. double

Solution

1. Step 1: Identify types of variables a and b

   a is int (10), b is double (3.5).

2. Step 2: Understand type inference of arithmetic operations

   Adding int and double results in a double in Dart.

3. Final Answer:

   double -> Option D

4. Quick Check:

   int + double = double [OK]

Hint: int plus double results in double type [OK]

Common Mistakes:

• Assuming result is int because a is int
• Choosing num instead of double
• Thinking result is String due to concatenation