Variables and type inference in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a variable in Flutter?

A variable is a name that holds a value. It can store data like numbers, text, or objects that your app uses.

beginner

How does type inference work in Flutter?

Type inference means Flutter can guess the type of a variable from the value you assign, so you don't always need to write the type explicitly.

beginner

What keyword do you use in Flutter for a variable whose type is inferred?

You use the keyword var to declare a variable with type inference.

intermediate

Can you change the type of a variable declared with var later?

No, once a variable is assigned a type by inference, you cannot assign a value of a different type to it later.

intermediate

What is the difference between var and dynamic in Flutter?

var infers the type and keeps it fixed, while dynamic allows the variable to hold any type and can change at runtime.

Which keyword lets Flutter guess the variable type automatically?

A var

B int

C final

D const

The var keyword tells Flutter to infer the type from the assigned value.

What happens if you assign a string to a var variable initially holding an int?

A The app crashes immediately

B It works fine

C The variable changes type to string

D Flutter throws a type error

Flutter fixes the type at the first assignment, so assigning a different type later causes a type error.

Which keyword allows a variable to hold any type and change it later?

A var

B final

C dynamic

D const

dynamic allows the variable to hold any type and change it at runtime.

What is the type of a variable declared as var name = 'Alice';?

A String

B int

C dynamic

D Object

Flutter infers the type String because the value is text.

Which keyword declares a variable that cannot be changed after it is set?

A var

B final

C dynamic

D const

final declares a variable that can only be set once and then cannot change.

Explain how Flutter uses type inference with the var keyword.

Think about how Flutter decides the variable's type without you writing it.

Describe the difference between var and dynamic variables in Flutter.

Consider how flexible each variable type is after declaration.

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