Variables and type inference in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with type inference holding the number 10.

Flutter

var number = [1];

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

A10.0

B"10"

Cten

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the number inside quotes makes it a string, not a number.

Using a decimal number when an integer is expected.

The keyword var lets Dart infer the type from the assigned value. Here, 10 is an integer, so number becomes an int.

2fill in blank

medium

Complete the code to declare a variable with type inference holding the text 'Hello'.

Flutter

var greeting = [1];

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

AHello

B"Hello"

C'Hello

Dhello

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around text makes Dart think it's a variable name.

Using mismatched or missing quotes causes errors.

Text values must be inside quotes. Double or single quotes work, but here double quotes are correct and complete.

3fill in blank

hard

Fix the error in the code by completing the variable declaration with type inference for a decimal number.

Flutter

var price = [1];

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

A100.5

B"100.5"

C100

Done hundred point five

Attempts:

3 left

💡 Hint

Common Mistakes

Putting decimal numbers inside quotes makes them strings.

Using words instead of numeric literals causes errors.

Decimal numbers need to be written as numeric literals with a dot, without quotes, so Dart infers a double type.

4fill in blank

hard

Fill both blanks to declare a variable with explicit type and assign it a boolean value.

Flutter

[1] isActive = [2];

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

Abool

Btrue

Cfalse

Dvar

Attempts:

3 left

💡 Hint

Common Mistakes

Using var when explicit type is asked.

Putting boolean values inside quotes makes them strings.

Explicitly declaring the type bool tells Dart this variable holds true/false values. Assigning true sets it to true.

5fill in blank

hard

Fill both blanks to declare a variable with type inference, assign a list of integers, and access the first element.

Flutter

var numbers = [1];
int first = numbers[2];
print(first);;

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

A[1, 2, 3]

B[0]

C;

D[1]

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets for lists or indexing.

Forgetting the semicolon at the end of statements.

The list [1, 2, 3] holds integers. Indexing uses square brackets starting at 0, so numbers[0] gets the first element. Statements end with a semicolon ;.

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