Variables and type inference in Flutter

Introduction

Variables store information you want to use later. Type inference lets the computer guess the kind of data automatically, so you write less code.

When you want to remember a user's name in your app.

When you need to keep track of a score in a game.

When you want to store a color or size for a button.

When you want to hold a list of items to show on the screen.

Syntax

Flutter

var name = 'Alice';
int age = 30;
double height = 1.75;
String greeting = 'Hello!';

var lets Dart guess the type from the value you give.

You can also write the type explicitly like int or String.

Examples

Dart guesses city is a String because of the quotes.

Flutter

var city = 'Paris';

Here, we say count is an int number explicitly.

Flutter

int count = 5;

Dart infers price as a double because of the decimal point.

Flutter

var price = 9.99;

We tell Dart message is a String directly.

Flutter

String message = 'Welcome!';

Sample App

This app shows how to use variables with and without type inference. It displays the user's name, age, and height on the screen.

Flutter

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    var userName = 'Sam'; // Dart infers String
    int userAge = 25; // Explicit int
    var userHeight = 1.82; // Dart infers double

    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Variables Example')),
        body: Center(
          child: Text('Name: $userName, Age: $userAge, Height: $userHeight m'),
        ),
      ),
    );
  }
}

Important Notes

Once a variable's type is set or inferred, you cannot assign a different type to it later.

Using var is handy for quick coding, but explicit types can help avoid mistakes.

Summary

Variables hold data you want to use later in your app.

Type inference lets Dart guess the variable type from the value you give.

You can also write the type explicitly for clarity.

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