Fixed Length List In Dart

Fixed-length lists in Dart provide a way to create lists of a specific length that cannot be changed once defined. This feature allows developers to optimize memory usage and improve performance in situations where a fixed number of elements is known in advance. In this tutorial, we will explore the concept of fixed-length lists in Dart, including syntax, practical examples, common mistakes to avoid, and best practices.

What is Fixed Length List in Dart?

A fixed-length list in Dart is a type of list where the number of elements is predetermined and cannot be changed after initialization. This means that once the list is created, you cannot add or remove elements from it. Fixed-length lists are useful when you know the exact number of elements the list will hold and want to ensure a fixed memory allocation size.

History/Background

Fixed-length lists have been a part of Dart since its early versions. They were introduced to provide developers with a way to efficiently manage memory and improve performance in scenarios where the size of the list is known in advance. By using fixed-length lists, developers can avoid unnecessary memory allocations and reallocations, leading to more efficient code execution.

Syntax

Creating a fixed-length list in Dart is straightforward. You can define a fixed-length list by specifying the size of the list within square brackets ` and using the List.filled` constructor to initialize the list with a specific value.

// Creating a fixed-length list with a size of 3
List<int> fixedList = List<int>(3);

// Initializing a fixed-length list with a specific value
List<String> initializedList = List.filled(4, 'Dart');

Key Features

• ✅ Elements in a fixed-length list are initialized with default values (0 for integers, null for objects) when created.
• ✅ Fixed-length lists have a fixed size that cannot be changed after creation.
• ✅ Fixed-length lists are memory-efficient and can improve performance in applications where the size of the list is known in advance.

Example 1: Basic Usage

Example
Copy
void main() {
  // Creating a fixed-length list with a size of 5
  List<int> numbers = List<int>(5);

  // Assigning values to the list
  numbers[0] = 10;
  numbers[1] = 20;
  numbers[2] = 30;
  numbers[3] = 40;
  numbers[4] = 50;

  print(numbers);
}
Try it Yourself

Output:

[10, 20, 30, 40, 50]

Example 2: Practical Application

void main() {
  // Initializing a fixed-length list with a specific value
  List<String> daysOfWeek = List.filled(7, '');

  // Assigning values to the list
  daysOfWeek[0] = 'Monday';
  daysOfWeek[1] = 'Tuesday';
  daysOfWeek[2] = 'Wednesday';
  daysOfWeek[3] = 'Thursday';
  daysOfWeek[4] = 'Friday';
  daysOfWeek[5] = 'Saturday';
  daysOfWeek[6] = 'Sunday';

  print(daysOfWeek);
}

Output:

[Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday]

Common Mistakes to Avoid

1. Forgetting Fixed Length

Problem: Beginners often forget that a fixed-length list cannot change its length after creation. They may try to add or remove elements, leading to runtime errors.

// BAD - Don't do this
void main() {
  var fixedList = List.filled(3, 0); // A fixed-length list with 3 elements
  fixedList.add(4); // Attempting to add an element
}

Solution:

// GOOD - Do this instead
void main() {
  var fixedList = List.filled(3, 0); // A fixed-length list with 3 elements
  print(fixedList); // Access elements without modifying the length
}

Why: Attempting to change the length of a fixed-length list will throw an UnsupportedError. To avoid this mistake, always remember that the size of a fixed-length list is immutable after its creation.

2. Misunderstanding List Initialization

Problem: Some beginners attempt to initialize a fixed-length list without using the List.filled constructor, thinking they can simply create a list with empty slots.

// BAD - Don't do this
void main() {
  var fixedList = List(3); // Incorrect initialization
  fixedList[0] = 1; // Not initialized properly
}

Solution:

// GOOD - Do this instead
void main() {
  var fixedList = List.filled(3, 0); // Properly initialized with a default value
  fixedList[0] = 1; // Now it's safe to assign values
}

Why: Using List(3) creates a list of nulls but doesn't set any initial values, which could lead to errors when trying to access uninitialized slots. Always use List.filled for fixed-length lists to ensure all slots are initialized.

3. Index Out of Range

Problem: Beginners may try to access an index that is out of the list's bounds, which will result in runtime exceptions.

// BAD - Don't do this
void main() {
  var fixedList = List.filled(3, 0);
  print(fixedList[3]); // Accessing an invalid index
}

Solution:

// GOOD - Do this instead
void main() {
  var fixedList = List.filled(3, 0);
  for (int i = 0; i < fixedList.length; i++) {
    print(fixedList[i]); // Accessing valid indices only
  }
}

Why: Accessing an invalid index causes an IndexError. Be mindful of the valid indices, which are from 0 to length - 1. Always use the list's length property to ensure you're within bounds.

4. Overlooking Type Safety

Problem: Beginners might create a fixed-length list without specifying the type, leading to potential issues with type safety.

// BAD - Don't do this
void main() {
  var fixedList = List.filled(3, 0); // Implicitly typed as dynamic
  fixedList[0] = "string"; // Assigning a string to an int list
}

Solution:

// GOOD - Do this instead
void main() {
  var fixedList = List<int>.filled(3, 0); // Explicitly typed as int
  fixedList[0] = 1; // Valid assignment
}

Why: Not specifying the type can lead to runtime errors when you mistakenly assign a value of a different type. Always declare the type of your list to enforce type safety.

5. Misusing List Methods

Problem: Beginners may try to use methods that are not applicable to fixed-length lists, such as sort or shuffle, without understanding their implications.

// BAD - Don't do this
void main() {
  var fixedList = List.filled(3, 0);
  fixedList.sort(); // Attempting to sort a fixed-length list
}

Solution:

// GOOD - Do this instead
void main() {
  var fixedList = [3, 1, 2]; // A mutable list
  fixedList.sort(); // Sorting a mutable list
}

Why: Fixed-length lists do not support certain operations that change their state. Use mutable lists if you need to perform operations like sorting or shuffling. Always be aware of the characteristics of the data structure you are using.

Best Practices

1. Use List.filled for Initialization

Using List.filled ensures that all elements in your fixed-length list are initialized to a specific value right from the start. This avoids potential null reference issues.

var fixedList = List.filled(5, 0); // All elements initialized to 0

Importance: It guarantees that your list is populated with valid data, avoiding errors when accessing elements.

2. Always Specify the Type

When creating a fixed-length list, always specify the type to enforce type safety, which helps catch errors at compile-time rather than runtime.

var fixedList = List<String>.filled(3, ""); // Explicitly typed as String

Importance: Helps prevent type-related errors and improves code readability.

3. Be Mindful of Index Bounds

Always check the bounds of your list when accessing elements. Use the length property to avoid IndexError.

for (var i = 0; i < fixedList.length; i++) {
  print(fixedList[i]);
}

Importance: Prevents runtime exceptions and makes your code more robust.

4. Prefer Immutable Operations

If you need to perform operations that change the list's content, consider creating a new list rather than modifying an existing fixed-length list.

var newList = [...fixedList, 4]; // Create a new list with an additional element

Importance: Promotes functional programming practices and avoids side effects.

5. Documenting Your Code

When using fixed-length lists, document your code to clarify the intended usage and constraints, especially if the list has a specific purpose or structure.

/// A fixed-length list of three integers representing [x, y, z] coordinates.
var coordinates = List<int>.filled(3, 0);

Importance: Enhances code maintainability and helps other developers understand your intent.

6. Use Assertions for Validations

If your application logic depends on certain conditions, use assertions to validate the state of the fixed-length list.

assert(fixedList.length == 3); // Ensures the list has the expected length

Importance: Catches potential issues during development, leading to more stable code.

Key Points