Array Vs. Linked List: Key Differences & Usages Explained in Detail

Data structures play a crucial role in organizing and managing data efficiently in programming. They help in storing, manipulating, and retrieving data in a way that is optimal for the task at hand. Two widely used data structures, arrays and linked lists, serve totally different purposes in various applications.

In this article, we will focus on the main differences between arrays and linked lists based on their structure, memory consumption, access strategy, and performance. Knowledge of these differences will help you make the right choice of data structures based on your own programming needs.

Brief Introduction to Arrays

An array is a data structure used to store a fixed-size sequence of elements of the same type. The advantage of arrays is that they can keep elements in contiguous memory locations, and they can also be accessed directly through an index. Arrays are useful in cases where the number of elements is known up front and the system requires random access to these elements.

Brief Introduction to Linked Lists

A linked list is a linear data structure where elements, called nodes, are stored in separate memory locations. Each node has a data part and a pointer (or reference) to another node in the sequence. Unlike an array, linked lists do not require contiguous spaces in memory, which means that they are far more flexible in their arrangement. Therefore, although access time has to be slower than that of an array since they are inherently sequential, the flexibility is greatly compensated for.

Differences Between Array and Linked List (Comparison Table)

Listed below are the key differences between an Array and a Linked List:

What is an Array?

An array is a basic data structure used to maintain an ordered collection of elements, usually of the same data type, in contiguous memory locations. Arrays allow for fast access to elements by their index and are widely used in programming for data list management.

An array is best when a bunch of values have to be stored and manipulated of the same type under a single identifier- it is static in most languages. Also, the size of the array has to be defined at the time of declaration.

Key Points Related to an Array:

• It stores multiple elements belonging to the same data type under one variable name.
• Elements are stored in contiguous locations in memory.
• Array indexing commonly starts at 0.
• Random access of elements is allowed using the index.
• Once declared, the size is fixed (this is the case in most languages like C/C++ and Java).

How to Declare an Array in C++?

In C++, while declaring an array, the programmer will usually specify the size as well as the type of objects it will contain. In other words, we must have the data type, the name of the array, and an index notation given in brackets [ ] specifying the number of elements the array can hold.

The following is the syntax for declaring an array in C++:

type arrayName[arraySize];

Here,

• The term type refers to the data type of elements in the array.
• The arrayName refers to the name given to the array being created.
• And the arraySize refers to the number of elements that will be in the entire collection or array.

Example: string names[2];

The snippet above, when used in a code, will declare an array with two string elements, just like a normal variable.

For Example: string names[2] = {"John", "Mark"};

How to Declare an Array in Java?

In Java, when you're declaring an array, you always have to first specify a data type and then give the size or initialize the array directly. In Java, arrays are treated as objects, so they are dynamically created through the use of the new keyword (for size-based arrays) or direct value initialization.

The following is the syntax for declaring an array in Java:

type[] arrayName = new type[arraySize];

Here,

• The term type refers to the data type of elements in the array.
• The arrayName refers to the name given to the array being created.
• new is used to dynamically allocate memory for the array.
• And the arraySize indicates the total number of elements.

Example: int[] numbers = new int[5];

This declares an integer array named numbers that can hold 5 integer values, all initialized to 0.

For Example (with values): String[] names = {"John", "Mark"};

This creates and initializes a String array with two values.

How to Declare an Array in Python?

Python does not have a specific array type, like C or Java; it uses lists, which are rather dynamic, flexible, and can hold multiple data types.

The following is the syntax for declaring a list (array equivalent) in Python:

array_name = [value1, value2, value3, ...]

Here,

• array_name is the variable name (your array equivalent).
• The square brackets [] are used to define a list.
• Elements can be added or removed freely.

Example: numbers = [1, 2, 3, 4, 5]

This creates a list of five integer elements.

For Example (with strings): names = ["John", "Mark"]

This creates a list containing two string elements.

Applications of Arrays

Some of the key applications of arrays in programming are:

Advantages & Disadvantages of Arrays

What is a Linked List?

A linked list is a linear data structure that uses pointers to connect various elements called nodes. It consists of two parts: a piece of information and the address, or pointer, to the next node, which makes it a sequence. Unlike arrays, linked lists do not require a contiguous memory location and can grow or shrink in size at runtime.

Linked lists come in different forms:

• Singly Linked List: Each node points to the next node only.
• Doubly Linked List: Each node contains two pointers — one to the next node and one to the previous.
• Circular Linked List: The last node links back to the first node, forming a loop.

The adaptable memory management and dynamic construct structure of linked lists are most viable in cases of frequent inserts and deletions.

How to Create a Linked List in C++?

So, what can you do with linked lists in C++? Instead of pre-defining specific types like arrays, you would create a structure (struct) to represent a node and use pointers to create and link each node dynamically.

The following is the basic structure of a singly linked list in C++:

struct Node {

int data;

Node* next;

};

Here,

• The struct keyword is used to define a new data type.
• Data holds the actual value stored in the node.
• Next is a pointer to the next node in the list.

Creating and linking nodes:

Node* head = new Node();

head->data = 10;

head->next = new Node();

head->next->data = 20;

head->next->next = nullptr;

In the example above:

• A new node (head) is created dynamically using new.
• We assign data to it and create a second node, linking the first node to the second.
• The last node points to nullptr, indicating the end of the list.

This creates a simple linked list like: [10] -> [20] -> NULL

Applications of Linked List

Advantages & Disadvantages of Linked List

Detailed Explanation of the Key Differences Between Array & Linked List

We cannot conclude which data structure is better, an array or a linked list. One data structure may suit one type of requirement, while the other may suit a different kind of requirement. In addition, there are many kinds of considerations that go into deciding which data structure to adopt for a given requirement. Therefore, we shall continue with our investigation into the differences between arrays and linked lists under a few parameters.

Cost of Accessing an Element

The access time to an array element is a constant, irrespective of the size of the array. Being contiguous, an array provides the user an easy way to determine the address of any array element as long as the base address from which the elements are starting is known.

In determining the address of any array element, a simple mathematical calculation would have to be performed. Time complexity-wise, the access cost for an array element is O(1).

For example, in the image shown below, the base address is 200, and we need to find the address of the ith element, i.e., i = 3.

Address of ith element = (Base address) + i * (memory space required)    

                     A[3] = 200 + 3*(4) = 212    {integer size: four bytes}

Linked lists do not store elements in a proper sequence. These are non-contiguous memory allocations that comprise several blocks present in a pool of memory, each of which contains two fields, and we only maintain the availability of the head node in the entire representation of a linked list. Therefore, to access any element in this list, we need to come from the head node itself. So, the average case for accessing any element from a linked list would be O(n).

So, accessing an element in an array will cost less than in a linked list. Therefore, if there is a requirement for quick access to the elements, one can go with an array rather than using a linked list. The next factor that differentiates an array from a linked list is the cost of inserting an element.

Cost of Inserting an Element

There are three insertion options possible for this type of data structure, and we shall discuss each one of them:

Inserting an Element at the Beginning

In order to place the new element at the beginning of an array, we must first shift to the right all the existing elements to vacate the first position. Hence, the time complexity becomes relative to the size of the list. If n is the array size, then the time complexity is O(n). The diagram describing the insertion at the beginning of an array illustrates why this operation will cost us O(n).

To insert an element, a newly created node in the context of a linked list will have an address of the first node linked to the reference field for the new node. Thus, the position of the first node will be pushed to the new node. Thus, the time complexity would not be proportional to the size of the list; it would be constant, i.e., O(1).

The following representation of linked lists would represent the scenario when insertion is done at the beginning.

Inserting an Element at the End

If the array is not full, an element can be added using the index in question explicitly: In this case, that operation runs in constant time, i.e., O(1). If the array is full, however, it has first to be replicated and a new element added into another array, which takes time O(n) in this case.

To insert an element at the end of a linked list, we must traverse all nodes in the linked list. If there are n elements in the linked list, the time complexity for inserting an element will be O(n).

Inserting an Element in Mid

If we insert an element in the ith place of the array, we have to right shift all n/2 elements available to that position so that our ith element can be left empty. Thus, the time complexity is made proportional to the number of elements. For the average case, time complexity is given as O(n).

In the case of a linked list, whenever we want to insert a new element into the list, we need to find the index at which the element has to be inserted. There will be no shifting of elements required in this case, but because we will have to traverse n/2 elements to reach this mid location, this will result in O(n) complexity.

Cost of Removing an Element

The time complexity for removing elements from both the array and the linked list is similar to the insertion scenario. The table below consolidates the time complexities pertaining to various deletion operations as they pertain to both array and linked list.

Memory Requirement

As already discussed, an array stores its elements in continuous memory locations. For a specific size in the array declaration, it will directly access that part of the memory pool from the memory area. For instance, if there are 5 integer elements in our array, the size taken would be equal to:

Utilized memory space = 5 * (integer size) = 5 * 4 = 20. Since a linked list requires additional memory space to store pointer variables, it will not be an ideal choice when we already know the size of items to be stored in the list. For example, a linked list with a size of five requires a size:

Utilized memory space = 10* (Integer size) = 10 * 4 = 40

Reallocation becomes inefficient when we do not know the size of the array at compile time; hence, it is not the best solution. Instead, a linked list would be a better choice, which allows dynamic resizing at runtime.

Hence, on the other hand, we conclude that an array is applicable when the size is finite or small, and a linked list can be used when the size is unknown or infinite.

Memory Locations

All elements of an array are stored in contiguous memory locations, which provide a direct index and quick access to the individual element. These contiguous locations ensure that all these elements are placed next to each other and aid the processor in fetching the data.

Memory Allocation

An array is fixed in size. It means that when you declare the array, memory is allocated to it and this size is determined at start. This memory is also contiguous, which makes memory allocation efficient, but once set, it cannot be changed, and this could prove to be a limitation in certain situations. Time complexity memory allocation of arrays is O(1) since memory is allocated in one block .

Memory Utilization

In an array, the memory may be wasted if the allocated space is not used completely. If the array is more extensive, memory goes to waste, whereas in the case of smaller arrays, when you are going to resize it, then troubles can occur. But arrays are more effectively used when the size is already known and remains constant during the entire execution of the program.

Element Access

Since an array is laid up in contiguous memory it makes access fast to the elements of the array. Any element's address is calculated by a simple mathematical formula and is based on a base address plus an index. Access to elements is hence a constant time O(1), which makes arrays very suitable for fast access scenarios.

Deletion of Operations

The deletion of an element from an array is, however, in reality, shifting all succeeding elements to fill that gap since arrays are contiguous memory location-based, and it is impossible to leave a gap. This is why deletion is considered a costly operation, requiring O(n) time to shift the elements. When dealing with a large number of arrays, this process becomes inefficient.

Shifting Elements After Deletion

After deletion, it is a major drawback that you need to shift elements. After the deletion of some elements, for all subsequent elements, they would have to be shifted one position to the left, filling that space. For this, you would need O(n) time, where n is the number of elements in the array. Moving elements is one of the main factors rendering the deletion operations for arrays ineffective, especially in the case of large datasets. 

Conclusion

In this tutorial, we concentrated on examining the distinctions between arrays and linked lists. We specified their characteristics in detail and examined their suitability for specific use cases. Furthermore, we discussed complexity analysis as it relates to different operations of these data structures.

Finally, we could present the difference between arrays and linked lists in a tabular form and then determine when to favor the adoption of one data structure over the other according to individual needs. The use of arrays and linked lists depends on the particular need in a given case. Distinguishing between them is, therefore, key when it comes to appropriate software choices.

Frequently Asked Questions (FAQs)

1. Does the linked list use more memory?

Yes, in general, linked lists tend to use more memory compared to arrays for storing the same amount of data. Here's why:

Pointers: Each node in a linked list stores not only the data itself but also a pointer to the next node in the sequence. This additional pointer adds overhead to the memory footprint of each node.

Scattered Memory Allocation: Unlike arrays that reside in a contiguous block of memory, linked list nodes are scattered throughout memory. While this allows for dynamic size, it can lead to less efficient memory utilization compared to the compact storage of arrays.

2. Why are arrays faster than linked lists?

Arrays are faster than linked lists for several reasons:

Contiguous Memory Allocation: In an array, elements are stored in contiguous memory locations. This allows the computer’s CPU to access array elements more quickly because once the starting memory location (i.e., the base address) is known, the computer can directly calculate the memory location of any element.

Direct Access: Arrays allow direct access to elements. This means that if you want to access the ith element of an array, you can do so by directly referring to its position (i.e., index). This is not the case with linked lists, where you have to traverse from the head node to the ith node, which takes more time.

Data Locality: Modern CPUs have a feature called ‘caching’, where they store recently used data in cache memory. Since array elements are stored contiguously, there’s a good chance that if one element is accessed, the next few elements could already be in the cache, leading to faster access times. This is not the case for concurrencies, where nodes are scattered throughout memory.

3. Why are linked lists better than arrays?

Linked lists have several advantages over arrays:

Dynamic Size: Unlike arrays, linked lists do not have a fixed size. This makes them flexible for situations where the amount of data is not known in advance.

Efficient Insertions and Deletions: Linked lists are more efficient than arrays when it comes to inserting and deleting elements. You can easily insert a new node or delete an existing node from any position of the linked list in O(1) time if you have the pointer to the node. This is not possible with arrays, where you may need to shift elements to fill up space or create space for a new element, which takes O(n) time.

No Memory Wastage: As each node is created independently, linked lists use just as much memory as they need. On the other hand, arrays can lead to wasted memory if they are not fully utilized.

Ease of Implementation: Data structures such as stacks and queues are easily implemented using linked lists.

4. Which type of memory is allocated to linked lists?

Linked lists use dynamic memory allocation. This means that memory for nodes is allocated at runtime, as and when required. Unlike arrays, linked lists do not allocate memory in advance. Each node is created independently when needed, which allows a linked list to grow and shrink in size during the execution of a program.

5. What is the difference between an array list and a linked list?

ArrayList:

•   Uses a resizable array for storage.
•   Fast random access to elements (O(1) complexity).
•   Slower insertions and deletions (O(n) complexity) due to element shifting.

LinkedList:

•   Uses a doubly linked list structure with nodes.
•   Slower random access (O(n) complexity) as it needs to traverse the list.
•   Faster insertions and deletions (O(1) complexity) by manipulating references.

In short:

•   Choose ArrayList for fast lookups.
• · Choose LinkedList for frequent additions/removals.

This article was contributed by Johns Joseph, Unstop Intern and Campus Ambassador.

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