Difference between array and linked list

Difference between array and linked list

In last post : Linked list data structure, we discussed basics of linked list, where I promised to go in details what is difference between array and linked list. Before going into post, I want to make sure that you understand that there is no such thing called one data structure is better than other. Based on your requirements and use cases, you chose one or the other. It depends on what is most frequent operation your algorithm would perform in it’s lifetime. That’s why they have data structure round in interview process to understand if you can chose the correct one for the problem.

What is an array?
Array is linear, sequential and contiguous collection of elements which can be addressed using index.

What is a linked list?
Linked list is linear, sequential and non-contiguous collection of nodes, each node store the reference to next node. To understand more, please refer to Linked list data structure.

Difference between arrays and linked list

Static Vs dynamic size

Size of an array is defined statically at the compile time where as linked list grows dynamically at run time based on need. Consider a case where you know the maximum number of elements algorithm would ever have, then you can confidently declare it as array. However, if you do not know, the linked list is better. There is a catch : What if there is a rare chance that number of elements will reach maximum, most of the time it will be way less than maximum? In this case, we would unnecessary allocating extra memory for array which may or may not be used. 

Memory allocation

An array is given contiguous memory in system. So, if you know the address of any of the element in array, you can access other elements based position of the element.

linked list vs arrays
Statically allocated contiguous memory

Linked list are not store contiguous on memory, nodes are scattered around on memory. So you may traverse forward in linked list, given node (using next node reference), but you can not access nodes prior to it.

arrays vs linked list
Dynamically allocated non-contiguous memory

Contiguous allocation of memory required sufficient memory before hand for an array to be stored, for example if want to store 20 integers in an array, we would required 80 bytes contiguous memory chunk. However, with linked list we can start with 8 bytes and request more memory as when required, which may be wherever. Contiguous allocation of memory makes it difficult to resize an array too. We have to look for different chunk of memory, which fits the new size, move all existing elements to that location. Linked list on other hand are dynamically size and can grow much faster without relocating existing elements.

Memory requirement

It’s good to have non-contiguous memory then? It comes with a cost. Each node of linked list has to store reference to next node in memory. This leads to extra payload of 4 bytes in each node. On the other hand, array do not require this extra payload. You  have to trade off extra space with advantages you are getting. Also, sometime, spending extra space is better that have cumbersome operations like shifting, adding and deleting operation on array. Or value stored in node is big enough to make these 4 bytes negligible in analysis.

Operation efficiency

We do operations of data structure to get some output. There are four basic operations we should be consider : read, search, insert/update and delete.

Read on array is O(1) where you can directly access any element in array given it’s index. By O(1), read on array does not depend on size of array.
Whereas, time complexity of read on linked list is O(n) where n is number of nodes. So, if you have a problem, which requires more random reads, array will over-weigh linked list.

Given the contiguous memory allocation of array, there are optimized algorithms like binary search to search elements on array which has complexity of O(log n). Search on linked list on other hand requires O(n).

Insert on array is O(1) again, if we are writing within the size of array. In linked list, complexity of insert depends where do you want to write new element at. If insert happens at head, then it O(1), on the other hand if insert happens at end, it’s O(n).

Insert node at start of linked list
Insert node at the tail of linked list

Update means here, changing size of array or linked list by adding one more element. In array it is costly operation, as it will require reallocation of memory and copying all elements on to it. Does not matter if you add element at end or start, complexity remains O(1).
For linked list, it varies, to update at end it’s O(n), to update at head, it’s O(1). 
In same vain, delete on array requires movement of all elements, if first element is deleted, hence complexity of O(n). However, delete on linked list O(1), if it’s head, O(n) if it’s tail.

To see the difference between O(1) and O(n), below graph should be useful.

difference between array and linked list
Complexity analysis graph

Key difference between array and linked list are as follows

  • Arrays are really bad at insert and delete operation due to internal reallocation of memory.
  • Statically sized at the compile time
  • Memory allocation is contiguous,  which make access elements easy without any additional pointers. Can jump around the array without accessing all the elements in between.
  • Linked list almost have same complexity when insert and delete happens at the end, however no memory shuffling happens
  • Search on linked list is bad.=, usually require scan with O(n) complexity
  • Dynamically sized on run time.
  • Memory allocation is non-contiguous, additional pointer is required to store neighbor node reference. Cannot jump around in linked list.

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