Today in class, we learned that retrieving an element from a list is O(1) in Python. Why is this the case? Suppose I have a list of four items, for example:
li = ["perry", 1, 23.5, "s"]
These items have different sizes in memory. And so it is not possible to take the memory location of li[0] and add three times the size of each element to get the memory location of li[3]. So how does the interpreter know where li[3] is without having to traverse the list in order to retrieve the element?
A list in Python is implemented as an array of pointers1. So, what's really happening when you create the list:
["perry", 1, 23.5, "s"]
is that you are actually creating an array of pointers like so:
[0xa3d25342, 0x635423fa, 0xff243546, 0x2545fade]
Each pointer "points" to the respective objects in memory, so that the string "perry" will be stored at address 0xa3d25342 and the number 1 will be stored at 0x635423fa, etc.
Since all pointers are the same size, the interpreter can in fact add 3 times the size of an element to the address of li[0] to get to the pointer stored at li[3].
1 Get more details from: the horse's mouth (CPython source code on GitHub).
When you say a = [...], a is effectively a pointer to a PyObject containing an array of pointers to PyObjects.
When you ask for a[2], the interpreter first follows the pointer to the list's PyObject, then adds 2 to the address of the array inside it, then returns that pointer. The same happens if you ask for a[0] or a[9999].
Basically, all Python objects are accessed by reference instead of by value, even integer literals like 2. There are just some tricks in the pointer system to keep this all efficient. And pointers have a known size, so they can be stored conveniently in C-style arrays.
Short answer: Python lists are arrays.
Long answer: The computer science term list usually means either a singly-linked list (as used in functional programming) or a doubly-linked list (as used in procedural programming). These data structures support O(1) insertion at either the head of the list (functionally) or at any position that does not need to be searched for (procedurally). A Python ``list'' has none of these characteristics. Instead it supports (amortized) O(1) appending at the end of the list (like a C++ std::vector or Java ArrayList). Python lists are really resizable arrays in CS terms.
The following comment from the Python documentation explains some of the performance characteristics of Python ``lists'':
It is also possible to use a list as a queue, where the first element added is the first element retrieved (“first-in, first-out”); however, lists are not efficient for this purpose. While appends and pops from the end of list are fast, doing inserts or pops from the beginning of a list is slow (because all of the other elements have to be shifted by one).
I am learning about arrays, single linked list and double linked list now a days and this question came that
" What is the best option between these three data structures when it comes to fast searching, less memory, easily insertion and updating of things "
As far I know array cannot be the answer because it has fixed size. If we want to insert a new thing. it wouldn't always be possible. Double linked list can do the task but there will be two pointers needed for each node so there will be memory problem, so I think single linked list will fulfill all given requirements. Am I right? Please correct me if I am missing any point. There is also one more question that instead of choosing one of them, can I make combination of one or more data structures given here to meet all the requirements?
"What is the best option between these three data structures when it comes to fast searching, less memory, easily insertion and updating of things".
As far as I can tell Arrays serve the purpose.
Fast search: You could do binary search if array is sorted. You dont get that option in linkedlist
Less memory: Arrays will take least memory (but contiguous memory )
Insertion: Inserting in array is a matter of a[i] = "value". If array size is exceeded then simply export data into a new array. That is exactly how HashMaps / ArrayLists work under covers.
Updating things: Only Arrays provide you with Random access. a[i] ="new value".. updated in O(1) time if you know the index.
Each of those has its own benefits and downsides.
For search speed, I'd say arrays are better suitable due to the quick lookup times.
Since an array is a sequence of same-size elements, retrieving the value at an index is just memoryLocation + index * elementSize. For a linked list, the whole list needs traversing.
Arrays also win in the "less memory" category, since there's no need to store extra pointers.
For insertions, arrays are slow. You'll need to traverse the array, copy contents to a new array, assign the new array, delete the old one...
Insertions go much quicker in linked- or double lists, because it's just a matter of changing one or two pointers.
In the end, it all just depends on the use case. Are you inserting a lot? Then you probably want to consider a non-array structure.
Do you need many quick lookups? Consider those arrays again. Etc..
See also this question.
A linked list is usually the best choice when we don’t know in advance the number of elements we will have to store or the number can change dynamically.
Arrays have slow insertion and deletion times. To insert an element to the front or middle of the array, the first step is to ensure that there is space in the array for the new element, otherwise, the array needs to be RESIZED. This is an expensive operation. The next step is to open space for the new element by shifting every element after the desired index. Likewise, for deletion, shifting is required after removing an element. This implies that insertion time for arrays is Big O of n (O(n)) as n elements must be shifted.
Using static arrays, we can save some extra memory in
comparison to linked lists because we do not need to store pointers to the next node
a doubly-linked list support fast insertion/removal at their ends. This is used in LRU cache, where you need to enter new item to front and remove the oldest item from the end.
I am very puzzled about this. Everywhere there is written "linked lists are faster than arrays" but no one makes the effort to say WHY. Using plain logic I can't understand how a linked list can be faster. In an array all cells are next to each other so as long as you know the size of each cell it's easy to reach one cell instantly. For example if there is a list of 10 integers and I want to get the value in the fourth cell then I just go directly to the start of the array+24 bytes and read 8 bytes from there.
In the other hand when you have a linked list and you want to get the element in the fourth place then you have to start from the beginning or end of the list(depending on if it's a single or double list) and go from one node to the other until you find what you're looking for.
So how the heck can going step by step be faster than going directly to an element?
This question title is misleading.
It asserts that linked lists are faster than arrays without limiting the scope well. There are a number of times when arrays can be significantly faster and there are a number of times when a linked list can be significantly faster: the particular case of linked lists "being faster" does not appear to be supported.
There are two things to consider:
The theoretical bounds of linked-lists vs. arrays in a particular operation; and
the real-world implementation and usage pattern including cache-locality and allocations.
As far as the access of an indexed element: The operation is O(1) in an array and as pointed out, is very fast (just an offset). The operation is O(k) in a linked list (where k is the index and may always be << n, depending) but if the linked list is already being traversed then this is O(1) per step which is "the same" as an array. If an array traversal (for(i=0;i<len;i++) is faster (or slower) depends upon particular implementation/language/run-time.
However, if there is a specific case where the array is not faster for either of the above operations (seek or traversal), it would be interesting to see to be dissected in more detail. (I am sure it is possible to find a language with a very degenerate implementation of arrays over lists cough Haskell cough)
Happy coding.
My simple usage summary: Arrays are good for indexed access and operations which involve swapping elements. The non-amortized re-size operation and extra slack (if required), however, may be rather costly. Linked lists amortize the re-sizing (and trade slack for a "pointer" per-cell) and can often excel at operations like "chopping out or inserting a bunch of elements". In the end they are different data-structures and should be treated as such.
Like most problems in programming, context is everything. You need to think about the expected access patterns of your data, and then design your storage system appropriately. If you insert something once, and then access it 1,000,000 times, then who cares what the insert cost is? On the other hand, if you insert/delete as often as you read, then those costs drive the decision.
Depends on which operation you are referring to. Adding or removing elements is a lot faster in a linked list than in an array.
Iterating sequentially over the list one by one is more or less the same speed in a linked list and an array.
Getting one specific element in the middle is a lot faster in an array.
And the array might waste space, because very often when expanding the array, more elements are allocated than needed at that point in time (think ArrayList in Java).
So you need to choose your data structure depending on what you want to do:
many insertions and iterating sequentially --> use a LinkedList
random access and ideally a predefined size --> use an array
Because no memory is moved when insertion is made in the middle of the array.
For the case you presented, its true - arrays are faster, you need arithmetic only to go from one element to another. Linked list require indirection and fragments memory.
The key is to know what structure to use and when.
Linked lists are preferable over arrays when:
a) you need constant-time insertions/deletions from the list (such as in real-time computing where time predictability is absolutely critical)
b) you don't know how many items will be in the list. With arrays, you may need to re-declare and copy memory if the array grows too big
c) you don't need random access to any elements
d) you want to be able to insert items in the middle of the list (such as a priority queue)
Arrays are preferable when:
a) you need indexed/random access to elements
b) you know the number of elements in the array ahead of time so that you can allocate the correct amount of memory for the array
c) you need speed when iterating through all the elements in sequence. You can use pointer math on the array to access each element, whereas you need to lookup the node based on the pointer for each element in linked list, which may result in page faults which may result in performance hits.
d) memory is a concern. Filled arrays take up less memory than linked lists. Each element in the array is just the data. Each linked list node requires the data as well as one (or more) pointers to the other elements in the linked list.
Array Lists (like those in .Net) give you the benefits of arrays, but dynamically allocate resources for you so that you don't need to worry too much about list size and you can delete items at any index without any effort or re-shuffling elements around. Performance-wise, arraylists are slower than raw arrays.
Reference:
Lamar answer
https://stackoverflow.com/a/393578/6249148
LinkedList is Node-based meaning that data is randomly placed in memory and is linked together by nodes (objects that point to another, rather than being next to one another)
Array is a set of similar data objects stored in sequential memory locations
The advantage of a linked list is that data doesn’t have to be sequential in memory. When you add/remove an element, you are simply changing the pointer of a node to point to a different node, not actually moving elements around. If you don’t have to add elements towards the end of the list, then accessing data is faster, due to iterating over less elements. However there are variations to the LinkedList such as a DoublyLinkedList which point to previous and next nodes.
The advantage of an array is that yes you can access any element O(1) time if you know the index, but if you don’t know the index, then you will have to iterate over the data.
The down side of an array is the fact that its data is stored sequentially in memory. If you want to insert an element at index 1, then you have to move every single element to the right. Also, the array has to keep resizing itself as it grows, basically copying itself in order to make a new array with a larger capacity. If you want to remove an element in the begging, then you will have to move all the elements to left.
Arrays are good when you know the index, but are costly as they grow.
The reason why people talk highly about linked lists is because the most useful and efficient data structures are node based.
This is an assignment question that I am having trouble wording an answer to.
"Suppose a tree may have up to k children per node. Let v be the average number of children per node. For what value(s) of v is it more efficient (in terms of space used) to store the child nodes in a linked list versus storage in an array? Why?"
I believe I can answer the "why?" more or less in plain English -- it will be more efficient to use the linked list because rather than having a bunch of empty nodes (ie empty indexes in the array if your average is lower than the max) taking up memory you only alloc space for a node in a linked list when you're actually filling in a value.
So if you've got an average of 6 children when your maximum is 200, the array will be creating space for all 200 children of each node when the tree is created, but the linked list will only alloc space for the nodes as needed. So, with the linked list, space used will be approximately(?) the average; with the array, spaced used will be the max.
...I don't see when it would ever be more efficient to use the array. Is this a trick question? Do I have to take into account the fact that the array needs to have a limit on total number of nodes when it's created?
For many commonly used languages, the array will require allocating storage k memory addresses (of the data). A singly-linked list will require 2 addresses per node (data & next). A doubly-linked list would require 3 addresses per node.
Let n be the actual number of children of a particular node A:
The array uses k memory addresses
The singly-linked list uses 2n addresses
The doubly-linked list uses 3n addresses
The value k allows you to determine if 2n or 3n addresses will average to a gain or loss compared to simply storing the addresses in an array.
...I don't see when it would ever be more efficient to use the array. Is this a trick question?
It’s not a trick question. Think of the memory overhead that a linked list has. How is a linked list implemented (vs. an array)?
Also (though this is beyond the scope of the question!), space consumption isn’t the only deciding factor in practice. Caching plays an important role in modern CPUs and storing the individual child nodes in an array instead of a linked list can improve the cache locality (and consequently the tree’s performance) drastically.
Arrays must pre-allocate space but we can use them to access very fast any entry.
Lists allocate memory whenever they create a new node, and that isn't ideal because
memory allocation costs on cpu time.
Tip: you can allocate the whole array at once, if you want to, but usually we allocate,
lets say, 4 entries and we resize it by doubling the size when we need more space.
I can imagine it could be a very good idea and very efficient in many scenarios to use a LinkedList if the data item one uses has intrinsic logic for a previous and next item anyway, for example a TimeTableItem or anything that is somehow time-related. These should implement an interface, so the LinkedList implementation can leverage that and doesn't have to wrap the items into its own node objects. Inserting and removing would be much more efficient here than using a List implementation which internally juggles arrays around.
You're assuming the array can't be dynamically re-allocated. If it can, the array wins, because it need be no bigger than k items (plus a constant overhead), and because it doesn't need per-item storage for pointers.
Why would someone want to use a linked-list over an array?
Coding a linked-list is, no doubt, a bit more work than using an array and one may wonder what would justify the additional effort.
I think insertion of new elements is trivial in a linked-list but it's a major chore in an array. Are there other advantages to using a linked list to store a set of data versus storing it in an array?
This question is not a duplicate of this question because the other question is asking specifically about a particular Java class while this question is concerned with the general data structures.
Another good reason is that linked lists lend themselves nicely to efficient multi-threaded implementations. The reason for this is that changes tend to be local - affecting only a pointer or two for insert and remove at a localized part of the data structure. So, you can have many threads working on the same linked list. Even more, it's possible to create lock-free versions using CAS-type operations and avoid heavy-weight locks altogether.
With a linked list, iterators can also traverse the list while modifications are occurring. In the optimistic case where modifications don't collide, iterators can continue without contention.
With an array, any change that modifies the size of the array is likely to require locking a large portion of the array and in fact, it's rare that this is done without a global lock across the whole array so modifications become stop the world affairs.
It's easier to store data of different sizes in a linked list. An array assumes every element is exactly the same size.
As you mentioned, it's easier for a linked list to grow organically. An array's size needs to be known ahead of time, or re-created when it needs to grow.
Shuffling a linked list is just a matter of changing what points to what. Shuffling an array is more complicated and/or takes more memory.
As long as your iterations all happen in a "foreach" context, you don't lose any performance in iteration.
Wikipedia has very good section about the differences.
Linked lists have several advantages
over arrays. Elements can be inserted
into linked lists indefinitely, while
an array will eventually either fill
up or need to be resized, an expensive
operation that may not even be
possible if memory is fragmented.
Similarly, an array from which many
elements are removed may become
wastefully empty or need to be made
smaller.
On the other hand, arrays allow random
access, while linked lists allow only
sequential access to elements.
Singly-linked lists, in fact, can only
be traversed in one direction. This
makes linked lists unsuitable for
applications where it's useful to look
up an element by its index quickly,
such as heapsort. Sequential access on
arrays is also faster than on linked
lists on many machines due to locality
of reference and data caches. Linked
lists receive almost no benefit from
the cache.
Another disadvantage of linked lists
is the extra storage needed for
references, which often makes them
impractical for lists of small data
items such as characters or boolean
values. It can also be slow, and with
a naïve allocator, wasteful, to
allocate memory separately for each
new element, a problem generally
solved using memory pools.
http://en.wikipedia.org/wiki/Linked_list
I'll add another - lists can act as purely functional data structures.
For instance, you can have completely different lists sharing the same end section
a = (1 2 3 4, ....)
b = (4 3 2 1 1 2 3 4 ...)
c = (3 4 ...)
i.e.:
b = 4 -> 3 -> 2 -> 1 -> a
c = a.next.next
without having to copy the data being pointed to by a into b and c.
This is why they are so popular in functional languages, which use immutable variables - prepend and tail operations can occur freely without having to copy the original data - very important features when you're treating data as immutable.
Besides inserting into the middle of the list being easier - it's also much easier to delete from the middle of a linked list than an array.
But frankly, I've never used a linked list. Whenever I needed fast insertion and deletion, I also needed fast lookup, so I went to a HashSet or a Dictionary.
Merging two linked lists (especially two doubly linked lists) is much faster than merging two arrays (assuming the merge is destructive). The former takes O(1), the latter takes O(n).
EDIT: To clarify, I meant "merging" here in the unordered sense, not as in merge sort. Perhaps "concatenating" would have been a better word.
A widely unappreciated argument for ArrayList and against LinkedList is that LinkedLists are uncomfortable while debugging. The time spent by maintenance developers to understand the program, e.g. to find bugs, increases and IMHO does sometimes not justify the nanoseconds in performance improvements or bytes in memory consumption in enterprise applicatons. Sometimes (well, of course it depends on the type of applications), it's better to waste a few bytes but have an application which is more maintainable or easier to understand.
For example, in a Java environment and using the Eclipse debugger, debugging an ArrayList will reveal a very easy to understand structure:
arrayList ArrayList<String>
elementData Object[]
[0] Object "Foo"
[1] Object "Foo"
[2] Object "Foo"
[3] Object "Foo"
[4] Object "Foo"
...
On the other hand, watching the contents of a LinkedList and finding specific objects becomes a Expand-The-Tree clicking nightmare, not to mention the cognitive overhead needed to filter out the LinkedList internals:
linkedList LinkedList<String>
header LinkedList$Entry<E>
element E
next LinkedList$Entry<E>
element E "Foo"
next LinkedList$Entry<E>
element E "Foo"
next LinkedList$Entry<E>
element E "Foo"
next LinkedList$Entry<E>
previous LinkedList$Entry<E>
...
previous LinkedList$Entry<E>
previous LinkedList$Entry<E>
previous LinkedList$Entry<E>
First of all, in C++ linked-lists shouldn't be much more trouble to work with than an array. You can use the std::list or the boost pointer list for linked lists. The key issues with linked lists vs arrays are extra space required for pointers and terrible random access. You should use a linked list if you
you don't need random access to the data
you will be adding/deleting elements, especially in the middle of the list
For me it is like this,
Access
Linked Lists allow only sequential access to elements. Thus the algorithmic complexities is order of O(n)
Arrays allow random access to its elements and thus the complexity is order of O(1)
Storage
Linked lists require an extra storage for references. This makes them impractical for lists of small data items such as characters or boolean values.
Arrays do not need an extra storage to point to next data item. Each element can be accessed via indexes.
Size
The size of Linked lists are dynamic by nature.
The size of array is restricted to declaration.
Insertion/Deletion
Elements can be inserted and deleted in linked lists indefinitely.
Insertion/Deletion of values in arrays are very expensive. It requires memory reallocation.
Two things:
Coding a linked list is, no doubt, a bit more work than using an array and he wondered what would justify the additional effort.
Never code a linked list when using C++. Just use the STL. How hard it is to implement should never be a reason to choose one data structure over another because most are already implemented out there.
As for the actual differences between an array and a linked list, the big thing for me is how you plan on using the structure. I'll use the term vector since that's the term for a resizable array in C++.
Indexing into a linked list is slow because you have to traverse the list to get to the given index, while a vector is contiguous in memory and you can get there using pointer math.
Appending onto the end or the beginning of a linked list is easy, since you only have to update one link, where in a vector you may have to resize and copy the contents over.
Removing an item from a list is easy, since you just have to break a pair of links and then attach them back together. Removing an item from a vector can be either faster or slower, depending if you care about order. Swapping in the last item over top the item you want to remove is faster, while shifting everything after it down is slower but retains ordering.
Eric Lippert recently had a post on one of the reasons arrays should be used conservatively.
Fast insertion and removal are indeed the best arguments for linked lists. If your structure grows dynamically and constant-time access to any element isn't required (such as dynamic stacks and queues), linked lists are a good choice.
Here's a quick one: Removal of items is quicker.
Linked-list are especially useful when the collection is constantly growing & shrinking. For example, it's hard to imagine trying to implement a Queue (add to the end, remove from the front) using an array -- you'd be spending all your time shifting things down. On the other hand, it's trivial with a linked-list.
Other than adding and remove from the middle of the list, I like linked lists more because they can grow and shrink dynamically.
Arrays Vs Linked List:
Array memory allocation will fail sometimes because of fragmented memory.
Caching is better in Arrays as all elements are allocated contiguous memory space.
Coding is more complex than Arrays.
No size constraint on Linked List, unlike Arrays
Insertion/Deletion is faster in Linked List and access is faster in Arrays.
Linked List better from multi-threading point of view.
No one ever codes their own linked list anymore. That'd be silly. The premise that using a linked list takes more code is just wrong.
These days, building a linked list is just an exercise for students so they can understand the concept. Instead, everyone uses a pre-built list. In C++, based the on the description in our question, that'd probably mean an stl vector (#include <vector> ).
Therefore, choosing a linked list vs an array is entirely about weighing the different characteristics of each structure relative to the needs of your app. Overcoming the additional programming burden should have zero impact on the decision.
It's really a matter of efficiency, the overhead to insert, remove or move (where you are not simply swapping) elements inside a linked list is minimal, i.e. the operation itself is O(1), verses O(n) for an array. This can make a significant difference if you are operating heavily on a list of data. You chose your data-types based on how you will be operating on them and choose the most efficient for the algorithm you are using.
Arrays make sense where the exact number of items will be known, and where searching by index makes sense. For example, if I wanted to store the exact state of my video output at a given moment without compression I would probably use an array of size [1024][768]. This will provide me with exactly what I need, and a list would be much, much slower to get the value of a given pixel. In places where an array does not make sense there are generally better data types than a list to deal with data effectively.
as arrays are static in nature, therefore all operations
like memory allocation occur at the time of compilation
only. So processor has to put less effort at its runtime .
Suppose you have an ordered set, which you also want to modify by adding and removing elements. Further, you need ability to retain a reference to an element in such a way that later you can get a previous or next element. For example, a to-do list or set of paragraphs in a book.
First we should note that if you want to retain references to objects outside of the set itself, you will likely end up storing pointers in the array, rather than storing objects themselves. Otherwise you will not be able to insert into array - if objects are embedded into the array they will move during insertions and any pointers to them will become invalid. Same is true for array indexes.
Your first problem, as you have noted yourself, is insertion - linked list allows inserting in O(1), but an array would generally require O(n). This problem can be partially overcome - it is possible to create a data structure that gives array-like by-ordinal access interface where both reading and writing are, at worst, logarithmic.
Your second, and more severe problem is that given an element finding next element is O(n). If the set was not modified you could retain the index of the element as the reference instead of the pointer thus making find-next an O(1) operation, but as it is all you have is a pointer to the object itself and no way to determine its current index in the array other than by scanning the entire "array". This is an insurmountable problem for arrays - even if you can optimized insertions, there is nothing you can do to optimize find-next type operation.
In an array you have the privilege of accessing any element in O(1) time. So its suitable for operations like Binary search Quick sort, etc. Linked list on the other hand is suitable for insertion deletion as its in O(1) time. Both has advantages as well as disadvantages and to prefer one over the other boils down to what you want to implement.
-- Bigger question is can we have a hybrid of both. Something like what python and perl implement as lists.
Linked List
Its more preferable when it comes about insertion! Basically what is does is that it deals with the pointer
1 -> 3 -> 4
Insert (2)
1........3......4
.....2
Finally
1 -> 2 -> 3 -> 4
One arrow from the 2 points at 3 and the arrow of 1 points at 2
Simple!
But from Array
| 1 | 3 | 4 |
Insert (2)
| 1 | 3 | | 4 |
| 1 | | 3 | 4 |
| 1 | 2 | 3 | 4 |
Well anyone can visualize the difference!
Just for 4 index we are performing 3 steps
What if the array length is one million then? Is array efficient?
The answer is NO! :)
The same thing goes for deletion!
In Linked List we can simply use the pointer and nullify the element and next in the object class!
But for array, we need to perform shiftLeft()
Hope that helps! :)
Linked List are more of an overhead to maintain than array, it also requires additional memory storage all these points are agreed. But there are a few things which array cant do. In many cases suppose you want an array of length 10^9 you can't get it because getting one continous memory location has to be there. Linked list could be a saviour here.
Suppose you want to store multiple things with data then they can be easily extended in the linked list.
STL containers usually have linked list implementation behind the scene.
1- Linked list is a dynamic data structure so it can grow and shrink at runtime by allocating and deallocating memory. So there is no need to give an initial size of the linked list. Insertion and deletion of nodes are really easier.
2- size of the linked list can increase or decrease at run time so there is no memory wastage. In the case of the array, there is a lot of memory wastage, like if we declare an array of size 10 and store only 6 elements in it then space of 4 elements is wasted. There is no such problem in the linked list as memory is allocated only when required.
3- Data structures such as stack and queues can be easily implemented using linked list.
Only reason to use linked list is that insert the element is easy (removing also).
Disadvatige could be that pointers take a lot of space.
And about that coding is harder:
Usually you don't need code linked list (or only once) they are included in
STL
and it is not so complicated if you still have to do it.
i also think that link list is more better than arrays.
because we do traversing in link list but not in arrays
Depending on your language, some of these disadvantages and advantages could be considered:
C Programming Language: When using a linked list (through struct pointers typically), special consideration must be made sure that you are not leaking memory. As was mentioned earlier, linked lists are easy to shuffle, because all were doing is changing pointers, but are we going to remember to free everything?
Java: Java has an automatic garbage collect, so leaking memory won't be an issue, but hidden from the high level programmer is the implementation details of what a linked list is. Methods such as removing a node from the middle of the list is more complicated of a procedure than some users of the language would expect it to be.
Why a linked list over an array ? Well as some have already said, greater speed of insertions and deletions.
But maybe we don't have to live with the limits of either, and get the best of both, at the same time... eh ?
For array deletions, you can use a 'Deleted' byte, to represent the fact that a row has been deleted, thus reorging the array is no longer necessary. To ease the burden of insertions, or rapidly changing data, use a linked list for that. Then when referring to them, have your logic first search one, then the other. Thus, using them in combination gives you the best of both.
If you have a really large array, you could combine it with another, much smaller array or linked list where the smaller one hold thes 20, 50, 100 most recently used items. If the one needed is not in the shorter linked list or array, you go to the large array. If found there, you can then add it to the smaller linked list/array on the presumption that 'things most recently used are most likey to be re-used' ( and yes, possibly bumping the least recently used item from the list ). Which is true in many cases and solved a problem I had to tackle in an .ASP security permissions checking module, with ease, elegance, and impressive speed.
While many of you have touched upon major adv./dis of linked list vs array, most of the comparisons are how one is better/ worse than the other.Eg. you can do random access in array but not possible in linked list and others. However, this is assuming link lists and array are going to be applied in a similar application. However a correct answer should be how link list would be preferred over array and vice-versa in a particular application deployment.
Suppose you want to implement a dictionary application, what would you use ?
Array : mmm it would allow easy retrieval through binary search and other search algo .. but lets think how link list can be better..Say you want to search "Blob" in dictionary. Would it make sense to have a link list of A->B->C->D---->Z and then each list element also pointing to an array or another list of all words starting with that letter ..
A -> B -> C -> ...Z
| | |
| | [Cat, Cave]
| [Banana, Blob]
[Adam, Apple]
Now is the above approach better or a flat array of [Adam,Apple,Banana,Blob,Cat,Cave] ? Would it even be possible with array ?
So a major advantage of link list is you can have an element not just pointing to the next element but also to some other link list/array/ heap/ or any other memory location.
Array is a one flat contigous memory sliced into blocks size of the element it is going to store.. Link list on the other hand is a chunks of non-contigous memory units (can be any size and can store anything) and pointing to each other the way you want.
Similarly lets say you are making a USB drive. Now would you like files to be saved as any array or as a link list ? I think you get the idea what I am pointing to :)