Arrays or linked-list for frequently random access?

问题

I'm using lists and arrays very often, I am wondering what is faster, array or list?

Let's say we have array of integers and linked-list, both hold same values.

int array_data[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

typedef struct node_data{
    int data;
    struct node_data * next;
} list_data;

                  ____    ____    ____    ____    ____    ____    ____    ____    ____    ____
list_data *d = -> | 1| -> | 2| -> | 3| -> | 4| -> | 5| -> | 6| -> | 7| -> | 8| -> | 9| -> |10| -> NULL

If I want to get value of array_data on index 6, I'll use array_data[6] and get value of 7. But what if I want same data from list? Should I go from start and count hops till I reach asked index get_data(d,6)? Or there is better way to do this?

int get_data(list_data *l, int index){
    int i = 0;
    while(l != NULL){
        if(index == i){
            return l -> data;
        }
        i++;
        l = l -> next;
    }
    return 0;
}

How about using array of pointers to list elements? Will be this best solution in case I have more then 100,000 records or even more to save, and each record contains more then one data type. I'll mostly need insertion to the end, and very frequently access to elements.

Thanks!

回答1:

You are correct to consider the question each time; when deciding whether to implement an array, linked-list (or other) structure.

ARRAY

• + Fast random access.
• + Dynamically allocated arrays can be re-sized using `realloc()`.
• + Sorted using `qsort()`.
• + For sorted arrays, a specific record can be located using `bsearch()`


• - Must occupy a contiguous block of memory.
• - For a long-lived applications, frequent enlargement of the the array can eventually lead to a fragmented memory space, and perhaps even eventual failure of `realloc()`.
• - Inserting and deleting elements is expensive. Inserting an element (in a sorted array) requires all elements of the array beyond the insertion point to be moved. A similar movement of elements is required when deleting an element.

LINKED-LIST

• + Does not require a contiguous block of memory.
• + Much more efficient than an array to re-size dynamically. Outperforms an array when it comes to fragmented memory usage
• + Sequential access is good, but perhaps still not as fast as an array (due to CPU cache misses, etc.).


• - Random access is not really possible.
• - Extra memory overhead for node pointers (priorNode, nextNode).

There are other structures that even combine arrays with linked list, such as hash tables, binary trees, n-trees, random-access-list, etc., each comes with various characteristics to consider.

回答2:

Arrays are constant in access time; i.e. it takes the same amount of time to access any element.

That's not true for lists: the average time taken is linear on the number of elements.

However, lists don't require a contiguous block of memory, unlike arrays. As such, appending to an array can cause memory reallocation which can wreak havoc with any pointers to array elements that you've stored.

These points are the principal considerations when choosing between an array and a list.

回答3:

There is a third way: a random access list (singly or doubly linked, or not linked at all). It has a worst case access of O(log(n)) and doesn't need ordered data. It isn't a Skip-list (not reliable). You can find info in this article: Random Access List

来源：https://stackoverflow.com/questions/23807759/arrays-or-linked-list-for-frequently-random-access