C program to make a second copy of a linked list

Question

I was writing a C code to copy the contents of a Linked List onto another list. I want to know if there is a more efficient way of doing this.

Which is better?

Answer 1

For speed, there may in fact be a better way of doing it. As always, the only real way to tell is to benchmark it.

• Depending on the relative cost of iteration
  • (which may itself depend on how and in what order you allocated your nodes, as well as cache and memory architecture)
• versus free store allocation
  • (which may depend on what state your heap is in, how many other threads are likely to be accessing it, the physical memory status in the host OS, etc.)

• it may be faster to:

  • spend one iteration counting the length of the source list

    int len = 0;
    for (start2 = start1; start2 != NULL; start2 = start2->link)
        ++len;
    

  • allocate space for all required nodes in a single block

    struct node *temp = malloc(len * sizeof(*temp));
    

  • and then spend a second iteration linking up your array of nodes

    int i = 0;
    for (start2 = start1; start2 != NULL; start2 = start2->link) {
        temp[i].info = start2->info;
        temp[i].link = &temp[i+1];
    }
    temp[len-1].link = NULL;
    

As I say, I'm not promising it will be faster (and it's certainly uglier), but it may be on some systems, with some compilers, under some conditions. Of course, it's a non-starter if the rest of your code assumes you can free individual nodes at will.

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For elegance, recursion naturally wins.

The simple iterative approach is usually going to be a good compromise, though, between elegant but might explode unless you have a fancy TCO-ing compiler and the above, which is frankly a bit ugly and would benefit from an explanatory comment.