From Python's Linked Lists to Dynamic Arrays

**hexbox** · 01-26-2005

Hi there,

I'm doing some conversion of python to C code. I'm having quite a bit of problems converting what used to be linked lists in to C arrays.

In the python code, the programmer loves to declare an empty list, appending items while he iterates through some set of tasks, and return the list as a function's output. This causes me some trouble..

It's a bit of a pain to determine the size of the list before having to work with it, but the real trouble I'm having is managing the storage of these sizes. Should my function return the size of the array as well as a pointer to the array itself? Should I declare new structs which contain the array as well as the array's size?

Any advice is appreciated; I tend to think object oriented-ly, so this is all new to me

**quzah** · 01-26-2005

So why are you converting to arrays at all? Use a linked list.

Quzah.

**hexbox** · 01-26-2005

Originally Posted by quzah

So why are you converting to arrays at all?

Because an array is faster than a linked list, basically.

The C code is eventually going to be parallelized and run on a beowulf cluster. I know that certain MPI functions require contiguous arrays for passing between processes, so using linked lists makes me nervous.. I may end up using them, though, just to save myself some sanity.

**itsme86** · 01-26-2005

Should my function return the size of the array as well as a pointer to the array itself?

Well, since a C function can only return one value you're not really even left with this option. One thing you might look at is doing it something like this:

Code:

int fill_array(int **array)
{
  int num = 7; // Some arbitrary number that's unknown to the calling function

  if(!(*array = malloc(sizeof(int) * num)))
    return -1;

  // Fill the array with whatever
  (*array)[0] = 1;
  (*array)[1] = 9;
  ...
  (*array)[6] = 4;

  return num;
}

int main(void)
{
  int *array = NULL;
  int n_array;

  if((n_array = fill_array(&array)) == -1)
  {
    puts("There was a problem!");
    return 1;
  }

  // Now the array is filled with data and you know how big it is

  free(array);

  return 0;
}

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