Question 173977 in PostgreSQL Question posted in
The official documentation can be found here.

Postgresql – How to create and iterate over a List of graphids (Apache AGE)?

I’m creating a function for Apache AGE, and I need to iterate over a List of graphids, how should I do it properly?

What I’m essentially trying to do is this:

graphid new_graphid = 0;
graphid vertex_to_connect = 0;
List* graphids_array = NIL;

(...)

for (int64 i=(int64)1;i<graph.graph_size;i++)
{  
        new_graphid = create_vertex(&graph);

        graphids_array = lappend_int(graphids_array, new_graphid);
        
        for(int64 j = 0; j<graphids_array->length-1; j++)
        {
            vertex_to_connect = list_nth_int(graphids_array, j);
            connect_vertexes_by_graphid(&graph,
                                        new_graphid,
                                        vertex_to_connect);
        }
}

(...)

The problem happening is that the vertex_to_connect variable is not receiving the proper graphid (which should be a big number like 844424930131970), instead, it’s getting small values that seems to be from the j variable.

Any help on showing where I could be mistaken is appreciated.

Tags:

2

Answers


  1. 0

    It looks like youre using the list_nth_int() function to get vertex_to_connect value from graphids_array. But list_nth_int() function gets an element from list by its position instead of its value. As a result you’re getting small values that vertex_to_connect gets small values that seem like they belong to j variable, because j is the index of the list element, not the actual value of the element.

    Login or Signup to reply.
  2. 0

    In order to iterate through a List you can use the foreach loop with a ListCell iterator. Here is an example of how you can do this:

    List *graphid_list = NIL;
ListCell *lc;

// Go through all cells in the list.
foreach (lc, graphid_list)
{
    // Code goes here.
}

    Alternatively, you could use the ListGraphId structure to create your list. Since it’s only graphids, I believe it will do the trick. Then, you would need to create an iterator to peek at each element of the list as a GraphIdNode. I believe it would be necessary to create a for or while loop for this, but then update at the end of the loop for the iterator to be the next value of the GraphIdNode. So, something like this would traverse the entire list:

        ListGraphId *container;
    GraphIdNode *current = get_list_head(container);
    GraphIdNode *previous = NULL;

    while (current != NULL)
    {
        previous = current;
        current = next_GraphIdNode(current);
    }

    You can find more about it at src/backend/utils/adt/age_graphid_ds.c and src/include/utils/age_grapid_ds.h.

    https://github.com/apache/age/blob/master/src/backend/utils/adt/age_graphid_ds.c

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search