C - Pointers, functions and recursion

Question

Just trying to get some code to work involving pointers, functions and recursion:

Node * root = malloc(sizeof(Node));
root->data = "1";
root->next = NULL;
root->child = NULL;

Node * G = NULL;
BuildGraph(root, &G);
printf("Root is: %d\n", root->data);
Print(G, ">>"); // Custom Print function

And BuildGraph:

void BuildGraph(Node * head, Node ** G) {
    if (head->child == NULL) { // No child
        printf("Head has no child! %d\n", head->data);
        Push(head, &G);
        Print(G, ">>");
        return;
    }
    BuildGraph(head->child, &G);
    return;
}

And so when I run the program, my output goes like this:

Head has no child! 1 // printf in BuildGraph
G: 1>> // Print(G, ">>") in BuildGraph
Root is: 1
G is empty! // Print(G, ">>") in main

Anyone know the reason G is not carrying over into main?

Thanks.

chux · Accepted Answer · 2013-08-27 15:02:23Z

2

Within void BuildGraph(), BuildGraph(head->child, &G); should be BuildGraph(head->child, G);. No &, Likely the same with Push(head, &G);

Within your build function, G is a Node **. Outside in main(), G is a Node *.

Consider using a different and more expansive variable name than G within BuildGraph(). Maybe something like

void BuildGraph(Node * head, Node ** AddressG) {
    if (head->child == NULL) { // No child
        printf("Head has no child! %d\n", head->data);
        Push(head, AddressG);
        Print(AddressG, ">>");
        return;
    }
    BuildGraph(head->child, AddressG);
    return;
}

I am confident that you compilation provided warning messages concerning this issue. If they did not recommend investigating how to turn them on.

edited Aug 27, 2013 at 15:02

answered Aug 27, 2013 at 14:48

chux

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8 Comments

Travv92 Over a year ago

Thanks for your reply. I removed all the & characters like you said and still the list yields empty after calling BuildGraph. What I'm trying to achieve is a recursive way to populate G using the child and next structures starting from some node head. I've implemented Push/Pop using Node ** somelist without having to return the new list which is nice, and I'm looking for a way to do that here. Any more ideas?

chux Over a year ago

Likely need to see 1) more of your code, 2) input data 3) compiler warning messages. That may need to be a re-vamped question.

Travv92 Over a year ago

Thanks for your reply. No compiler warning messages when I run it, and input data is root node. As for more code, there shouldn't be any need for it. The original post has outlined the problem. The output for G in the last line should be G: 1>> (in main) as it is in the BuildGraph loop, but instead it is giving me G is empty!.

chux Over a year ago

I don't see where "is empty!" comes from. You must be getting warnings as root->data = "1"; implies data is char * yet printf("Head has no child! %d\n", head->data) implies data is int. Or your warnings are not turned on.

Travv92 Over a year ago

Don't worry about the Print function, that's a simple custom one that works fine.. The real problem is that G is NOT being updated in main when BuildGraph pushes to it.

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