Question

I’m doing a orders program and I have a binary search tree for products, item contain product name, stock quantity, order quantity, and price. When I creeate an order, the order quantity in the selected item is increased according to the order.

I am required to create a function, called batchUpdate, which goes goes through the tree and substracts the stock quantity by the order quantity for each item, and obviously setting the order quantity to 0 again. The order quantity is being updated when a new order is created, but for some reason nothing is happening. here are a few snippets:

void BatchUpdate(PTree * pt) //PTree is a typedef, the tree
{
    if (PTreeIsEmpty(pt))
        puts("Nothing to update!");
    else
    {
        puts("test"); //just for debug, it is still being displayed
        TraverseP(pt,MinusStock); //Traverses the tree, and applies function 
                                  //MinusStock to each item in the node
    }
}

void MinusStock(Prdct C) //Prdct is a typedef of struct containing details
{
    C.StockQ = C.StockQ - C.OrderQ;
    C.OrderQ = 0;

}

void TraverseP(const PTree * ptree, void (* pfun)(Prdct item))
{
    if (ptree != NULL)
        InOrderP(ptree->root,pfun);
}

static void InOrderP(const PNode * root, void (* pfun)(Prdct item))
{
    if (root != NULL)
    {
        (*pfun)(root->item);
        InOrderP(root->left, pfun);
        InOrderP(root->right, pfun);
    }
}

no error is being given, it just ignores the traverse, and puts(“test”) is being displayed

Answer 1

void MinusStock(Prdct C) //Prdct is a typedef of struct containing details
{
    C.StockQ = C.StockQ - C.OrderQ;
    C.OrderQ = 0;

}

This is the problem, you are taking it by value, this means that any changes are NOT affecting the tree elements, since you are operating on a copy of the data which is destroyed after this function terminates.

This can be fixed by making the below change.

void MinusStock(Prdct* C){
                     ^

This means you take the product struct as a pointer and thus any changes to it are persistant.

To do this you also need to change both how you call, (*pfun)(&root->item); and how you access the variable, C->OrderQ = 0;, and how you define the function pointer, void (* pfun)(Prdct* item).

Thank you @MathewHall for correctly pointing out that this is a C question.

To summarise, the traverse IS happening, but it is not changing the tree, instead it is creating copies of each element before it operates on them, leaving the tree as-is.