What is wrong with this recursive function for searching a word match inside a non binary tree?

Question

I have this snippet of code inside my project that has to search for an exact match inside an XML tree parsed with RapidXML. It does find the match but at the end it's always a null pointer match. I can't figure out the order in which I should code the function can somebody help me out?

xml_node<>* processNode(xml_node<>* node, char* lookout)
    {
        for (xml_node<>* child = node->first_node(); child; child = child->next_sibling())
        {
            cout << "processNode: child name is " << child->name() << " comparing it with " << lookout << endl;
            if (strcmp(child->name(), lookout) == 0) {
                return child;
            }
            processNode(child, lookout);
        }
    
        return nullptr;
    }

@tkausl I can't figure out how to not ignore it... could you show me a quick fix? — randomizzatore, Mar 27 '23 at 16:14
change `xml_node<>* processNode` to `[[nodiscard]] xml_node<>* processNode`. You don't want the results from this function to be discarded, and yet you are doing that. — Drew Dormann, Mar 27 '23 at 16:31
Recursive functions are exactly the same as non-recursive functions. You would never miss out a `return` on a normal function, so why do it on a recursive function? — john, Mar 27 '23 at 16:43

Karen Baghdasaryan · Answer 1 · 2023-03-27T16:27:15.770

You should have written

return processNode(child, lookout);

You should specify that you want to return the result of the recursive call.

When you write function call without return statement, what it does is just creating local temporary object, which is returned by the function processNode, and it immediately deletes it. Imagine you wanted to return the result of f function from g function, you would write.

Some_Type f()
{
    if(.....)
    {
        ....
    }
    return g();
}

Same is for recursive calling. You want to return the result of the next call. So you final function should be

xml_node<>* processNode(xml_node<>* node, char* lookout)
    {
        for (xml_node<>* child = node->first_node(); child; child = child->next_sibling())
        {
            cout << "processNode: child name is " << child->name() << " comparing it with " << lookout << endl;
            if (strcmp(child->name(), lookout) == 0) {
                return child;
            }
            return processNode(child, lookout); // Here is the sub.
        }
    
        return nullptr;
    }

What should I substitute that with? – randomizzatore Mar 27 '23 at 16:21 — randomizzatore, Mar 27 '23 at 16:21

score 0 · Accepted Answer · answered Mar 27 '23 at 16:33

You need to return the result of the recursion – recursive functions work exactly like non-recursive function and return to the place where they were called.

However, you can't return it directly, because then you would only ever look in one of the children.
You need to first store the value, and then return it only if you found what you were looking for.
Something like this:

    for (xml_node<>* child = node->first_node(); child; child = child->next_sibling())
    {
        if (strcmp(child->name(), lookout) == 0) {
            return child;
        }
        xml_node<>* result = processNode(child, lookout);
        if (result != nullptr) {
            return result;
        }
    }

What is wrong with this recursive function for searching a word match inside a non binary tree?

2 Answers2