How do a move a dynamically allocated element to another node in C?

Question

I'm creating a huffman tree in C. I store all the characters with their frequencies as a queue_elem element. I put all those elements in a priority queue then take the least frequent elements, allocate a new queue_elem element with its left and right child being those 2 elements then add that new parent node to the pariotiy queue and lastly detele+free those 2 child nodes from the priority queue. My probem is that I'm struggling to "move" those 2 elements into the children nodes of the parent node.

I either add them, free those 2 nodes, but they are then freed entierly (from both the priority queue and parent's children nodes) or I try allocating memory like I did here (this is a small test program I wrote to test how I would do it in the real file):

#include <stdio.h>
#include <stdlib.h>

typedef struct queue_elem queue_elem;

// The type for elements in the priority queue
struct queue_elem {
    //How often a chracter is repeated
    int freq;
    struct queue_elem *left_child;
    struct queue_elem *right_child;
    unsigned char val;
};
queue_elem *create_pq_elem(int val, int freq);
void delete_nodes(queue_elem *qe);

queue_elem *create_pq_elem(int val, int freq) {
    queue_elem *qe = malloc(sizeof(queue_elem));
    qe->freq = freq;
    qe->val = val;
    qe->left_child = NULL;
    qe->right_child = NULL;
    return qe;
}

void delete_nodes(queue_elem *qe)
{
    if(qe != NULL)
    {
        if(qe->left_child != NULL || qe->right_child != NULL)
        {
            delete_nodes(qe->left_child);
            delete_nodes(qe->right_child);
        }
        free(qe);
        qe = NULL;
    }
}

int main() {
    queue_elem *qe1 = create_pq_elem(97, 5);
    queue_elem *qe2 = create_pq_elem(65, 3);

    queue_elem *qe3 = create_pq_elem(0, qe1->freq + qe2->freq);
    qe3->left_child =create_pq_elem(qe2->val, qe2->freq);
    qe3->right_child = create_pq_elem(qe1->val, qe1->freq);
    free(qe2);
    free(qe1);
    queue_elem *qe4 = create_pq_elem(0, qe3->freq);
    qe4->left_child =create_pq_elem(qe3->left_child->val, qe3->left_child-freq);
    // Here, I'm supossed to delete+free the child node from the priority queue after it's been added.
   free(qe3);
   delete_nodes(qe4);
    return 0;
}

But as you may have noticed, I end up losing the memory I allocated for qe3 left and right child.

I tried doing it like this, but I just free the child nodes.

#include <stdio.h>
#include <stdlib.h>

typedef struct queue_elem queue_elem;

// The type for element in the priority queue
struct queue_elem {
    //How often a chracter is repeated
    int freq;
    queue_elem *left_child;
    queue_elem *right_child;
    //the character
    unsigned char val;
};

queue_elem *create_pq_elem(int val, int freq) {
    queue_elem *qe = malloc(sizeof(queue_elem));
    qe->freq = freq;
    qe->val = val;
    qe->left_child = NULL;
    qe->right_child = NULL;
    return qe;
}

int main() {
    queue_elem *qe1 = create_pq_elem(97, 5);
    queue_elem *qe2 = create_pq_elem(65, 3);
    queue_elem *qe3 = create_pq_elem(0, qe1->freq + qe2->freq);
    qe3->left_child = qe2;
    qe3->right_child = qe1;
    free(qe2);
    free(qe1);
    printf("%c has a frequency of %d", qe3->left_child->val, qe3->right_child->freq);
    return 0;
}

Anyone got a solution for this? TL;DR, I wanna move the child nodes to a new parent node but can't seem to figure it out.

score 0 · Answer 1 · answered Mar 15 '23 at 12:02

The above code is missing the logic of adding the elements to the priority queue. The elements need to be added to the queue after

int main() {
   queue_elem *qe1 = create_pq_elem(97, 5);
   queue_elem *qe2 = create_pq_elem(65, 3);
   queue_elem *qe3 = create_pq_elem(0, qe1->freq + qe2->freq);
   qe3->left_child = qe2;
   qe3->right_child = qe1;
   // Add qe1, qe2, qe3 to a priority queue

How do a move a dynamically allocated element to another node in C?

1 Answers1