Question

Please I need someone help. I’m having trouble with my homework.

This homework is very simple. Create a list(1,2,3) and delete the middle number by creating a function delnode. But it must use the function free().

Right now,I have created the list (1,2,3) by using linked list method. I want to delete the number 2 but it doesn’t work. It should be comes out with (1,3) but it comes out with (2,3).

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


struct node{
    int number;
    struct node* next;
};

typedef struct node node;

//prototype function
node* allocateMemory(void);
node* insertNode(node*);
void delnode(node*);

int main(){
    int i,num;
    node* entr = allocateMemory();
    node* p = NULL;

    entr->number = 1;
    entr->next = NULL;

    num = 3;

    for(i=1;i<num;i++){
        if(!p){
            p=insertNode(entr);
        }else{
            p=insertNode(p);
        }

        p->number = i+1;
    }

    while(entr){
        if(entr->number == 2){
            entr->number == NULL;
            break;
        }
        entr=entr->next;
    }

    while(entr){
        printf("%d\n",entr->number);
        entr=entr->next;
    }

    return 0;
}

node* insertNode(node* current){
    node* newNode = allocateMemory();
    current->next = newNode;
    newNode->next = NULL;
    return newNode;
}


void delnode(node* current){
    node* temp = allocateMemory();
    temp = current->next->next;
    free(current->next);
    current = temp;
    free(temp);
    return ;
}

node* allocateMemory(void){
    return (node*)malloc(sizeof(node));
}

Answer 1

Ok, this function delnode is all kinds of messed up. You don’t need to allocate more memory when you’re trying to delete a node. You need to link the previous to the next, and delete the current.

| prev |      | current |      | next |
|------|      |---------|      |------|
   /\              X               /\
   ||              X               ||
   |================================|

It should look more like this.

void delnode(node* current)
{
  node* next = current->next;
  // Now we need to find the node previous to this.
  node* prev = entr; // From the start.
  while(prev->next != current) { prev = prev->next; };
  free(current); // delete the current node.
  prev->next = next; // Link the previous node to the next in the list.
}

If you’re trying to add three nodes, you need to change your loop.

node* current = entr;
for(i=0;i<3;i++)
{
  int newCount = current->number++; // Increment count.
  current = insertNode(current); // Returns the new node.
  current->number = newCount; // Assign new count.
}

If you want to delete the node with number of 2:

node* idx = entr;
while(idx)
{
  if(idx->number == 2) { delnode(idx); break; } // Delete node and break.
  idx = idx->next; // Else, go to next node.
}