Below is the code please help I am new to c

#include <iostream>
using namespace std;

struct node
{
    int data;
    struct node *next;
} *first = NULL;

void Create(int A[], int n)
{
    int i;
    struct node *t, *last;
    first = new struct node *(sizeof(struct node));
    first->data = A[0];
    first->next = NULL;
    last = first;

    for (i = 1; i < n; i  )
    {
        t = new struct node *(sizeof(struct node));
        t->data = A[i];
        t->next = NULL;
        last->next = t;
        last = t;
    }
}

void Display(int *p)
{
    while (p != NULL)
    {
        cout << p.data;
        p = p.next;
    }
}

int main()
{
    int A[] = {3, 5, 7, 9, 15}
    // struct node n1;
    Create(A, 5);
    Display(first);
    return 0;
}

The errors are

1. invalid conversion from 'unsigned int' to 'node*' [-fpermissive] first = new struct node* (sizeof(struct node));
2. 14:50: error: cannot convert 'node**' to 'node*' in assignment first = new struct node* (sizeof(struct node));

CodePudding user response：

First off, this looks like you're trying to convert C code to C .

I'm assuming the original C code looks something like this:

first = (struct node*) malloc(sizeof(struct node));

C's malloc accepts a size in bytes. C 's new will know the size of the node struct automatically:

first = new node();

To explain the error message: new struct node *(sizeof(struct node)) is trying to allocate a pointer to node node* using an unsigned int sizeof(struct node).

Also keep in mind that many things considered normal in C are weird in C . Here are some suggestions:

• Use nullptr instead of NULL. NULL can easily turn into an integer 0 accidentally.
• You don't need struct when declaring variables, struct node *t, *last; can be changed to node *t, *last;
• You can initialize the whole struct all at the same time, instead of setting the members afterwards: first = new node{A[0], nullptr};
• Arrays are NOT null-terminated, so the loop inside Display will glitch out. A better way to do this is to use std::array, std::vector, or just pass a count into the function. Also, this function tries to use ints as nodes? Assuming it isn't complete yet.
• Read about 'smart pointers' as it will lead to safer code that doesn't leak memory.

CodePudding user response：

The problem is that you are using new incorrectly. You are asking it to create a new node* pointer initialized with the return value of sizeof, which is wrong. You should instead be asking it to create a new node object.

Also, your Create() can be simplified. And your Display() is coded wrong. And you are leaking the list.

Try this instead:

#include <iostream>
using namespace std;

struct node
{
    int data;
    node *next = nullptr;
};

void Create(node* &first, int A[], int n)
{
    node **curr = &first;
    for (int i = 0; i < n;   i)
    {
        node *t = new node;
        t->data = A[i];
        *curr = t;
        curr = &(t->next);
    }
}

void Destroy(node *p)
{
    while (p)
    {
        node *n = p->next;
        delete p;
        p = n;
    }
}

void Display(node *p)
{
    while (p)
    {
        cout << p->data << ' ';
        p = p->next;
    }
}

int main()
{
    int A[] = {3, 5, 7, 9, 15}
    node *n1;
    Create(n1, A, 5);
    Display(n1);
    Destroy(n1);
    return 0;
}

That being said, you really should use std::list instead:

#include <iostream>
#include <list>
using namespace std;

void Display(list<int> &l)
{
    for (int val : l)
    {
        cout << val << ' ';
    }
}

int main()
{
    int A[] = {3, 5, 7, 9, 15};
    list<int> n1(begin(A), end(A));
    Display(n1);
    return 0;
}