infinite loop on linked list

The while statements are kept on running even when I set the statement != 0.
The displayLinkedList uses iteration which works normally,
the displayLinkedListRe uses recursion, which is running as an infinite loop. Also, the function sizeOfLinkedList which is supposed to return a linked list is also running as an infinite loop for some reason.

#include <iostream>

using namespace std;

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

void createLinkedList(int a[], int n) {
    int i;
    struct node *last, *t;
    first = (struct node *)malloc(sizeof(struct node));
    first->data = a[0];
    first->p = NULL;
    cout << first->p << endl;
    last = first;

    for (i = 1; i <= n; i++) {
        t = (struct node *)malloc(sizeof(struct node));
        t->data = a[i];
        t->p = NULL;
        last->p = t;
        last = t;
    }
}

void displayLinkedList(struct node *p) {
    while (p->p != 0) {
        cout << "the element is " << p->data << endl;
        p = p->p;
    }
}

int sizeOfLinkedList(struct node *pointer) {
    int n{ 0 };
    while (pointer->p != NULL) {
        n++;
    }
    return n;
}

int displayLinkedListRe(struct node *pointer) {
    while (pointer->p != NULL) {
        cout << pointer->data << endl;
        displayLinkedListRe(pointer->p);
    }
    return 0;
}

int main() {
    int array[] = { 1, 2, 3, 4, 5, 6, 7 };
    createLinkedList(array, 7);
    displayLinkedList(first);
    displayLinkedListRe(first);
    cout << "the size of linked list is " << sizeOfLinkedList(first);
}

>Solution :

There are multiple problems:

• The loop in createLinkedList runs one time too much: you should write

  for (i = 1; i < n; i++)
  

• The functions displayLinkedList() stops too soon. You should write:

  void displayLinkedList(struct node *p) {
      while (p != NULL) {
          cout << "the element is " << p->data << endl;
          p = p->p;
      }
  }
  

• in function sizeOfLinkedList(), you do not update p in the loop body, so the loop runs forever.

• displayLinkedListRe recurses inside the loop and you do not update p either! either use a loop or recursion but not both.

• Furthermore, allocating objects with malloc() in C++ is not recommended, especially without including <stdlib.h>.

Here is a modified version:

#include <iostream>

using namespace std;

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

void createLinkedList(const int a[], int n) {
    if (n <= 0)
        return;

    node *first = new node;
    first->data = a[0];
    first->p = NULL;
    node *last = first;

    for (int i = 1; i < n; i++) {
        node *t = new node;
        t->data = a[i];
        t->p = NULL;
        last->p = t;
        last = t;
    }
}

void displayLinkedList(const struct node *p) {
    while (p != NULL) {
        cout << "the element is " << p->data << endl;
        p = p->p;
    }
}

int sizeOfLinkedList(const struct node *p) {
    int n = 0;
    while (p != NULL) {
        n++;
        p = p->p;
    }
    return n;
}

void displayLinkedListRe(const struct node *p) {
    if (p != NULL) {
        cout << p->data << endl;
        displayLinkedListRe(p->p);
    }
}

int main() {
    int array[] = { 1, 2, 3, 4, 5, 6, 7 };
    createLinkedList(array, 7);
    displayLinkedList(first);
    displayLinkedListRe(first);
    cout << "the size of linked list is " << sizeOfLinkedList(first);
}