Why does my linked-list program print out a loop of that "first number" I entered, infinitely?

Question

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

struct Node{
    int data;
    struct Node* next; 
};

struct Node* head;

void Insert(int x){ 
    struct Node* temp = (struct Node*)malloc(sizeof(struct Node));
    temp->data  = x; 
    temp->next = head;
    head = temp;
    if(head != NULL)
            temp->next = head;
    head = temp;
}

void Print(){
    struct Node* temp = head; 
    printf("List is: ");
    while(temp != NULL){
        printf("%d", temp->data);
        temp = temp->next;
    }
    printf("\n");
}

int main(){
    head = NULL; //empty list
    printf("How many numbers?\n");
    int n, i;
    scanf("%d", &n);
    for(int i=0; i < n; i++){
        printf("Enter the number\n");
        scanf("%d", &x);
        Insert(x); 
        Print();
    }

Example of the wrong output that I got from my compiler:

How many numbers?

My input: 2

Enter the number:

My input: 1

List is: 1111111111111111111111111111111111111111111111111111111111111111111111111111111111111 and it goes on & on & on

I didn't even have the chance to enter the 2nd number, since I put 2 numbers as the amount of numbers.

yeah that worked, thanks!! However the list currently only prints out the latest number I entered instead of the full linked list... How can I make it print the whole linked-list? Like instead of it printing out just one number... — hamburgerwhoselflearns, Jan 14 '22 at 02:15
`void Insert(int x) { struct Node* temp = malloc(sizeof(struct Node)); temp->data = x; temp->next = head; head = temp; }` — 001, Jan 14 '22 at 02:21
But doesn't malloc return a void pointer hence we need to typecast it as we need a pointer to node? — hamburgerwhoselflearns, Jan 14 '22 at 02:40
There is a difference of opinion on that: [Do I cast the result of malloc?](https://stackoverflow.com/q/605845) — 001, Jan 14 '22 at 02:41
I've just read it, I see. I removed the typecasting from my code but it still just prints out one number which is the latest number I entered... — hamburgerwhoselflearns, Jan 14 '22 at 03:02

mohammed yaqub · Answer 1 · 2022-01-14T15:48:47.520

I refactored the code and it's worked.

The infinite loop was due to the first node pointing to the head and the head pointing to the first node.

temp->next = head;
head = temp;

So when you call Print() the while(temp != NULL) condition never fails.

#include <stdio.h>
#include<stdlib.h>
struct Node{
    int data;
    struct Node* next; 
};

struct Node* head;
void push_front(int x) { 
    struct Node *current = malloc( sizeof( struct Node ) ); 
    if (current==NULL) { printf("No available space on heap\n"); exit(1); }
    current->data = x; 
    current->next = head; 
    head = current; 
} 
void append(int x){ 
    struct Node* temp = malloc(sizeof(struct Node));
    if (temp==NULL) { printf("No available space on heap\n"); exit(1); }
    temp->data  = x; 
    temp->next = NULL;
     //check if we have to insert at begining(initial node) or empty list
    if (head==NULL){
        head=temp;
        return ;
    }
    //traverse till the last node to insert new node
    //should not modify the head
    struct Node *t=head;
    while(t->next != NULL) t=t->next;
    t->next= temp;
}

void Print(){
    struct Node* temp = head; 
    printf("List is: ");
    while(temp != NULL){
        printf("%d->", temp->data);
        temp = temp->next;
    }
    printf("null\n");
}
void destroy_mem(struct Node *head)
{//we need destroy the used memory on heap for further use
    struct Node *curr = NULL;
    while ((curr = head) != NULL)
    {                      // set curr to head, stop if list empty.
        head = head->next; // moving head to next element.
        free(curr);        // delete saved pointer.
    }
    head = NULL;
}
int main(){
    head = NULL; //empty list
    printf("How many numbers?\n");
    int n, i,x;
    scanf("%d", &n);
    for(int i=0; i < n; i++){
        printf("Enter the number\n");
        scanf("%d", &x);
        push_front(x); 
       
    }
  Print();  
  destroy_mem(head);
}

Why does my linked-list program print out a loop of that "first number" I entered, infinitely?

1 Answers1