scanf not running multiple times in loops C

Question

so this is my code for creating and printing a linked list. I wrote this code in atom ide but when i run the code there is a problem with asking input as shown in output below.

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

typedef struct node {
    int data;
    struct node *next;
};

struct node * createl(int n);

void printlist(struct node* head2);

int main() {
    struct node *mainhead;
    int n;

    printf("Enter the number of nodes : " );
    scanf("%d",&n );
    mainhead=createl(n);
    printlist(mainhead);
    getch();
    return 0;
}

void printlist(struct node * head2)
{
    struct node *ptr;
    ptr=head2;
    printf("\nLinked List : \n" );
    while (ptr!=NULL) {
        printf("%d => ",ptr->data);
        ptr=ptr->next;
    }

}


struct node * createl(int n)
{
    struct node *head=NULL,*iso=NULL,*p=NULL;
    int i=0;
    while(i<n){
        setbuf(stdout,NULL);
        iso=(struct node*)malloc(sizeof(struct node));
        printf("\nEnter data in node no. %d :",i+1);
 
        scanf("%d",&(iso->data));
        iso->next=NULL;

        if (head=NULL)
        {
            head=iso;
        } else {
            p=head;
            while (p->next!=NULL) {
                p=p->next;
            }
            p->next=iso;
        }
        i++;
    }
    return head;
}

The expected Output should be:

Enter number of node : 5
Enter data in node no 1 : 1
Enter data in node no 2 : 2
Enter data in node no 3 : 3
Enter data in node no 4 : 4
Enter data in node no 5 : 5

Linked List : 1 => 2 => 3 => 4 => 5

But the actual output it shows is :

Enter the number of nodes : 4
Enter data in node no. 1 : 1
program ends

You do not check the return value from `scanf` so how do you know the input succeeded? — klutt, Aug 10 '20 at 13:39
@kartik30369 Save time and enable all warnings. A good compiler would warn about `if (head=NULL)`. — chux - Reinstate Monica, Aug 10 '20 at 15:13

ryyker · Answer 1 · 2020-08-10T16:36:54.137

The code is setting head to NULL when it should be evaluating it for NULL:

Change this

if (head=NULL)//assignment, leaves head NULL when executed

to

if (head==NULL)//evaluates head, but does not change it.

3 additional suggestions:

When memory created using malloc() is no longer needed, it should be freed.

....
getch();
//free Each node that was created
Casting the return of malloc() in C is not recommended.

//iso=(struct node*)malloc(sizeof(struct node));
iso=malloc(sizeof(struct node));
getch() is POSIX only, and is not portable. A portable alternative is getchar()

score 1 · Answer 2 · edited Aug 10 '20 at 14:02

1

The problem is in your createl function, in the if case. You need to change from if (head = NULL) to if (head == NULL) and it will work.

Another 2 things:

In the function printlist, your last element will print x => and not x.
You need to free your struct node* mainhead at the end of the main.

edited Aug 10 '20 at 14:02

Adrian Mole

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answered Aug 10 '20 at 13:43

gera verbun

285
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And also free all the nodes that `mainhead` points to. – Jonathan Leffler Aug 10 '20 at 14:22

scanf not running multiple times in loops C

2 Answers2