Why does my link list have the wrong data?

Question

When printing out my linked list, what is displayed is not what I assumed it to be. How do I get the right output?

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

struct node *newNode(int data){ 
    struct node *new_node=(struct node *) malloc(sizeof(struct node)); 
    new_node->data=data; 
    new_node->next=NULL; 
    return new_node; 
} 

void push(struct node*** head, int data){ 
    struct node* new_node=newNode(data); 
    new_node->next=(**head); 
    (**head)=new_node; 
} 

void create(struct node **number, char num[]){ 
    int x=0; 
    while(x<strlen(num)){ 
        int d=(int)(num[x]); 
        push(&number, d); 
        x++; 
   } 
}

void printList(struct node *number){ 
    while(number!=NULL){ 
       printf("%d", number->data); 
       number=number->next; 
    } 
    printf("\n"); 
} 

int main (void){ 
    struct node *first; 
    char num1[10]; 
    scanf("%s", num1); 
    create(&first, num1); 
    printList(first); 
    return 0; 
}

Examples

Input          : 1
Expected Output: 1
Actual Output  : 49

Input          : 12345
Expected Output: 12345
Actual Output  : 5352515049

I think it is printing where the value is stored, not the value itself. Correct me on that if that's wrong. Anyways how do I get the expected output I want.

[Don't cast `malloc()`](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) — Iharob Al Asimi, Feb 14 '15 at 23:53
Can it be a coincidence that the ASCII value of `'1'` is `49`? — Weather Vane, Feb 14 '15 at 23:53
In `create()` that (my last comment) is what is happening with the 1-char string with `int d=(int)(num[x]);` — Weather Vane, Feb 14 '15 at 23:58
@iharob Why should I not cast malloc( )? Every example I have found online on how to create a linked list has malloc( ). Why is strlen inefficient? And the part where i call create( ) is posted, just scroll down on the code. — dsb, Feb 15 '15 at 00:02
Futhermore, your second example input `"12345"` output `"5352515049"` - is the 2-digit ASCII values of the input string in reverse order. — Weather Vane, Feb 15 '15 at 00:03
@WeatherVane, yeah I see that. I feel like that will be as easy fix. I'm just really stuck on getting the numbers I entered not the ASCII values I'm apparently getting. — dsb, Feb 15 '15 at 00:10

Iharob Al Asimi · Accepted Answer · 2015-02-15T00:21:51.677

The problem is that you are reading ascii values, and then trying to print integers, and since you need to store integers and not the ascii values all you need is a simple mathematical operation, i.e. subtract the ascii value of the digit '0', so to convert the ascii value of a digit to it's integer value all you need is

integer = ascii - '0';

here I fixed your code, because you were appending the value to the head of the list instead of it's tail

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

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

struct node *newNode(int data)
{
    struct node *new_node;

    new_node = malloc(sizeof(struct node));
    if (new_node == NULL) /* always check that malloc succeeded */
        return NULL;
    new_node->data = data;
    new_node->next = NULL;

    return new_node;
}

struct node *push(struct node *tail, int data)
{
    struct node *new_node;

    new_node = newNode(data);
    if (tail != NULL)
        return tail->next = new_node;
    return new_node;
}

struct node *create(char *numbers)
{
    size_t       i;
    struct node *head;
    struct node *tail;

    i    = 0;
    head = NULL;
    tail = NULL;
    /* since strings are 'nul' terminated, you just need to loop,
     * until you find the 'nul' byte, strlen() expects that byte
     * anyway.
     */
    while (numbers[i] != '\0')
    {
        tail = push(tail, numbers[i++] - '0');
        if (head == NULL)
            head = tail;
    }

    return head;
}

void printList(struct node *number)
{
    while (number != NULL)
    {
       printf("%d", number->data);
       number = number->next;
    }
    printf("\n");
}

void freeList(struct node *number)
{
    while (number != NULL)
    {
        struct node *last;

        last   = number;
        number = number->next;

        free(last);
    }
}

int main(void)
{
    struct node *first;
    char         numbers[10];

    /* '%9s' prevents buffer overflow */
    if (scanf("%9s", numbers) != 1)
        return -1;
    first = create(numbers);

    printList(first);
    freeList(first);

    return 0;
}

I am using a char array because i need to do arbitrary precision integer arithmetic. I'm not using int for cases that exceeds ints max value. I used 10 as just an arbitrary length. — dsb, Feb 15 '15 at 00:08
Thank you so much for the help. It took me awhile but I understand it now. Thanks again. — dsb, Feb 15 '15 at 01:32

Weather Vane · Answer 2 · 2015-02-15T00:15:50.043

You have a linked list which ends up in reverse order, and this is why your second example comes out in reverse order. If you print the linked list like this:

printf("%d ", number->data);

with a space, it would be clearer what is happening, output is ASCII values

53 52 51 50 49

But you are also confusing character values with numeric values. If you correct this,

printf("%c ", number->data);

you will get your original input characters in reverse order.

5 4 3 2 1

Why does my link list have the wrong data?

2 Answers2