Why am I getting a zero at the end and how can I do the same thing without reversing the array?

Question

I am new to C, just try to understand a very tiny part of a problem I have a simple program -1 represents the end of the array :

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

typedef struct List list_t;
struct List {
    void * head;
    list_t * tail;
};

list_t * makelist(void * x, list_t * xs){
    list_t * ans = (list_t *) malloc(sizeof(list_t));
    ans->head =x;
    ans->tail = xs;
    return ans;
}

list_t * makeListFromIntArray(int list[]){
    list_t * ans = (list_t *) malloc(sizeof(list_t));
    int i = 0;
    while(list[i]!=-1){
        ans = makelist((void *)(intptr_t)list[i],ans);
        i++;
    }
    return ans;
}

void printList(list_t * xs){
    while(xs != NULL){
        printf("%lu\n",((intptr_t)xs->head));
        xs = xs->tail;
    }
}

int main() {
    int s[] = {1,2,3,4,5,-1};
    list_t * xs = makeListFromIntArray(s);
    printList(xs);
    return 0;
}

The main problem is when printList executes I am getting a zero at the end.

5
4
3
2
1
0

Process returned 0 (0x0)   execution time : 0.001 s
Press ENTER to continue.

This is my main problem and the minor problem is the array is reversed which is understandable. I can overcome this by reversing before feeding into makelistfromintarray. However I am open more elegant solutions.

Your abuse of pointers scares me. I understand that type punning is a thing, but that's what unions are for. — Wug, Apr 11 '13 at 18:02
Also remove all your type-casts until your compiler comlains. — Kninnug, Apr 11 '13 at 18:05
It's all part of the learning experience, but this is some crazy bad code. You have `malloc` — Matt Phillips, Apr 11 '13 at 18:06
Do not assume that an `int` can be stored in a `void*`. Although most of the time that's true, the behavior is undefined. — Sergey Kalinichenko, Apr 11 '13 at 18:06
It is a part of lecture about programming languages forcing C to use closures. Ugly I guess... Thanks for your suggestions. — cgon, Apr 11 '13 at 18:12

score 3 · Accepted Answer · edited May 23 '17 at 12:18

3

The issue is here...

 list_t * ans = (list_t *) malloc(sizeof(list_t));

You are creating an empty node at the head of the list in MakeListFromArray. That line should read

 list_t * ans = NULL;

PS: You should never cast malloc() in C!

edited May 23 '17 at 12:18

Community

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answered Apr 11 '13 at 18:03

K Scott Piel

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Why am I getting a zero at the end and how can I do the same thing without reversing the array?

1 Answers1