C generic array implementation

Question

I am trying to implement a generic array list in C. However, when the data type is any type other than int, the list wont contain the correct data. For example, like 123.1234 as a double, when the double is passed into the list, it will become 000.0000. One when the data type is int, it will have correct value. I don't know which part of the code is wrong, can anyone give me a hint? Thanks

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "genericADT.h"

struct list_type {
   void *data;
   int elementSize;
   int size;
   int capacity;
};

ListType create(int elementSize) {
   ListType listptr = malloc(sizeof(struct list_type));

   if (listptr != NULL) {
      listptr->size = 0;
      listptr->capacity = 10;
      listptr->elementSize = elementSize;
      listptr->data = malloc(10 * (listptr->elementSize));
      if (listptr->data == NULL) {
         free(listptr);
         listptr = NULL;
      }
   }
   return listptr;
}

void push(ListType listptr, void *item) {
   if (listptr->size >= listptr->capacity) {
      void *temp = realloc(listptr->data, listptr->elementSize * (listptr->capacity + 100));
      if (temp != NULL) {
         listptr->capacity += 100;
         listptr->data = temp;

         memcpy(listptr->data + (listptr->size) * (listptr->elementSize), item, sizeof(listptr->elementSize));
         listptr->size++;
      }
   } else {
      memcpy(listptr->data + (listptr->size) * (listptr->elementSize), item, sizeof(listptr->elementSize));
      listptr->size++;
   }
}

void *get(ListType listptr, int index) {
   return listptr->data + index * (listptr->elementSize);
}

int size_is(ListType listptr) {
   return listptr->size;
}

malloc() returns a pointer, why are you assigning a pointer to listptr? — babon
– babon, Commented May 28, 2016 at 8:22
suggest have to array element contains a void* to the actual data. Then the program that is using that list it can interpret the data in what ever manner fits the application. — user3629249
– user3629249, Commented May 28, 2016 at 20:10

Serge Ballesta · Accepted Answer · 2016-05-28 08:54:06Z

6

There are minor problems in your code, but it correctly processes double values.

First as noticed by @n.m., you really want to use listptr->elementSize instead of sizeof(listptr->elementSize)

Next, as you want to do pointer arithmetic, you should declare data as char * and not void *

Last as a elementary optimization, you pull the actual insertion code after the test for capacity instead of duplicating it in both branches.

But after those fixes, this main correctly stores and extract double:

int main() {
    ListType ls = create(sizeof(double));
    double f1=1.5, f2=3.6;
    push(ls, &f1);
    push(ls, &f2);
    printf("Got %f %f\n", *((double *) get(ls, 0)), *((double *) get(ls, 1)));
    return 0;
}

it prints as expected:

Got 1.500000 3.600000

edited May 28, 2016 at 8:54

answered May 28, 2016 at 8:30

Serge Ballesta

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2 Comments

Ming Over a year ago

Thanks for correcting me. I remove the sizeof(), it now works for double, but string type (char *) still doesn't work, it keeps printing weird symbol

Serge Ballesta Over a year ago

Beware, if you only store the pointer to the string, you must ensure the array is not destroyed before you use it (this is called dangling pointer, just google for that...)

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