Question

I am slowly learning how to program generic functions in C and get into trouble now and so often. I am making a program that makes a union of two arrays, in this implementation two int arrays. The first problem, which also leads to the second one, is that the compareints (function) does not access one of the passed arguments (void *): I can’t figure out why? I been staring at the screen for to long time now…

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

//makes union of two generic arrays and eliminates duplicates if there are some...
void **
unite(int (*comp)(void *f, void *s), void **first, void **second, int f_size, int s_size,     int bytes, int *x)
{
     int i;
     void **arr=malloc(bytes*(f_size+s_size));
     for(i=0; i<f_size+s_size; i++)
     {
         /* first bigger */
         if(((*comp)(*first, *second))>0)
         {
            *(arr+i)=*(first++);
         }
         /* second bigger */
         else if(((*comp)(*first, *second))<0)
         {
            *(arr+i)=*((second++));
         }
         /* equal => just one copy */
         else
         {
            *(arr+i)=*(first++);
            second++;
         }
     }
     *x=i;
     return arr;
 }

 int
 compareints(void *first, void *second)
 {
     if(*((int *)first)>*((int *)second)) //can't access the memoryloc in second...
         return 1;
     else if(*((int *)first)<*((int *)second))
         return -1;
     else
         return 0;
 }

 int main(int argc, const char * argv[])
 {

     int arr[10]={1, 2, 4, 12, 22, 29, 33, 77, 98};
     int arr2[5]={3, 5, 7, 8, 9};
     void **first=malloc(sizeof(int *)*10);
     void **second=malloc(sizeof(int *)*5);
     //make pointers to static arrays in dynamic arrays
     int f_ind, s_ind;
     for(f_ind=0; f_ind<10; f_ind++)
         first[f_ind]=&arr[f_ind];
     for(s_ind=0; s_ind<5; s_ind++)
         second[s_ind]=&arr2[s_ind];
     int i;
     //make union of the two arrays and print out the result
     void **ret=unite(&compareints, first, second, 10, 5, sizeof(int), &i);
     for(int k=0; k<i; k++)
         printf("%d  ", *((int *)ret[k]));
     return 0;
 }

Answer 1

I tried an approach thanks to @WhozCraigs post about the index going out of bounds. So I made some small mods and now the program does what it intends to.

void **
unite(int (*comp)(void *f, void *s), void **first, void **second, int f_size, int s_size, int bytes, int *x)
{
    int i;
    int f_ind=0, s_ind=0;
    void **arr=malloc(bytes*(f_size+s_size));
    for(i=0; i<f_size+s_size; i++)
    {
        /* first bigger */
        if(((*comp)(*first, *second))>0)
        {
            s_ind++;
            if(s_ind<s_size)
                *(arr+i)=*(second++);
            else
            {
                f_ind++;
                if(f_ind<f_size)
                    *(arr+i)=*(first++);
                else
                    break;
            }
        }
         /* second bigger */
        else if(((*comp)(*first, *second))<0)
        {
            f_ind++;
            if(f_ind<f_size)
                *(arr+i)=*(first++);
            else
            {
                s_ind++;
                if(s_ind<s_size)
                    *(arr+i)=*(second++);
                else
                    break;
            }
        }
        /* equal => just one copy */
        else
        {
            f_ind++;
            s_ind++;
            if(f_ind<f_size && s_ind==s_size)
            {
                *(arr+i)=*(first++);
            }
            else if(f_ind==f_size && s_ind<s_size)
            {
                *(arr+i)=*(second++);
            }
            else
            {
                *(arr+i)=*(first++);
                second++;
            }  
        }
    }
    *x=i;
    return arr;
}