Issue implementing dynamic array of structures - c

Question

I am having an issue creating a dynamic array of structures. I have seen and tried to implement a few examples on here and other sites, the examples as well as how they allocate memory tend to differ, and I can't seem to get any of them to work for me. Any help would be greatly appreciated.
typedef struct node {
int index;
int xmin, xmax, ymin, ymax;
} partition;
partition* part1 = (partition *)malloc(sizeof(partition) * 50);
I can't even get this right. It gives me the following error:
error: initializer element is not constant
If anyone could explain how something like this should be implemented I would greatly appreciate it.
Also, once I have that part down, how would I add values into the elements of the structure? Would something like the below work?
part1[i]->index = x;

Answer 1

The compiler is complaining because you're doing:
partition* part1 = (partition *)malloc(sizeof(partition) * 50);
Do this instead:
partition* part1;
int
main(void)
{
part1 = (partition *)malloc(sizeof(partition) * 50);
...
}
Your version used an initializer on a global, which in C must be a constant value. By moving the malloc into a function, you are "initializing the value" with your code, but you aren't using an initializer as defined in the language.
Likewise, you could have had a global that was initialized:
int twenty_two = 22;
Here 22 is a constant and thus allowable.
UPDATE: Here's a somewhat lengthy example that will show most of the possible ways:
#define PARTMAX 50
partition static_partlist[PARTMAX];
partition *dynamic_partlist;
int grown_partmax;
partition *grown_partlist;
void
iterate_byindex_static_length(partition *partlist)
{
int idx;
for (idx = 0; idx < PARTMAX; ++idx)
do_something(&partlist[idx]);
}
void
iterate_byptr_static_length(partition *partlist)
{
partition *cur;
partition *end;
// these are all equivalent:
// end = partlist + PARTMAX;
// end = &partlist[PARTMAX];
end = partlist + PARTMAX;
for (cur = partlist; cur < end; ++cur)
do_something(cur);
}
void
iterate_byindex_dynamic_length(partition *partlist,int partmax)
{
int idx;
for (idx = 0; idx < partmax; ++idx)
do_something(&partlist[idx]);
}
void
iterate_byptr_dynamic_length(partition *partlist,int partmax)
{
partition *cur;
partition *end;
// these are all equivalent:
// end = partlist + partmax;
// end = &partlist[partmax];
end = partlist + partmax;
for (cur = partlist; cur < end; ++cur)
do_something(cur);
}
int
main(void)
{
partition *part;
dynamic_partlist = malloc(sizeof(partition) * PARTMAX);
// these are all the same
iterate_byindex_static_length(dynamic_partlist);
iterate_byindex_static_length(dynamic_partlist + 0);
iterate_byindex_static_length(&dynamic_partlist[0]);
// as are these
iterate_byptr_static_length(static_partlist);
iterate_byptr_static_length(static_partlist + 0);
iterate_byptr_static_length(&static_partlist[0]);
// still the same ...
iterate_byindex_dynamic_length(dynamic_partlist,PARTMAX);
iterate_byindex_dynamic_length(dynamic_partlist + 0,PARTMAX);
iterate_byindex_dynamic_length(&dynamic_partlist[0],PARTMAX);
// yet again the same ...
iterate_byptr_dynamic_length(static_partlist,PARTMAX);
iterate_byptr_dynamic_length(static_partlist + 0,PARTMAX);
iterate_byptr_dynamic_length(&static_partlist[0],PARTMAX);
// let's grow an array dynamically and fill it ...
for (idx = 0; idx < 10; ++idx) {
// grow the list -- Note that realloc is smart enough to handle
// the fact that grown_partlist is NULL on the first time through
++grown_partmax;
grown_partlist = realloc(grown_partlist,
grown_partmax * sizeof(partition));
part = &grown_partlist[grown_partmax - 1];
// fill in part with whatever data ...
}
// once again, still the same
iterate_byindex_dynamic_length(grown_partlist,grown_partmax);
iterate_byindex_dynamic_length(grown_partlist + 0,grown_partmax);
iterate_byindex_dynamic_length(&grown_partlist[0],grown_partmax);
// sheesh, do things ever change? :-)
iterate_byptr_dynamic_length(grown_partlist,grown_partmax);
iterate_byptr_dynamic_length(grown_partlist + 0,grown_partmax);
iterate_byptr_dynamic_length(&grown_partlist[0],grown_partmax);
}
There are two basic ways to interate through an array: by index and by pointer. It does not matter how the array was defined (e.g. global/static --> int myary[37]; or via malloc/realloc --> int *myptr = malloc(sizeof(int) * 37);). The "by index" syntax and "by pointer" syntaxes are interchangeable. If you wanted the 12th element, the following are all equivalent:
myary[12]
*(myary + 12)
*(&myary[12])
myptr[12]
*(myptr + 12)
*(&myptr[12])
That's why all of the above will produce the same results.