C: Make generic iteration function? - c

Question

I have an struct which defines an array of structs, each of which contain a couple more arrays. These inner arrays define my 'datasets'.
In numerous places, I wish to iterate over all the datasets. To avoid 3 nested loops, I loop over the total number of datasets and have a few if statements to keep track of which array I need to access. It looks like this:
int datasetCount = 0;
int defendedDatasetCount = 0;
int i;
Dataset *ds;
for (i = 0 ; i < totalDatasets ; i++)
{
// If we have passed the number of datasets in this group, move to the next group
if (datasetCount == (dg->nDatasets + dg->nDefDatasets))
{
dg++;
datasetCount = 0;
defendedDatasetCount = 0;
}
// If we have gone through all the (undefended) datasets, read from thte defended datasets
if (datasetCount >= dg->nDatasets)
{
ds = &(dg->defDatasets[defendedDatasetCount]);
defendedDatasetCount++;
}
else
{
ds = &(dg->datasets[datasetCount]);
}
where dg is the pointer to a struct which is simply an array of structs and a size counter.
I find myself repeating this boilerplate code for iterating through the datasets in a few functions which do different things.
I'm struggling to be able to come up with something like this:
while (give_me_next_dataset(dg) == TRUE)
{
...
}
is it possible?

Answer 1

It's possible, but the proposed API isn't very nice. You're going to have to put iteration state somewhere, and you're not leaving any room for that.
I would propose something like:
struct DgIter {
struct Dg *dg;
size_t index;
size_t dataSet;
};
Then you can have functions like:
struct DgIter iter_start(struct Dg *dg)
{
const struct DgIter iter = { dg, 0, 0 };
return iter;
}
void * iter_next_dataset(struct DgIter *iter)
{
// check if there is a current dataset, else return NULL
// update fields in iter as necessary
}