Generally iterating over a seq and deleting elements does not work. Nim gives a
warning when length of seq changes while we iterate over it with a for loop.
There exist situations where we have to process elements of seq and conditional
have to delete them from the list. In this case a plain filter template does
not work. C++ example:
void
QuickhullDisk::remove_pre_and_post_apex_disks_and_contained_disks_in_one_of_the_two_from_candidate_apex_disks_if_exist(
CircularDisk2D* preApexDisk_dp, CircularDisk2D* postApexDisk_dq,
list<pair<Point2D, CircularDisk2D*>>& candidateTriangleApexNDiskPairs,
list<CircularDisk2D*>& containedDisksInPreOrPostApexDisk) const
{
list<pair<Point2D, CircularDisk2D*>>::const_iterator
it_TriangleApexNDisk = candidateTriangleApexNDiskPairs.begin();
while (it_TriangleApexNDisk != candidateTriangleApexNDiskPairs.end())
{
CircularDisk2D* candidateApexDisk = (*it_TriangleApexNDisk).second;
if (candidateApexDisk == preApexDisk_dp || candidateApexDisk ==
postApexDisk_dq)
{
it_TriangleApexNDisk =
candidateTriangleApexNDiskPairs.erase(it_TriangleApexNDisk);
continue;
}
if (preApexDisk_dp->contain(*candidateApexDisk, _resNeg6) ||
postApexDisk_dq->contain(*candidateApexDisk, _resNeg6))
{
containedDisksInPreOrPostApexDisk.push_back(candidateApexDisk);
it_TriangleApexNDisk =
candidateTriangleApexNDiskPairs.erase(it_TriangleApexNDisk);
}
else
{
++it_TriangleApexNDisk;
}
}
}
Run
The C++ iterator works because erase() returns the next iterator. For this
example there is not only deletion of elements involved, as the line
"containedDisksInPreOrPostApexDisk.push_back(candidateApexDisk);" does an
additional action. So I assume that a filter template may not work. How do we
such operations in Nim best?
