问题
I would like to check all Elements of an vector against each other. By checking a condition an element should be removed.
One approach was to erase the elements by nested for loops
for (int a = 0; a < rs.size(); a++)
{
Point A = rs[a];
for (int b = 1; b <= rs.size(); b++)
{
Point B = rs2[b];
float distance = sqrt(pow(B.x - A.x, 2) + pow(B.y - A.y, 2) * 1.0);
if (distance < 10.0)
{
if (distance > 0)
{
rs.erase(rs.begin() + b);
}
}
}
}
but this would effect the vector and his size in runtime.
A second approach was to collect the index of b in an unordered_set but how can I delete the elements with the correspondig index in the original vector?
unordered_set<int> index;
for (int a = 0; a < rs.size(); a++)
{
Point A = rs[a];
for (int b = 0; b < rs.size(); b++)
{
Point B = rs2[b];
float distance = sqrt(pow(B.x - A.x, 2) + pow(B.y - A.y, 2) * 1.0);
if (distance < 10.0)
{
if (distance > 0)
{
index.insert(b);
}
}
}
}
As you might expect, this approach does not work either:
for (const int& idx : index)
{
rs.erase(rs.begin() + idx);
}
Any help?
回答1:
You can delete indices from a vector with a reverse loop in the last suggestion you made. Just make that index
a vector. Let's call it toRemove
.
for (int i = toRemove.size() - 1; i >= 0; i--)
{
rs.erase(rs.begin() + toRemove[i]);
}
Just be careful that this loop has to have a signed index. Otherwise you may underflow. You can make this "nicer" with a reverse iterator. This is just a proof of concept.
回答2:
To be as robust as possible, I would go with std::for_each. In the function performed on each object, roughly:
- perform the calculation necessary
- check for your condition of whether to keep the element or not
- if you want to keep it, add it to a new out-vector, else do not
Then clear the original vector when your are done and swap the in and out vector. Move the objects to (smart!)-pointers for improved efficiency (less copying going on).
For_each combined with creating a new vector should make this very robust against potential size changes and re-allocations that might occur when using std::vector.
回答3:
Sorry guys I changed the distance checking to the point of inserting...
vector<Point> rois;
for (int y = 0; y < result.rows; y++)
{
for (int x = 0; x < result.cols; x++)
{
float qt = result.at<float>(y, x);
if (qt >= threshVal)
{
Point A = Point(x, y);
bool isNear = true;
if (rois.size() > 0)
{
for (Point &P : rois)
{
float distance = sqrt(pow(P.x - A.x, 2) + pow(P.y - A.y, 2) * 1.0);
if (distance < 10.0)
{
isNear = false;
break;
}
}
if (isNear)
{
rois.push_back(A);
}
}
else
{
rois.push_back(A);
}
}
}
}
That is much more easier than the approach before.
But thanks for the answers.
来源:https://stackoverflow.com/questions/54038997/c-nested-for-loop-with-erasing-elements