C++ nested for loop with erasing elements

Question

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?

How are vectors declared, how does the data look like, what is the condition for removal? That info will help to give better answers. — jrok
– jrok, Commented Jan 4, 2019 at 12:37
The simplest way seems to create a new vector with the non erased elements — Damien
– Damien, Commented Jan 4, 2019 at 12:42

The Quantum Physicist · Accepted Answer · 2019-01-04 12:37:25Z

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.

d-karl · Accepted Answer · 2019-01-04 13:01:44Z

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.

manuel · Accepted Answer · 2019-01-04 14:06:06Z

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.

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