C++中基于范围的for循环歧义

Question

These two pieces of code:

A)

void KMeans<N>::assign_pixels_to_clusters()
{
    for (auto cluster : clusters)
    {
        cluster.clear_pixels();
    }

    for (auto pixel : pixels)
    {
        float min_distance = FLT_MAX;
        Cluster<N> c;

        for (auto cluster : clusters)
        {
            float distance = norm(
                pixel.get_value(),
                cluster.get_centroid()
            );

            if (distance < min_distance)
            {
                min_distance = distance;
                c = cluster;
            }
        }

        c.add_pixel(pixel);
    }
}

and B)

template <size_t N>
void KMeans<N>::assign_pixels_to_clusters()
{
    for (auto cluster : clusters)
    {
        cluster.clear_pixels();
    }

    for (auto pixel : pixels)
    {
        float min_distance = FLT_MAX;
        int idx = 0;

        for (int i = 0; i < no_of_clusters; i++)
        {
            float distance = norm(
                pixel.get_value(),
                clusters[i].get_centroid()
            );

            if (distance < min_distance)
            {
                min_distance = distance;
                idx = i;
            }
        }

        clusters[idx].add_pixel(pixel);
    }
}

seem similar to me, yet only B) works the way I want to. In the case of A), the pixels are not assigned to clusters at all. After the piece of code A) is run, the clusters are empty and have no pixels assigned to them. Can you help me understand why, please?

Answer 1

In your A version, you are making a copy of the cluster on this line:

c = cluster;

So when you do

c.add_pixel(pixel);

you are not changing the clusters range.

For the same reason, this loop:

for (auto cluster : clusters)
    {
        cluster.clear_pixels();
    }

doesn't actually clear pixels of any cluster in clusters , since each cluster is a copy. If you actually want to refer to each cluster in clusters , you need to do this:

for (auto & cluster : clusters)
    {
        cluster.clear_pixels();
    }