sparse_hash_map<int, double> very slow

[ben...@gmail.com]

Hi all,

I am trying to use the sparse_hash_map with int as keys and double as values, but it is really slow.
It takes about 34 secs to store an image (only the non-zero values) vs 2 secs when using unordered_map.
I'm using the following code:

  sparse_hash_map<int, double> himg;
  himg.set_deleted_key(-1);

    for (int idx = 0; idx < numvoxels; ++idx) {
        val=image->Get(idx);
        if(val){
            himg[idx]=val;
        }
    }

What could be causing this performance?

Thank you!
Antonis

[Geoff Pike]

sparse_hash_map is slower than most hash-based maps, especially for
inserting. You may be better off using a different data structure.
When the keys are clumpy (as they probably are in your case, since I
bet image->Get(idx) is correlated with image->Get(idx+1)), a trie
might be worthwhile.

Geoff