How to get number of edge changes between this tree and the next?

[hyanwong]

Given a tree, is there a way to find the number of edge changes to the next (or previous) tree? I can use the .next() method to get to the next one, but I can't see a way to find the number of changes involved. If there isn't one, I presume that I simply get the entire array by using [len(ed.edges_in) for ed in ts.edge_diffs()]?

[benjeffery]

[len(ed.edges_in) for ed in ts.edge_diffs()] only gets you the new edges, if you wanted number of changes don't you want len(ed.edges_in)+len(ed.edges_out)?

[hyanwong]

Ah, good point. I'm not sure if deleting one edge and then grafting the subtree somewhere else is a single "change" or two changes?

[minesrebollo]

So just to check I got this right, if I want to see how many edges changed from ts.at_index(1000) to ts.at_index(1001) I would do [len(ed.edges_in)+len(ed.edges_out) for ed in ts.edge_diffs()][1001] ?

[jeromekelleher]

There's no way of knowing this in the Tree API (the information isn't stored by the low-level code). The total number of in-and-out edges from the diffs provides a reasonable, easy, definition. If you start reasoning about what the edges do you'll get into all sorts of semantic awkwardnesses (esp. in cases where multiple recombinations happen).

[hyanwong]

Thanks Jerome. When you estimated that the number of edges changed between trees in the UKBB TS was ~40 (I think), was this the in+out numbers then? I assume that means that in an infinite-recombination-site TS, we should expect the number of edge "changes" between trees to be e.g. 6 (rather than 3 as we tend to quote)

[jeromekelleher]

I can't remember tbh, I had a look in the repo but couldn't find where I did it. I guess you could also just do ts.num_edges / ts.num_trees as the number of edges per tree