[srcreigh]

It’s more like 2NM where M is loading the data from disk/memory. One pass is 2N+M.

Why go to the store and back twice to buy two things instead of buying two things in one trip? ;p

[jiggawatts]

I think you meant 2N+2M vs 2N+M, but yes, that’s the point: not reading twice is cheaper because unlike in traditional big-O analysis compute is cheap but data access is very expensive.

[srcreigh]

Yep, thx.