Work Note 13, DM206, fall 2008

Exercises December 3

1. Consider an algorithm which needs a really large array, initialized to all zeros, in order to perform random memory access to a few entries (which are unknown before execution). In that case, we would prefer not to have to initialize such a large array in advance since we are only planning to use a few entries. Inspired by the timestamp verification procedure from the lecture, consider how to avoid this complete initialization by being able to verify if an entry has been set before it is used. It is possible to obtain this by using three arrays instead of just the one array, and we refer to this as virtual initialization. Hint: one of the arrays is used as a stack.
   This technique makes sense when initialization would ruin the complexity. In practical scenarios, some languages would insist on initializing. In those cases, it can still be usefull when a process is repeated many times. Then the arrays would be permanently allocated and could each time, after the first, be initialized in constant time.
2. In [WT89], could we just as well use union by rank as union by weight (size)?
3. In [WT89], instead of letting find move references of nodes such that they point to the grandparent before the move (if it exists), what would happen if we move them to point to their great grandparents or great great grandparents (if they exist)?
   Let us refer to the parent of a node as its 1-ancestor, its grandparent as its 2-ancestor, etc. What would happen if we change pointers to point to their max(floor(log₁₀₀₀n), 2)-ancestor (if it exists)?
4. In the lecture on [WT89], we aimed at an amortized time complexity of O(log n/loglog n). In the arguments, other complexities showed up, and the final result depended on these other complexities belonging to O(log n/loglog n).
   Show that for 0<c<1, (log n)^c belongs to O(log n/loglog n) (this was the worst-case cost for a union).
   Show that loglog n belongs to O(log n/loglog n) (this was the extra time we had to spend in the bottom of the structure when we abandonned the stacks of pointers in order to get space usage down to O(n)).