[racket] Looking for feedback on code style

On Sep 13, 2010, at 3:07 PM, David Van Horn wrote:

>> In fact, ANY method to do this (even with a vector) takes Omega(n log n)
>> time. It needs to be able to produce any of n! different answers, so it
>> needs to make at least log(n!) decisions, which is Theta(n log n).
>> Otherwise there wouldn't be enough leaves on the decision tree.
> 
> Doesn't it make n decisions?  1 decision for each element (the decision being, where does the element go in the shuffled list)?

Each of these is an n-way decision, which takes log(n) bits to specify.  Any algorithm that solves this problem MUST generate at least n log(n) random bits of information, and therefore MUST take at least n log(n) time (ignoring parallelism).

Ryan Culpeper points out:
> I think the discrepancy comes from the fact that each of those decisions takes O(log n) bits, but we customarily pretend that "indexes" are O(1).

Exactly.  As long as your n fits into a machine word, and you have at least n cells of truly-random-access memory, you can treat array indexing and random-number-generation as taking O(1) time.

> Is this right? Does complexity analysis have a notation for distinguishing "true" complexity from "for up to k bits" complexity?

Not exactly a "notation" AFAIK, but people do routinely talk about whether the computational model allows bit operations in O(1) time, or integer operations in O(1) time, or whatever.


Stephen Bloch
sbloch at adelphi.edu