Ricardo Aráoz <[EMAIL PROTECTED]> wrote: ... > Alex Martelli wrote: > > <[EMAIL PROTECTED]> wrote: > > ... > >> In my case of have done os.listdir() on two directories. I want to see > >> what files are in directory A that are not in directory B. > > > > So why would you care about WHERE, in the listdir of B, are to be found > > the files that are in A but not B?! You should call .index only if you > > CARE about the position. > > > > def inAnotB(A, B): > > inA = os.listdir(A) > > inBs = set(os.listdir(B)) > > return [f for f in inA if f not in inBs] > > > > is the "one obvious way to do it" (the set(...) is just a simple and > > powerful optimization -- checking membership in a set is roughly O(1), > > while checking membership in a list of N items is O(N)...). > > And what is the order of passing a list into a set? O(N)+?
Roughly O(N), yes (with the usual caveats about hashing costs, &c;-). So, when A has M files and B has N, your total costs are roughly O(M+N) instead of O(M*N) -- a really juicy improvement for large M and N! Alex -- http://mail.python.org/mailman/listinfo/python-list
