You seem to be attributing a general principle where I don't think there is one. For example, see the iterators in base/range.jl; there are not two calls to next.
--Tim On Tuesday, July 22, 2014 02:29:12 PM vava...@uwaterloo.ca wrote: > Dear Julia users, > > As I have mentioned in earlier posts, I am working on a 2-3 tree > implementation of a sort-order dict, that is, a dict in which the (key, > value) pairs can be retrieved in the sort-order of the keys. According to > section 2.1.7 of the manual, "for i = I; <body> ; end" translates to: > > state = start(I) > while !done(I, state) > (i,state) = next(I,state) > <body> > end > > The more obvious way to implement the loop would be to put the body BEFORE > the 'next' statement, and at first I thought that maybe the manual has a > typo. But then I checked the file dict.jl, and I found indeed the code: > > done(t::ObjectIdDict, i) = is(next(t,i),()) > > So this means that every loop iteration over an ObjectIdDict requires two > calls to 'next', one as part of the call to 'done', and a second one to > actually advance the loop. > > I also looked at the code for 'done' for Dict in dict.jl, and it appears to > be buggy(??). It does not check whether everything after the current i is > deleted(??) > > For a 2-3 tree, the 'next' operation is logarithmic time, so requiring it > twice per loop iteration is undesirable. > > Can anyone shed light on why the Julia loop construction is implemented > this way, so that two separate calls to 'next' are necessary? I suppose > that it is possible with additional code complexity to cache the result of > the first call to 'next' inside the state, but what exactly is the > rationale for the current design? > > Thanks, > Steve Vavasis
