Subject: Re: [boost] [mpl]iter_fold_if Forward Backward rationale?
From: Larry Evans (cppljevans_at_[hidden])
Date: 2009-04-04 03:05:45
On 04/03/09 21:31, David Abrahams wrote:
> on Thu Apr 02 2009, Larry Evans <cppljevans-AT-suddenlink.net> wrote:
>> One reason I'm interested is that I can't figure out how reverse_fold
>> would work on a sequence, S, such that begin<S>::type with just a
>> forward iterator. I would have thought that you'd need a
>> bidirectional iterator in order to traverse backwards; yet:
>> shows that it works on lists.
> More later, but remember there's recursion involved. You apply the
> forward function "on the way in" and the reverse one "on the way out" of
> the recursion.
I thought it would be interesting to compare the mpl folds with the
From pp. 116-117 of:
the following is a trace of the execution of foldl
and foldr where the argument order is modified to reflect that of mpl
fold argument order. IOW:
[x1,x2,x3] represents an mpl sequence
z represents the initial state, e.g. list<>.
F represents the binary operation, e.g.push_front.
Now, the foldl trace:
which looks like mpl::fold since, when F == push_front, the resulting
sequence is reversed, which is what happens with mpl::fold.
Then, the foldr trace:
which looks like mpl::fold_reverse since, when F == push_front, the
resulting sequence is the same is the start sequence, which is what
happens with mpl::fold_reverse. OOPS, push_front takes the sequence
as 1st arg, but F(x3,z) has the sequence (e.g. list<> ) as the 2nd
Now, is the F in the foldr applied "on the way out" and the F in the
foldl applied " on the way in"?
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