Re: remove_if with a mask

eLVa <>
Tue, 8 Jun 2010 07:16:47 -0700 (PDT)
On 8 juin, 10:09, "Leigh Johnston" <> wrote:

"Victor Bazarov" <v.baza...@comcast.invalid> wrote in message


On 6/8/2010 9:13 AM, eLVa wrote:

I have a simple problem : you are given a container, let's say a
vector of some objects, and a vector of bool which is to be treated as
a mask. You have to remove every element from the container for which
the mask is true.

basically, two iterators (one over the container and one over the
mask) iterates through their respective containers, removing elements
from the first when the second is true. I tried a few solutions, using
remove_if but I was not succesful !

Post what you have, post what didn't work, post your explanation on how
you expected it to work. Otherwise this looks too much like homework=

, and

we don't do homework.

Ok ! Wait a minute there, it's not an homework, it's a question I'm
trying to resolve.
I thought that it was not wise to keep an iterator into a functor used
in remove_if. I wanted a direction, that's all.

So that's what I got :

template <class Z> struct maskChecker {
        maskChecker(const vector<bool> &mask) : it(mask.begin()) {}
        bool operator()(Z &) const { return *it ++; }
        mutable vector<bool>::const_iterator it;

template <class T> template<class Z>
void Selector<T>::compact(vector<Z> &data, const vector<bool> &mask) {
    if (data.size() != mask.size()) throw UnmatchedSize(data.size(),
    typename vector<Z>::iterator end = remove_if(data.begin(),
data.end(), maskChecker<Z>(mask));
    data.erase(end, data.end());

That's kinda working, what's your opinion ?

Does anybody have a solution that only uses stl, (i.e I don't want to
use boost) !

The solution involves a predicate that should keep the second iterator
(and increment it every time the operator() is called). The predicat=


can probably be const, but has to keep non-const reference to the mask
iterator. Dereference the mask iterator to check and let the op() of=


predicate essentially return that dereferenced bit.

I do not respond to top-posted replies, please don't ask

Fail. Your solution will not work on all implementations, it will fail=


VC++ for example as VC++ first does a find_if to determine if it should
continue which results in more than N operator()'s being called.

I'm not sure I understand what you're saying, but I think it is what I
read somewhere about having an iterator inside a functor as not a very
good idea ... Is that it ?


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