Re: remove_if with a mask

From:

eLVa <elvadrias@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 8 Jun 2010 08:11:54 -0700 (PDT)

Message-ID:

<e63ffaea-3e1a-4fb9-8885-9f147f4364a2@c33g2000yqm.googlegroups.com>

On 8 juin, 11:00, "Leigh Johnston" <le...@i42.co.uk> wrote:

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

news:hulljv$mc1$1@news.eternal-september.org...

On 6/8/2010 10:16 AM, eLVa wrote:

On 8 juin, 10:09, "Leigh Johnston"<le...@i42.co.uk> wrote:

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

news:hulh6q$46f$1@news.eternal-september.org...

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=

a mask. You have to remove every element from the container for whi=

the mask is true.

basically, two iterators (one over the container and one over the
mask) iterates through their respective containers, removing elemen=

from the first when the second is true. I tried a few solutions, us=

ing

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 homew=

ork,

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.

Sure, I trust your word. Not a homework. Absolutely. And if yo=

u *were*

a student, you would definitely admit that it's homework, yes? That's
settled, then.

I thought that it was not wise to keep an iterator into a functor used
in remove_if.

I am not sure how to respond when somebody says that something "isn't
wise". You must be wiser than I am... It's probably wise for you =

not to

use my advice then. :-)

I wanted a direction, that's all.

OK. Did you get it? If you did, happy for you. If you didn't, =

sorry.

Better luck next time.

So that's what I got :

template<class Z> struct maskChecker {
public:
maskChecker(const vector<bool> &mask) : it(mask.=

begin()) {}

         bool operator()(Z&) const { return *it ++; }
     private:
         mutable vector<bool>::const_iterator it;

Instead of keeping a mutable iterator by value, I'd probably keep a
reference to an iterator. But, yes, that's the general idea.

Apologies Victor I didn't see your use of reference in your original repl=

but to clarify you *must* use a reference as an implementation is free to
make copies of a predicate. The complexity requirements imply that thi=

should work on most (if not all) implementations, I still feel nervous ab=

out

it though as some algorithms are free to parallelize operations on a
sequence for example. Storing state in a predicate should be approache=

with caution.

/Leigh

Ok, I understand about the reference to the iterator, that's what I'll
use.
As you said, an implementation is free to parallelise, so there's
still the possibility that it could fail, right ?

Is there a safer way to do this ?