Re: Advancing Through std::vector

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 19 Sep 2013 08:08:30 -0700 (PDT)

Message-ID:

<7aa6df51-d802-4664-a94b-8f5c4f329b37@googlegroups.com>

On Thursday, 12 September 2013 10:07:09 UTC+1, F.Zwarts wrote:

"Jorgen Grahn" wrote in message
news:slrnl31m36.2m9.grahn+nntp@frailea.sa.invalid...

On Wed, 2013-09-11, Mike Copeland wrote:

   I have a std::vector of some size (I know I can get the size with the
.size() function, but that doesn't help my question here). I have logic
which processes "chunks" of this data, 20 at a time. Let's say that I
have 32 objects in the container, and my first "page" of data is 1-20.
   The next "chunk" can't be 20, and I obviously have to protect against
trying to access object(s) that don't exist. Normal incrementing of the
iterator (e.g. advance) or direct indexing don't seem to offer ways to
avoid accessing beyond the data set. That is, I can't find any STL
functions that give me ways to avoid problems here.
   Yes, I could do some convoluted math that works with .size(), but if
I'm using an iterator and advance, the work seems messy and ugly. 8<{{
   So I asking here if there is some way to use/convert the iterator
position and apply it to see if it's "gone beyond" the container's data
set. Perhaps there's a function that returns an indicator, or maybe
someone who's dealt with this problems has a good solution. TIA

No worries. I'd just do something like this:

   vector<Foo> v;
   vector<Foo>::iterator a = v.begin();

   while(a!=v.end()) {
       vector<Foo>::iterator b = v.size()>20 ? a+20 : v.end();

Shouldn't that be something like
vector<Foo>::iterator b = v.end()-a >20 ? a+20 : v.end();

Or even:
ptrdiff_t const chuckSize = 20;
vector<Foo>::iterator b = a + std::min( chuckSize, v.end() - a );

If you add "v.end() - a" to "a", you're guaranteed to be
a v.end();

       process(a, b);
       a = b;
   }

--
James