Re: Advancing Through std::vector
On Wednesday, 11 September 2013 21:01:13 UTC+1, red floyd wrote:
On 9/11/2013 12:36 PM, 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
Why doesn't size() help you?
cur_index = &*it - &vec.begin();
Why not just "it - vec.begin()".
int items_left = vec.size() - 20;
or if this is what you're after:
int items_left = vec.end() - it;
chunk_size = items_left > 20 ? 20 : items_left;
or the simplest:
chunk_size = std::min( vec.end() - it, static_cast<ptrdiff_t>(20) );
(The static_cast is only necessary because we don't know the
real type of "vec.end() - it". It's ptrdiff_t, but that may be
int, long or long long, depending on the implementation.)
--
James