Re: STL container question
James Kanze wrote:
On Oct 1, 7:31 pm, Erik Wikstr??m <Erik-wikst...@telia.com> wrote:
On 2008-10-01 18:57, Rolf Magnus wrote:
Jeff Schwab wrote:
Boltar wrote:
I need to store a number of integer values which I will
then search on later to see if they exist in my container.
Can someone tell me which container would be quickest for
finding these values? I can't use a plain C array (unless
I make it 2^32 in size!) since I don't know the max
integer value.
Sorted vector. See Effective STL, Item 23.
For the record, you wouldn't 2^32 integers, just 2^32 bits
= 500 MiB. It's actually not that much RAM, depending on
your target system, and would let you check for integers
with O(1) complexity (rather than O(log N)).
However, it can still be slower, since it's more or less the
worst thing you can do to the cache.
Still, you should only get one cache-miss when looking for a
value, if you use a set or vector you will probably get more.
One thing I don't understand here: both a C style array and
std::vector use a single block of contiguous memory. How could
cache performance be any different for them?
We're not talking about C style arrays vs. vector, but about different
algorithms. One is to storing the values in a sorted array/vector, then
doing a binary search for the value. The other is like a trivial kind of
hash table, where each possible integer value has a boolean entry, so you
can simply use the value as index. Saves the search, but needs a big amount
of memory that is much larger than the cache.
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