Searching in std::set without iterator C++

Question

I have std::set which contains come int values. Now i use iterator to find out whether set contans value.

But my application use this search very ofter and search using iterator too slow, can i do something like that:

std::set<int> fdsockets;

void myfunc(int fd)
{
    if(fdsockets[fd] != fdsockets.end())
    {
            // my code
    }
}

But i have error when compile using G++

no match for 'operator[]' in 'fdsockets[fd]'

Maybe i can use something instead of std::set?

Thanks!

The reason to prefer `find()` over a loop over the set is that `find()` is an O(log(N)) algorithm. A loop is worse than O(N) (I suspect it's an O(N*log(N)) algorithm) because `operator++()` is quite complex for those associative containers. — David Hammen, Nov 09 '12 at 13:52

score 4 · Accepted Answer · edited Nov 11 '12 at 19:49

4

std::unorered_set or an ordered vector with binary search are more effective for simple membership test. If the maximum value of intergers is low a lookup table might be an alternative.

edited Nov 11 '12 at 19:49

answered Nov 09 '12 at 13:15

hansmaad

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A sorted `vector` with binary search is only more effective if you rarely add or remove elements from the vector. Each insert or removal takes nlgn time. But the searches will be much faster. – Yakk - Adam Nevraumont Nov 09 '12 at 13:18
You are right. binary search or static tables are for static data only. – hansmaad Nov 09 '12 at 13:20
@Yakk That's not certain. It depends on the size of the data set as well. And copying `int` is a very cheap operation; insertions and deletions, even in a moderately large data set, may be cheaper using a sorted vector than using `std::set`: you can copy a lot of data for the price of one allocation (and finding where to insert is also cheaper). – James Kanze Nov 09 '12 at 14:26

score 4 · Answer 2 · answered Nov 09 '12 at 13:17

4

It sounds like you want set::find()

if( fdsockets.find(fd) != fdsockets.end() )
{
      // my code
}

answered Nov 09 '12 at 13:17

Benj

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score 2 · Answer 3 · answered Nov 09 '12 at 13:17

2

There is no operator[] in std::set.

You probably mean

if(fdsockets.find(fd) != fdsockets.end())

answered Nov 09 '12 at 13:17

Emilio Garavaglia

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score 0 · Answer 4 · answered Nov 09 '12 at 14:16

0

If you don't need the iterator that set::find returns (you're just testing existence, not actually going to access the fdsocket), here's an alternative:

if(fdsockets.count(fd))
{
        // my code
}

answered Nov 09 '12 at 14:16

Cubbi

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Searching in std::set without iterator C++

4 Answers4