How to check that an element is in a std::set?

Question

How do you check that an element is in a set?

Is there a simpler equivalent of the following code:

myset.find(x) != myset.end()

Answer 1

I use

if(!my_set.count(that_element)) //Element is present...
;

But it is not as efficient as

if(my_set.find(that_element)!=my_set.end()) ....;

My version only saves my time in writing the code. I prefer it this way for competitive coding.

Answer 2

If you were going to add a contains function, it might look like this:

#include <algorithm>
#include <iterator>

template<class TInputIterator, class T> inline
bool contains(TInputIterator first, TInputIterator last, const T& value)
{
    return std::find(first, last, value) != last;
}

template<class TContainer, class T> inline
bool contains(const TContainer& container, const T& value)
{
    // This works with more containers but requires std::begin and std::end
    // from C++0x, which you can get either:
    //  1. By using a C++0x compiler or
    //  2. Including the utility functions below.
    return contains(std::begin(container), std::end(container), value);

    // This works pre-C++0x (and without the utility functions below, but doesn't
    // work for fixed-length arrays.
    //return contains(container.begin(), container.end(), value);
}

template<class T> inline
bool contains(const std::set<T>& container, const T& value)
{
    return container.find(value) != container.end();
}

This works with std::set, other STL containers, and even fixed-length arrays:

void test()
{
    std::set<int> set;
    set.insert(1);
    set.insert(4);
    assert(!contains(set, 3));

    int set2[] = { 1, 2, 3 };
    assert(contains(set2, 3));
}

Edit:

As pointed out in the comments, I unintentionally used a function new to C++0x (std::begin and std::end). Here is the near-trivial implementation from VS2010:

namespace std {

template<class _Container> inline
    typename _Container::iterator begin(_Container& _Cont)
    { // get beginning of sequence
    return (_Cont.begin());
    }

template<class _Container> inline
    typename _Container::const_iterator begin(const _Container& _Cont)
    { // get beginning of sequence
    return (_Cont.begin());
    }

template<class _Container> inline
    typename _Container::iterator end(_Container& _Cont)
    { // get end of sequence
    return (_Cont.end());
    }

template<class _Container> inline
    typename _Container::const_iterator end(const _Container& _Cont)
    { // get end of sequence
    return (_Cont.end());
    }

template<class _Ty,
    size_t _Size> inline
    _Ty *begin(_Ty (&_Array)[_Size])
    { // get beginning of array
    return (&_Array[0]);
    }

template<class _Ty,
    size_t _Size> inline
    _Ty *end(_Ty (&_Array)[_Size])
    { // get end of array
    return (&_Array[0] + _Size);
    }

}

Answer 3

You can also check whether an element is in set or not while inserting the element. The single element version return a pair, with its member pair::first set to an iterator pointing to either the newly inserted element or to the equivalent element already in the set. The pair::second element in the pair is set to true if a new element was inserted or false if an equivalent element already existed.

For example: Suppose the set already has 20 as an element.

 std::set<int> myset;
 std::set<int>::iterator it;
 std::pair<std::set<int>::iterator,bool> ret;

 ret=myset.insert(20);
 if(ret.second==false)
 {
     //do nothing

 }
 else
 {
    //do something
 }

 it=ret.first //points to element 20 already in set.

If the element is newly inserted than pair::first will point to the position of new element in set.

Answer 4

I was able to write a general contains function for std::list and std::vector,

template<typename T>
bool contains( const list<T>& container, const T& elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

template<typename T>
bool contains( const vector<T>& container, const T& elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

// use:
if( contains( yourList, itemInList ) ) // then do something

This cleans up the syntax a bit.

But I could not use template template parameter magic to make this work arbitrary stl containers.

// NOT WORKING:
template<template<class> class STLContainer, class T>
bool contains( STLContainer<T> container, T elt )
{
  return find( container.begin(), container.end(), elt ) != container.end() ;
}

Any comments about improving the last answer would be nice.

Answer 5

The typical way to check for existence in many STL containers is:

const bool is_in = container.find(element) != container.end();

Answer 6

Another way of simply telling if an element exists is to check the count()

if (myset.count(x)) {
   // x is in the set, count is 1
} else {
   // count zero, i.e. x not in the set
}

Most of the times, however, I find myself needing access to the element wherever I check for its existence.

So I'd have to find the iterator anyway. Then, of course, it's better to simply compare it to end too.

set< X >::iterator it = myset.find(x);
if (it != myset.end()) {
   // do something with *it
}