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()

12 Answers

The typical way to check for existence in many STL containers such as std::map, std::set, ... is:

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

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
}

C++ 20

In C++20 set gets a contains function, so the following becomes possible as mentioned at:

if (myset.contains(x)) { // x is in the set
} else { // no x
}

Just to clarify, the reason why there is no member like contains() in these container types is because it would open you up to writing inefficient code. Such a method would probably just do a this->find(key) != this->end() internally, but consider what you do when the key is indeed present; in most cases you'll then want to get the element and do something with it. This means you'd have to do a second find(), which is inefficient. It's better to use find directly, so you can cache your result, like so:

auto it = myContainer.find(key);
if (it != myContainer.end())
{ // Do something with it, no more lookup needed.
}
else
{ // Key was not present.
}

Of course, if you don't care about efficiency, you can always roll your own, but in that case you probably shouldn't be using C++... ;)

In C++20 we'll finally get std::set::contains method.

#include <iostream>
#include <string>
#include <set>
int main()
{ std::set<std::string> example = {"Do", "not", "panic", "!!!"}; if(example.contains("panic")) { std::cout << "Found\n"; } else { std::cout << "Not found\n"; }
}

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); }
}

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.

Since C++20, there is simply (and at last!) bool std::contains(const K&)

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.

Write your own:

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

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.

It's this, by a mile.

bool once(uintptr_t val) { return visited.emplace(val).second;
}

How could it be otherwise?

func5(unsigned long): sub rsp, 24 mov QWORD PTR [rsp+8], rdi lea rsi, [rsp+8] mov edi, OFFSET FLAT:visited2 call std::pair<std::_Rb_tree_iterator<unsigned long>, bool> std::_Rb_tree<unsigned long, unsigned long, std::_Identity<unsigned long>, std::less<unsigned long>, std::allocator<unsigned long> >::_M_emplace_unique<unsigned long&>(unsigned long&) add rsp, 24 mov eax, edx ret

//general Syntax

 set<int>::iterator ii = find(set1.begin(),set1.end(),"element to be searched");

/* in below code i am trying to find element 4 in and int set if it is present or not*/

set<int>::iterator ii = find(set1.begin(),set1.end(),4); if(ii!=set1.end()) { cout<<"element found"; set1.erase(ii);// in case you want to erase that element from set. }

Hype Blast

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

12 Answers

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