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.
//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.
}
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));
}
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);
}
}
Write your own:
template<class T>
bool checkElementIsInSet(const T& elem, const std::set<T>& container)
{
return container.find(elem) != 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: https://stackoverflow.com/a/54197839/895245
if (myset.contains(x)) {
// x is in the set
} else {
// no x
}
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";
}
}
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++... ;)
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.
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.
Source: Stackoverflow.com