All I want to do is to check whether an element exists in the vector or not, so I can deal with each case.
if ( item_present )
do_this();
else
do_that();
转载于:https://stackoverflow.com/questions/571394/how-to-find-out-if-an-item-is-present-in-a-stdvector
You can use std::find from <algorithm>:
std::find(vector.begin(), vector.end(), item) != vector.end()
This returns a bool (true if present, false otherwise). With your example:
#include <algorithm>
if ( std::find(vector.begin(), vector.end(), item) != vector.end() )
do_this();
else
do_that();
Use find from the algorithm header of stl.I've illustrated its use with int type. You can use any type you like as long as you can compare for equality (overload == if you need to for your custom class).
#include <algorithm>
#include <vector>
using namespace std;
int main()
{
typedef vector<int> IntContainer;
typedef IntContainer::iterator IntIterator;
IntContainer vw;
//...
// find 5
IntIterator i = find(vw.begin(), vw.end(), 5);
if (i != vw.end()) {
// found it
} else {
// doesn't exist
}
return 0;
}
Use the STL find function.
Keep in mind that there is also a find_if function, which you can use if your search is more complex, i.e. if you're not just looking for an element, but, for example, want see if there is an element that fulfills a certain condition, for example, a string that starts with "abc". (find_if would give you an iterator that points to the first such element).
As others have said, use the STL find or find_if functions. But if you are searching in very large vectors and this impacts performance, you may want to sort your vector and then use the binary_search, lower_bound, or upper_bound algorithms.
Bear in mind that, if you're going to be doing a lot of lookups, there are STL containers that are better for that. I don't know what your application is, but associative containers like std::map may be worth considering.
std::vector is the container of choice unless you have a reason for another, and lookups by value can be such a reason.
You can try this code:
#include <algorithm>
#include <vector>
// You can use class, struct or primitive data type for Item
struct Item {
//Some fields
};
typedef std::vector<Item> ItemVector;
typedef ItemVector::iterator ItemIterator;
//...
ItemVector vtItem;
//... (init data for vtItem)
Item itemToFind;
//...
ItemIterator itemItr;
itemItr = std::find(vtItem.begin(), vtItem.end(), itemToFind);
if (itemItr != vtItem.end()) {
// Item found
// doThis()
}
else {
// Item not found
// doThat()
}
If your vector is not ordered, use the approach MSN suggested:
if(std::find(vector.begin(), vector.end(), item)!=vector.end()){
// Found the item
}
If your vector is ordered, use binary_search method Brian Neal suggested:
if(binary_search(vector.begin(), vector.end(), item)){
// Found the item
}
binary search yields O(log n) worst-case performance, which is way more efficient than the first approach. In order to use binary search, you may use qsort to sort the vector first to guarantee it is ordered.
If you wanna find a string in a vector:
struct isEqual
{
isEqual(const std::string& s): m_s(s)
{}
bool operator()(OIDV* l)
{
return l->oid == m_s;
}
std::string m_s;
};
struct OIDV
{
string oid;
//else
};
VecOidv::iterator itFind=find_if(vecOidv.begin(),vecOidv.end(),isEqual(szTmp));
I use something like this...
#include <algorithm>
template <typename T>
const bool Contains( std::vector<T>& Vec, const T& Element )
{
if (std::find(Vec.begin(), Vec.end(), Element) != Vec.end())
return true;
return false;
}
if (Contains(vector,item))
blah
else
blah
...as that way it's actually clear and readable. (Obviously you can reuse the template in multiple places).
template <typename T> bool IsInVector(T what, std::vector<T> * vec)
{
if(std::find(vec->begin(),vec->end(),what)!=vec->end())
return true;
return false;
}
You can use the find function, found in the std namespace, ie std::find. You pass the std::find function the begin and end iterator from the vector you want to search, along with the element you're looking for and compare the resulting iterator to the end of the vector to see if they match or not.
std::find(vector.begin(), vector.end(), item) != vector.end()
You're also able to dereference that iterator and use it as normal, like any other iterator.
You can use count too. It will return the number of items present in a vector.
int t=count(vec.begin(),vec.end(),item);
In C++11 you can use any_of. For example if it is a vector<string> v; then:
if (any_of(v.begin(), v.end(), bind2nd(equal_to<string>(), item)))
do_this();
else
do_that();
Here's a function that will work for any Container:
template <class Container>
const bool contains(const Container& container, const typename Container::value_type& element)
{
return std::find(container.begin(), container.end(), element) != container.end();
}
Note that you can get away with 1 template parameter because you can extract the value_type from the Container. You need the typename because Container::value_type is a dependent name.
Another sample using C++ operators.
#include <vector>
#include <algorithm>
#include <stdexcept>
template<typename T>
inline static bool operator ==(const std::vector<T>& v, const T& elem)
{
return (std::find(v.begin(), v.end(), elem) != v.end());
}
template<typename T>
inline static bool operator !=(const std::vector<T>& v, const T& elem)
{
return (std::find(v.begin(), v.end(), elem) == v.end());
}
enum CODEC_ID {
CODEC_ID_AAC,
CODEC_ID_AC3,
CODEC_ID_H262,
CODEC_ID_H263,
CODEC_ID_H264,
CODEC_ID_H265,
CODEC_ID_MAX
};
void main()
{
CODEC_ID codec = CODEC_ID_H264;
std::vector<CODEC_ID> codec_list;
codec_list.reserve(CODEC_ID_MAX);
codec_list.push_back(CODEC_ID_AAC);
codec_list.push_back(CODEC_ID_AC3);
codec_list.push_back(CODEC_ID_H262);
codec_list.push_back(CODEC_ID_H263);
codec_list.push_back(CODEC_ID_H264);
codec_list.push_back(CODEC_ID_H265);
if (codec_list != codec)
{
throw std::runtime_error("codec not found!");
}
if (codec_list == codec)
{
throw std::logic_error("codec has been found!");
}
}
With boost you can use any_of_equal:
#include <boost/algorithm/cxx11/any_of.hpp>
bool item_present = boost::algorithm::any_of_equal(vector, element);
Using Newton C++ it's easier, self-documented and faster than with std::find because of return a bool directly.
bool exists_linear( INPUT_ITERATOR first, INPUT_ITERATOR last, const T& value )
bool exists_binary( INPUT_ITERATOR first, INPUT_ITERATOR last, const T& value )
I think it's obvious what the functions do.
include <newton/algorithm/algorithm.hpp>
if ( newton::exists_linear(first, last, value) )
do_this();
else
do_that();