C++ std::vector Finding an Element in std::vector
Example
The function std::find, defined in the <algorithm> header, can be used to find an element in a std::vector.
std::find uses the operator== to compare elements for equality. It returns an iterator to the first element in the range that compares equal to the value.
If the element in question is not found, std::find returns std::vector::end (or std::vector::cend if the vector is const).
C++11
static const int arr[] = {5, 4, 3, 2, 1};
std::vector<int> v (arr, arr + sizeof(arr) / sizeof(arr[0]) );
std::vector<int>::iterator it = std::find(v.begin(), v.end(), 4);
std::vector<int>::difference_type index = std::distance(v.begin(), it);
// `it` points to the second element of the vector, `index` is 1
std::vector<int>::iterator missing = std::find(v.begin(), v.end(), 10);
std::vector<int>::difference_type index_missing = std::distance(v.begin(), missing);
// `missing` is v.end(), `index_missing` is 5 (ie. size of the vector)
C++11
std::vector<int> v { 5, 4, 3, 2, 1 };
auto it = std::find(v.begin(), v.end(), 4);
auto index = std::distance(v.begin(), it);
// `it` points to the second element of the vector, `index` is 1
auto missing = std::find(v.begin(), v.end(), 10);
auto index_missing = std::distance(v.begin(), missing);
// `missing` is v.end(), `index_missing` is 5 (ie. size of the vector)
If you need to perform many searches in a large vector, then you may want to consider sorting the vector first, before using the binary_search algorithm.
To find the first element in a vector that satisfies a condition, std::find_if can be used. In addition to the two parameters given to std::find, std::find_if accepts a third argument which is a function object or function pointer to a predicate function. The predicate should accept an element from the container as an argument and return a value convertible to bool, without modifying the container:
C++11
bool isEven(int val) {
return (val % 2 == 0);
}
struct moreThan {
moreThan(int limit) : _limit(limit) {}
bool operator()(int val) {
return val > _limit;
}
int _limit;
};
static const int arr[] = {1, 3, 7, 8};
std::vector<int> v (arr, arr + sizeof(arr) / sizeof(arr[0]) );
std::vector<int>::iterator it = std::find_if(v.begin(), v.end(), isEven);
// `it` points to 8, the first even element
std::vector<int>::iterator missing = std::find_if(v.begin(), v.end(), moreThan(10));
// `missing` is v.end(), as no element is greater than 10
C++11
// find the first value that is even
std::vector<int> v = {1, 3, 7, 8};
auto it = std::find_if(v.begin(), v.end(), [](int val){return val % 2 == 0;});
// `it` points to 8, the first even element
auto missing = std::find_if(v.begin(), v.end(), [](int val){return val > 10;});
// `missing` is v.end(), as no element is greater than 10
