std::find

Defined in header <algorithm>

template< class InputIt, class T >
InputIt find( InputIt first, InputIt last, const T& value );

(1)

template< class InputIt, class UnaryPredicate >
InputIt find_if( InputIt first, InputIt last, 
                 UnaryPredicate p );

(2)

template< class InputIt, class UnaryPredicate >
InputIt find_if_not( InputIt first, InputIt last, 
                     UnaryPredicate q );

(3)

(since C++11)

Returns the first element in the range [first, last) that satisfies specific criteria:

1) find searches for an element equal to value

2) find_if searches for an element for which predicate p returns true

3) find_if_not searches for an element for which predicate q returns false

Parameters

first, last	-	the range of elements to examine
value	-	value to compare the elements to
p	-	unary predicate which returns `true` for the required element. The signature of the predicate function should be equivalent to the following: `bool pred(const Type &a);` The signature does not need to have `const &`, but the function must not modify the objects passed to it. The type `Type` must be such that an object of type `InputIt` can be dereferenced and then implicitly converted to `Type`.
q	-	unary predicate which returns `false` for the required element. The signature of the predicate function should be equivalent to the following: `bool pred(const Type &a);` The signature does not need to have `const &`, but the function must not modify the objects passed to it. The type `Type` must be such that an object of type `InputIt` can be dereferenced and then implicitly converted to `Type`.
Type requirements
- `InputIt` must meet the requirements of `InputIterator`.
- `UnaryPredicate` must meet the requirements of `Predicate`.

Return value

Iterator to the first element satisfying the condition or last if no such element is found.

Complexity

At most last - first applications of the predicate.

Possible implementation

First version

template<class InputIt, class T>
InputIt find(InputIt first, InputIt last, const T& value)
{
    for (; first != last; ++first) {
        if (*first == value) {
            return first;
        }
    }
    return last;
}

Second version

template<class InputIt, class UnaryPredicate>
InputIt find_if(InputIt first, InputIt last, UnaryPredicate p)
{
    for (; first != last; ++first) {
        if (p(*first)) {
            return first;
        }
    }
    return last;
}

Third version

template<class InputIt, class UnaryPredicate>
InputIt find_if_not(InputIt first, InputIt last, UnaryPredicate q)
{
    for (; first != last; ++first) {
        if (!q(*first)) {
            return first;
        }
    }
    return last;
}

If you do not have C++11, an equivalent to std::find_if_not is to use std::find_if with the negated predicate.

template<class InputIt, class UnaryPredicate>
InputIt find_if_not(InputIt first, InputIt last, UnaryPredicate q)
{
    return std::find_if(first, last, std::not1(q));
}

Example

The following example finds an integer in a vector of integers.

#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
  
int main()
{
    int n1 = 3;
    int n2 = 5;
  
    std::vector<int> v{0, 1, 2, 3, 4};
  
    auto result1 = std::find(std::begin(v), std::end(v), n1);
    auto result2 = std::find(std::begin(v), std::end(v), n2);
  
    if (result1 != std::end(v)) {
        std::cout << "v contains: " << n1 << '\n';
    } else {
        std::cout << "v does not contain: " << n1 << '\n';
    }
  
    if (result2 != std::end(v)) {
        std::cout << "v contains: " << n2 << '\n';
    } else {
        std::cout << "v does not contain: " << n2 << '\n';
    }
}

Output:

v contains: 3
v does not contain: 5

adjacent_find	finds the first two adjacent items that are equal (or satisfy a given predicate) (function template)
find_end	finds the last sequence of elements in a certain range (function template)
find_first_of	searches for any one of a set of elements (function template)
mismatch	finds the first position where two ranges differ (function template)
search	searches for a range of elements (function template)
std::experimental::parallel::find (parallelism TS)	parallelized version of `std::find` (function template)
std::experimental::parallel::find_if (parallelism TS)	parallelized version of `std::find_if` (function template)
std::experimental::parallel::find_if_not (parallelism TS)	parallelized version of `std::find_if_not` (function template)

Links:

http://en.cppreference.com/w/cpp/algorithm/find

doc_CPP

2025-01-10 15:47:30

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