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std::find, std::find_if, std::find_if_not

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
findfind_iffind_if_not
(C++11)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Permutations
Numeric operations
C library
 
Defined in header <algorithm>
(1)
template< class InputIt, class T >
InputIt find( InputIt first, InputIt last, const T& value );
(until C++20)
template< class InputIt, class T >
constexpr InputIt find( InputIt first, InputIt last, const T& value );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class T >
ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );
(2) (since C++17)
(3)
template< class InputIt, class UnaryPredicate >

InputIt find_if( InputIt first, InputIt last,

                 UnaryPredicate p );
(until C++20)
template< class InputIt, class UnaryPredicate >

constexpr InputIt find_if( InputIt first, InputIt last,

                           UnaryPredicate p );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >

ForwardIt find_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,

                 UnaryPredicate p );
(4) (since C++17)
(5)
template< class InputIt, class UnaryPredicate >

InputIt find_if_not( InputIt first, InputIt last,

                     UnaryPredicate q );
(since C++11)
(until C++20)
template< class InputIt, class UnaryPredicate >

constexpr InputIt find_if_not( InputIt first, InputIt last,

                               UnaryPredicate q );
(since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >

ForwardIt find_if_not( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,

                     UnaryPredicate q );
(6) (since C++17)

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

1) find searches for an element equal to value
3) find_if searches for an element for which predicate p returns true
5) find_if_not searches for an element for which predicate q returns false
2,4,6) Same as (1,3,5), but executed according to policy. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Contents

[edit] Parameters

first, last - the range of elements to examine
value - value to compare the elements to
policy - the execution policy to use. See execution policy for details.
p - unary predicate which returns ​true for the required element.

The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy (since C++11). ​

q - unary predicate which returns ​false for the required element.

The expression q(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy (since C++11). ​

Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
UnaryPredicate must meet the requirements of Predicate.

[edit] Return value

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

[edit] Complexity

At most last - first applications of the predicate

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the three standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

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

[edit] Notes

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));
}

[edit] 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

[edit] See also

finds the first two adjacent items that are equal (or satisfy a given predicate)
(function template) [edit]
finds the last sequence of elements in a certain range
(function template) [edit]
searches for any one of a set of elements
(function template) [edit]
finds the first position where two ranges differ
(function template) [edit]
searches for a range of elements
(function template) [edit]