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

From cppreference.com
< cpp‎ | algorithm
 
 
 
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 element for which predicate q returns false

Contents

[edit] 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.

[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] 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));
}

[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 two identical (or some other relationship) items adjacent to each other
(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]