Difference between revisions of "cpp/algorithm/find"

Revision as of 15:14, 8 January 2018

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, constexpr 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, constexpr 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 participates in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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 Template:par req concept Template:par req concept Template:par req concept

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

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 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.

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.

Run this code

#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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

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

cppreference.com

Namespaces

Variants

Views

Actions

Difference between revisions of "cpp/algorithm/find"

Revision as of 15:14, 8 January 2018

Contents

Parameters

Return value

Complexity

Exceptions

Possible implementation

Example

See also

Navigation

Toolbox

@@ Line 3: / Line 3: @@
 {{dcl begin}}
 {{dcl header | algorithm}}
-{{dcl | since= | num= 1 |1=
+{{dcl rev multi | num=1 | since1= | until1=c++20 |dcl1=
 template< class InputIt, class T >
 InputIt find( InputIt first, InputIt last, const T& value );
+|dcl2=
+template< class InputIt, class T >
+constexpr InputIt find( InputIt first, InputIt last, const T& value );
 }}
-{{dcl | since=c++17 | num= 2 |1=
+{{dcl | since=c++17 | until= | num=2 |1=
 template< class ExecutionPolicy, class ForwardIt, class T >
-ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );}}
+ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value );
-{{dcl | since= | num= 3 |1=
+}}
+{{dcl rev multi | num=3 | since1= | until1=c++20 |dcl1=
 template< class InputIt, class UnaryPredicate >
 InputIt find_if( InputIt first, InputIt last,
                   UnaryPredicate p );
+|dcl2=
+template< class InputIt, class UnaryPredicate >
+constexpr InputIt find_if( InputIt first, InputIt last,
+constexpr                  UnaryPredicate p );
 }}
-{{dcl | since=c++17 | num= 4 |1=
+{{dcl | since=c++17 | until= | num=4 |1=
 template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
 ForwardIt find_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,
-                  UnaryPredicate p );}}
+                  UnaryPredicate p );
-{{dcl | since=c++11 | num= 5 |1=
+}}
+{{dcl rev multi | num=5 | since1=c++11 | until1=c++20 |dcl1=
 template< class InputIt, class UnaryPredicate >
 InputIt find_if_not( InputIt first, InputIt last,
                       UnaryPredicate q );
+|dcl2=
+template< class InputIt, class UnaryPredicate >
+constexpr InputIt find_if_not( InputIt first, InputIt last,
+constexpr                      UnaryPredicate q );
 }}
-{{dcl | since=c++17 | num= 6 |1=
+{{dcl | since=c++17 | until= | num=6 |1=
 template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
 ForwardIt find_if_not( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,
-                      UnaryPredicate q );}}
+                      UnaryPredicate q );
+}}
 {{dcl end}}