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std::for_each_n

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< cpp‎ | algorithm
 
 
Algorithm library
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for_each_n
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(C++11)                (C++11)(C++11)

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(C++11)
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Defined in header <algorithm>
template< class InputIt, class Size, class UnaryFunc >
InputIt for_each_n( InputIt first, Size n, UnaryFunc f );
(1) (since C++17)
(constexpr since C++20)
template< class ExecutionPolicy,

          class ForwardIt, class Size, class UnaryFunc >
ForwardIt for_each_n( ExecutionPolicy&& policy,

                      ForwardIt first, Size n, UnaryFunc f );
(2) (since C++17)

Applies the given function object f to the result of dereferencing every iterator in the range [first, first + n). If f returns a result, the result is ignored.

1) f is applied in order starting from first.
If UnaryFunc is not MoveConstructible, the behavior is undefined.
2) f might not be applied in order. The algorithm is executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v <std::decay_t <ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v <std::remove_cvref_t <ExecutionPolicy>> is true.

(since C++20)
If UnaryFunc is not CopyConstructible, the behavior is undefined.

If n >= 0 is not true, the behavior is undefined.

If the iterator type (InputIt/ForwardIt) is mutable, f may modify the elements of the range through the dereferenced iterator.

Unlike the rest of the parallel algorithms, for_each_n is not allowed to make copies of the elements in the sequence even if they are TriviallyCopyable.

Contents

[edit] Parameters

first - the beginning of the range to apply the function to
n - the number of elements to apply the function to
policy - the execution policy to use
f - function object, to be applied to the result of dereferencing every iterator in the range [first, first + n)

The signature of the function should be equivalent to the following:

 void fun(const Type &a);

The signature does not need to have const &.
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 LegacyInputIterator.
-ForwardIt must meet the requirements of LegacyForwardIterator.
-Size must be convertible to an integral type.

[edit] Return value

An iterator equal to first + n, or more formally, to std::advance (first, n).

[edit] Complexity

Exactly n applications of f.

[edit] Exceptions

The overload with a template parameter named ExecutionPolicy reports 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.

[edit] Possible implementation

See also the implementation in libstdc++, libc++ and MSVC stdlib. 

template<class InputIt, class Size, class UnaryFunc>
InputIt for_each_n(InputIt first, Size n, UnaryFunc f)
{
 for (Size i = 0; i < n; ++first, (void) ++i)
 f(*first);
 
 return first;
}

[edit] Example

Run this code
#include <algorithm>
#include <iostream>
#include <vector>
 
void println(auto const& v)
{
 for (auto count{v.size()}; const auto& e : v)
 std::cout << e << (--count ? ", " : "\n");
}
 
int main()
{
 std::vector <int> vi{1, 2, 3, 4, 5};
 println(vi);
 
 std::for_each_n(vi.begin(), 3, [](auto& n) { n *= 2; });
 println(vi);
}

Output: 

1, 2, 3, 4, 5
2, 4, 6, 4, 5

[edit] See also

applies a function to a range of elements, storing results in a destination range
(function template) [edit]
range-for loop (C++11) executes loop over range[edit]
applies a unary function object to elements from a range
(function template) [edit]
applies a function object to the first N elements of a sequence
(algorithm function object)[edit]
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