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Difference between revisions of "cpp/algorithm/for each n"

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< cpp‎ | algorithm
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For both overloads, if the iterator type is mutable, {{tt|f}} may modify the elements of the range through the dereferenced iterator. If {{tt|f}} returns a result, the result is ignored. If {{tt|n}} is less than zero, the behavior is undefined.
 
For both overloads, if the iterator type is mutable, {{tt|f}} may modify the elements of the range through the dereferenced iterator. If {{tt|f}} returns a result, the result is ignored. If {{tt|n}} is less than zero, the behavior is undefined.
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Unlike the rest of the parallel algorithms, {{tt|for_each_n}} is not allowed to make copies of the elements in the sequence even if they are trivially copyable.
  
 
===Parameters===
 
===Parameters===
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Revision as of 00:35, 5 February 2020

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
for_each_n
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
(1)
template< class InputIt, class Size, class UnaryFunction >
InputIt for_each_n( InputIt first, Size n, UnaryFunction f );
 (since C++17)
(until C++20)
template< class InputIt, class Size, class UnaryFunction >
constexpr InputIt for_each_n( InputIt first, Size n, UnaryFunction f );
 (since C++20)
template< class ExecutionPolicy, class ForwardIt, class Size, class UnaryFunction2 >
ForwardIt for_each_n( ExecutionPolicy&& policy, ForwardIt first, Size n, UnaryFunction2 f );
 (2) (since C++17)
1) Applies the given function object f to the result of dereferencing every iterator in the range [first, first + n), in order.
2) Applies the given function object f to the result of dereferencing every iterator in the range [first, first + n) (not necessarily in order). The algorithm is executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

For both overloads, if the iterator type is mutable, f may modify the elements of the range through the dereferenced iterator. If f returns a result, the result is ignored. If n is less than zero, the behavior is undefined.

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 trivially copyable.

Contents

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. See execution policy for details.
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.
-
UnaryFunction must meet the requirements of MoveConstructible. Does not have to be CopyConstructible
-
UnaryFunction2 must meet the requirements of CopyConstructible.

Return value

first + n

Complexity

Exactly n applications of f

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.

Possible implementation

See also the implementation in libstdc++ and libc++. 

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

Example

Run this code
#include <algorithm>
#include <iostream>
#include <vector>
 
int main()
{
    std::vector<int> ns{1, 2, 3, 4, 5};
    for (auto n: ns) std::cout << n << ", ";
    std::cout << '\n';
    std::for_each_n(ns.begin(), 3, [](auto& n){ n *= 2; });
    for (auto n: ns) std::cout << n << ", ";
    std::cout << '\n';
}

Output: 

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

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 function to a range of elements
(function template) [edit]
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