std::fill_n

 

C++

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Concepts library (C++20)

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Ranges library (C++20)

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Numerics library

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Technical Specifications

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Concepts and utilities: std::Sortable, std::projected, ...
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseqexecution::unseq (C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++20)
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	findfind_iffind_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_3way (C++20)
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy (C++17)	uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) destroy_n (C++17)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17) compare_3way (C++20)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

Defined in header <algorithm>
	(1)
template< class OutputIt, class Size, class T > void fill_n( OutputIt first, Size count, const T& value );			(until C++11)
template< class OutputIt, class Size, class T > OutputIt fill_n( OutputIt first, Size count, const T& value );			(since C++11) (until C++20)
template< class OutputIt, class Size, class T > constexpr OutputIt fill_n( OutputIt first, Size count, const T& value );			(since C++20)
template< class ExecutionPolicy, class ForwardIt, class Size, class T > ForwardIt fill_n( ExecutionPolicy&& policy, ForwardIt first, Size count, const T& value );		(2)	(since C++17)

1) Assigns the given value to the first count elements in the range beginning at first if count > 0. Does nothing otherwise.

2) Same as (1), but executed according to policy. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Parameters

first	-	the beginning of the range of elements to modify
count	-	number of elements to modify
value	-	the value to be assigned
policy	-	the execution policy to use. See execution policy for details.
Type requirements
- OutputIt must meet the requirements of LegacyOutputIterator.
- ForwardIt must meet the requirements of LegacyForwardIterator.

Return value

(none)	(until C++11)
Iterator one past the last element assigned if count > 0, first otherwise.	(since C++11)

Complexity

Exactly count assignments, for count > 0.

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

template<class OutputIt, class Size, class T> OutputIt fill_n(OutputIt first, Size count, const T& value) { for (Size i = 0; i < count; i++) { *first++ = value; } return first; }

Example

The following code uses fill_n() to assign -1 to the first half of a vector of integers:

Run this code

#include <algorithm>
#include <vector>
#include <iostream>
#include <iterator>
 
int main()
{
    std::vector<int> v1{0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
 
    std::fill_n(v1.begin(), 5, -1);
 
    std::copy(begin(v1), end(v1), std::ostream_iterator<int>(std::cout, " "));
    std::cout << "\n";
}

Output:

-1 -1 -1 -1 -1 5 6 7 8 9

See also

fill	copy-assigns the given value to every element in a range (function template)