std::replace, std::replace_if

 

C++

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Feature test macros (C++20)

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Ranges library (C++20)

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Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
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)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Defined in header <algorithm>
	(1)
template< class ForwardIt, class T > void replace( ForwardIt first, ForwardIt last,               const T& old_value, const T& new_value );			(constexpr since C++20) (until C++26)
template< class ForwardIt, class T = typename std::iterator_traits                                          <ForwardIt>::value_type > constexpr void replace( ForwardIt first, ForwardIt last,                         const T& old_value, const T& new_value );			(since C++26)
		(2)
template< class ExecutionPolicy, class ForwardIt, class T > void replace( ExecutionPolicy&& policy,               ForwardIt first, ForwardIt last,               const T& old_value, const T& new_value );				(since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt,           class T = typename std::iterator_traits                         <ForwardIt>::value_type > void replace( ExecutionPolicy&& policy,               ForwardIt first, ForwardIt last,               const T& old_value, const T& new_value );				(since C++26)
			(3)
template< class ForwardIt, class UnaryPred, class T > void replace_if( ForwardIt first, ForwardIt last,                  UnaryPred p, const T& new_value );					(constexpr since C++20) (until C++26)
template< class ForwardIt, class UnaryPred,           class T = typename std::iterator_traits                         <ForwardIt>::value_type> > constexpr void replace_if( ForwardIt first, ForwardIt last,                            UnaryPred p, const T& new_value );					(since C++26)
				(4)
template< class ExecutionPolicy,           class ForwardIt, class UnaryPred, class T > void replace_if( ExecutionPolicy&& policy,                  ForwardIt first, ForwardIt last,                  UnaryPred p, const T& new_value );						(since C++17) (until C++26)
template< class ExecutionPolicy,           class ForwardIt, class UnaryPred,           class T = typename std::iterator_traits                         <ForwardIt>::value_type> > void replace_if( ExecutionPolicy&& policy,                  ForwardIt first, ForwardIt last,                  UnaryPred p, const T& new_value );						(since C++26)

Replaces all elements in the range [first, last) with new_value if they satisfy specific criteria.

1) Replaces all elements that are equal to old_value (using operator==).

3) Replaces all elements for which predicate p returns true.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if

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 *first = new_value is invalid(until C++20)new_value is not writable to first(since C++20), the program is ill-formed.

Parameters

first, last	-	the range of elements to process
old_value	-	the value of elements to replace
policy	-	the execution policy to use. See execution policy for details.
p	-	unary predicate which returns ​true if the element value should be replaced. 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 ForwardIt, 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). ​
new_value	-	the value to use as replacement
Type requirements
- ForwardIt must meet the requirements of LegacyForwardIterator.
- UnaryPred must meet the requirements of Predicate.

Return value

(none)

Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons using operator==.

3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

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.

Notes

Because the algorithm takes old_value and new_value by reference, it can have unexpected behavior if either is a reference to an element of the range [first, last).

Feature-test macro	Value	Std	Feature
__cpp_lib_algorithm_default_value_type	202403	(C++26)	List-initialization for algorithms (1-4)

Possible implementation

replace
template<class ForwardIt, class T = typename std::iterator_traits<ForwardIt>::value_type> void replace(ForwardIt first, ForwardIt last, const T& old_value, const T& new_value) { for (; first != last; ++first) if (*first == old_value) *first = new_value; }
replace_if
template<class ForwardIt, class UnaryPred, class T = typename std::iterator_traits<ForwardIt>::value_type> void replace_if(ForwardIt first, ForwardIt last, UnaryPred p, const T& new_value) { for (; first != last; ++first) if (p(*first)) *first = new_value; }

Example

Run this code

#include <algorithm>
#include <array>
#include <complex>
#include <functional>
#include <iostream>
 
void println(const auto& seq)
{
    for (const auto& e : seq)
        std::cout << e << ' ';
    std::cout << '\n';
}
 
int main()
{
    std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
 
    // Replace all occurrences of 8 with 88.
    std::replace(s.begin(), s.end(), 8, 88);
    println(s);
 
    // Replace all values less than 5 with 55.
    std::replace_if(s.begin(), s.end(), 
                    std::bind(std::less<int>(), std::placeholders::_1, 5), 55);
    println(s);
 
    std::array<std::complex<double>, 2> nums{{{1, 3}, {1, 3}}};
    #ifdef __cpp_lib_algorithm_default_value_type
        std::replace(nums.begin(), nums.end(), {1, 3}, {4, 2});
    #else
        std::replace(nums.begin(), nums.end(), std::complex<double>{1, 3},
                                               std::complex<double>{4, 2});
    #endif
    println(nums);
}

Output:

5 7 4 2 88 6 1 9 0 3
5 7 55 55 88 6 55 9 55 55
(4,2), (4,2)

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 283	C++98	T was required to be CopyAssignable (and EqualityComparable for replace), but the value type of ForwardIt is not always T and T is not always writable to ForwardIt	required *first = new_value to be valid instead

See also

replace_copyreplace_copy_if	copies a range, replacing elements satisfying specific criteria with another value (function template)
ranges::replaceranges::replace_if (C++20)(C++20)	replaces all values satisfying specific criteria with another value (niebloid)