std::replace, std::replace_if

 

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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)
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lower_bound	upper_bound	binary_search	equal_range
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merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
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is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
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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
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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 ForwardIt, class T > void replace( ForwardIt first, ForwardIt last,               const T& old_value, const T& new_value );			(until C++20)
template<class ForwardIt, class T > constexprvoid replace( ForwardIt first, ForwardIt last,                         const T& old_value, const T& new_value );			(since C++20)
template<class ExecutionPolicy, class ForwardIt, class T > void replace( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,               const T& old_value, const T& new_value );		(2)	(since C++17)
		(3)
template<class ForwardIt, class UnaryPredicate, class T > void replace_if( ForwardIt first, ForwardIt last,                  UnaryPredicate p, const T& new_value );				(until C++20)
template<class ForwardIt, class UnaryPredicate, class T > constexprvoid replace_if( ForwardIt first, ForwardIt last,                            UnaryPredicate p, const T& new_value );				(since C++20)
template<class ExecutionPolicy, class ForwardIt, class UnaryPredicate, class T > void replace_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,                  UnaryPredicate p, const T& new_value );			(4)	(since C++17)

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

1) Replaces all elements that are equal to old_value.

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

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

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.
- UnaryPredicate must meet the requirements of Predicate.

Return value

(none)

Complexity

Exactly 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.

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).

Possible implementation

First version
template<class ForwardIt, class T>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;}}}
Second version
template<class ForwardIt, class UnaryPredicate, class T>void replace_if(ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value){for(; first != last;++first){if(p(*first)){*first = new_value;}}}

Example

The following code at first replaces all occurrences of 8 with 88 in a vector of integers. Then it replaces all values less than 5 with 55.

Run this code

#include <algorithm>#include <array>#include <iostream>#include <functional>   int main(){std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};   std::replace(s.begin(), s.end(), 8, 88);   for(int a : s){std::cout<< a <<" ";}std::cout<<'\n';   std::replace_if(s.begin(), s.end(), std::bind(std::less<int>(), std::placeholders::_1, 5), 55);for(int a : s){std::cout<< a <<" ";}std::cout<<'\n';}

Output:

5 7 4 2 88 6 1 9 0 3 5 7 55 55 88 6 55 9 55 55

See also

replace_copyreplace_copy_if

copies a range, replacing elements satisfying specific criteria with another value (function template)