std::replace, std::replace_if

From cppreference.com
< cpp‎ | algorithm
 
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
(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
replacereplace_if
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 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 N as std::distance(first, last):

1,2) Exactly N comparisons using operator==.
3,4) Exactly 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

 copies a range, replacing elements satisfying specific criteria with another value
(function template)
(C++20)(C++20)
 replaces all values satisfying specific criteria with another value
(niebloid)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/replace&oldid=171914"