std::replace,std::replace_if (3) - Linux Manuals

std::replace,std::replace_if: std::replace,std::replace_if

NAME

std::replace,std::replace_if - std::replace,std::replace_if

Synopsis

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

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.
              unary predicate which returns true if the element value should be replaced.
p - 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