std::replace, std::replace_if

< cpp‎ | algorithm
Revision as of 09:33, 7 May 2014 by (Talk)

Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
Modifying sequence operations
(until C++17)
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations

Numeric operations
C library
Defined in header <algorithm>
template< class ForwardIt, class T >

void replace( ForwardIt first, ForwardIt last,

              const T& old_value, const T& new_value );
template< class ForwardIt, class UnaryPredicate, class T >

void replace_if( ForwardIt first, ForwardIt last,

                 UnaryPredicate p, const T& new_value );

Replaces all elements satisfying specific criteria with new_value in the range [first, last). The first version replaces the elements that are equal to old_value, the second version replaces elements for which predicate p returns true.



first, last - the range of elements to process
old_value - the value of elements to replace
p - unary predicate which returns ​true if the element value should be replaced.

The signature of the predicate function should be equivalent to the following:

 bool pred(const Type &a);

The signature does not need to have const &, but the function must not modify the objects passed to it.
The type Type must be such that an object of type ForwardIt can be dereferenced and then implicitly converted to Type. ​

new_value - the value to use as replacement
Type requirements
ForwardIt must meet the requirements of ForwardIterator.
UnaryPredicate must meet the requirements of Predicate.

Return value



Exactly last - first applications of the predicate.

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;


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.

#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';


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

See also

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