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# std::replace, std::replace_if

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class ForwardIt, class T > void replace( ForwardIt first, ForwardIt last, const T& old_value, const T& new_value ); | (1) |
template< class ForwardIt, class UnaryPredicate, class T > void replace_if( ForwardIt first, ForwardIt last, UnaryPredicate p, const T& new_value ); | (2) |

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.

### [edit] Parameters

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`.

### [edit] Return value

(none)

### [edit] Complexity

Exactly `last - first` applications of the predicate.

### [edit] 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;
            }
        }
    }

### [edit] 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

### [edit] See also

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

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