std::string::replace_copy(), std::string::replace_copy_if in C++

replace_copy

replace_copy() is a combination of copy() and replace(). 

• It copies the elements in the range [first, last) to the range beginning at result, replacing the appearances of old_value by new_value.
• The range copied is [first, last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
• The ranges shall not overlap in such a way that result points to an element in the range [first, last).
• The function uses operator== to compare the individual elements to old_value.

template <class InputIterator, class OutputIterator, class T>
  OutputIterator replace_copy (InputIterator first, InputIterator last,
                               OutputIterator result,
                               const T& old_value, const T& new_value); 
first, last : Input iterators to the initial and final positions 
in a sequence. 
old_value : Value to be replaced.
new_value : Replacement value.

Returns the position after the last copied element in the destination
range (the first element that is not overwritten). 

// CPP to illustrate
// replace_copy

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

// Function to replace 'A' at v.begin() with Z and copy it
// to v.begin() position
void replace_copyDemo(vector<char>& v)
{
    replace_copy(v.begin(), v.begin()+1, 
                    v.begin(), 'A', 'Z' );
}

// Function to print content of vector
void print(vector<char>& v)
{
    int len = v.size();
    for (int i = 0; i < len; i++)
        cout << v[i] << " ";
    cout << endl;
}

// Driver code
int main()
{

    vector<char> v;

    for (int i = 0; i <= 6; i++)
        v.push_back('A' + i);
    cout << "Before replace_copy " <<": "; 
    print(v);
    replace_copyDemo(v);
    
    cout << "After replace_copy " << ": ";
    print(v);

    return 0;
}

Output: 

Before replace_copy : A B C D E F G 
After replace_copy : Z B C D E F G 

replace_copy_if

replace_copy_if() is a combination of copy() and replace_if().  

• Copies the elements in the range [first, last) to the range beginning at result, replacing those for which pred returns true by new_value.
• In simpler terms, it Copies the elements of a sequence to another same-size sequence replacing any elements that satisfies a predicate, with another value.
• Returns the position after the last copied element in the destination range (the first element that is not overwritten). 

Template <class InputIterator, class OutputIterator, class UnaryPredicate, class T>
  OutputIterator replace_copy_if (InputIterator first, InputIterator last,
                                  OutputIterator result, UnaryPredicate pred,
                                  const T& new_value);
first, last : Input iterators to the initial and final positions 
in a sequence. 
old_value : Value to be replaced.
new_value : Replacement value.
pred : Unary function that accepts an element in the range as argument, 
and returns a value convertible to bool. The value returned indicates whether the 
element is to be replaced in the copy (if true, it is replaced).

// CPP code to illustrate
// replace_copy_if
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

// Function to check if number is even
int IsEven(int i)
{
    return ((i % 2) == 0);
}

// Function to print content of vector
void print(vector<int>& v)
{
    int len = v.size();
    for (int i = 0; i < len; i++)
        cout << v[i] << " ";
    cout << endl;
}

// Function to replace all 
// even numbers from vector v1 and
// copying them to v2
void replace_copy_ifDemo(vector<int>& v1, 
                            vector<int>& v2)
{
    replace_copy_if(v1.begin(), v1.end(), 
                       v2.begin(), IsEven, 0);
}

// Driver Code
int main()
{
    vector<int> v1, v2;

    for (int i = 1; i <= 10; i++)
        v1.push_back(i);

    cout << "Before replace_copy_if : ";
    print(v1);

    v2.resize(v1.size()); // allocate space
    replace_copy_ifDemo(v1, v2);

    cout << "After replace_copy_if : ";
    print(v2);

    return 0;
}

Output: 

Before replace_copy_if : 1 2 3 4 5 6 7 8 9 10 
After replace_copy_if : 1 0 3 0 5 0 7 0 9 0 

NOTE: Both algorithms i.e. replace_copy and replace_copy_if, returns the position after the last copied element in the destination range (the first element that is not overwritten).
Related Article : std::string::replace, std::string::replace_if
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