std::replace and std::replace_if in C++ (original) (raw)

Last Updated : 26 Apr, 2022

std::replace

Assigns new_value to all the elements in the range [first, last) that compare to old_value. The function use operator == to compare the individual elements to old_value

Function Template :

void replace (ForwardIterator first, ForwardIterator last, const T& old_value, const T& new_value) first, last : Forward iterators to the initial and final positions in a sequence of elements. old_value : Value to be replaced. new_value : Replacement value.

Return Value :
This function do not return any value. If elements that needs to be replace is found then element
replaced otherwise remain unchanged.

Examples:

Input : 10 20 30 30 20 10 10 20 Output : 10 99 30 30 99 10 10 99 // Replaced value 20 in vector to 99.

Input : 3 5 7 8 9 5 4 Output : 3 5 7 12 9 5 4
// Replaced value 8 by 12.

CPP `

// CPP program to find and replace the value // with another value in array // using std::replace #include <bits/stdc++.h> using namespace std;

// Driver code int main() { int arr[] = { 10, 20, 30, 30, 20, 10, 10, 20 }; int n = sizeof(arr) / sizeof(arr[0]);

// variable containing the old and new values
int old_val = 20, new_val = 99;

// print old array
cout << "Original Array:";
for (int i = 0; i < n; i++)
    cout << ' ' << arr[i];
cout << '\n';

// Function used to replace the values
replace(arr, arr + n, old_val, new_val);

// new array after using std::replace
cout << "New Array:";
for (int i = 0; i < n; i++)
    cout << ' ' << arr[i];
cout << '\n';

return 0;

}

`

Output

Original Array: 10 20 30 30 20 10 10 20 New Array: 10 99 30 30 99 10 10 99

std::replace_if

Assigns new_value to all the elements in range [first, last) for which pred returns true.
Function Template :

void replace_if (ForwardIterator first, ForwardIterator last, UnaryPredicate pred, const T& new_value) first, last : Forward iterators to the initial and final positions in a sequence of elelments. pred : Unary function that accepts an element in the range as argument, and returns a value convertible to bool.The returned value indicate whether the element is to be replaced (if true, it is replaced). The function shall not modify its argument. old_value : Value to be replaced. new_value : Replacement value.

Examples:

Input : 1 2 3 4 5 6 7 8 9 10
Output : 0 2 0 4 0 6 0 8 0 10
// Replaced all odd values to 0.

Input : 10 20 30 30 20 10 10 20 Output : 10 4 30 30 4 10 10 4
// Replaced all number divisible by 4 to 4.

CPP `

// CPP code to find all the elements that are odd // and replace them with 0. // using std::replace_if #include <bits/stdc++.h> using namespace std;

// Function that is used in std::replace_if // If number is odd return 1, else 0 // 1 (True) means replace the number // 0 (False) means does not replace bool IsOdd(int i) { return ((i % 2) == 1); }

// Driver code int main() { int arr[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; int n = sizeof(arr) / sizeof(arr[0]);

// print old array
cout << "Original Array:";
for (int i = 0; i < n; i++)
    cout << ' ' << arr[i];
cout << '\n';

// replacement value
int new_val = 0;

// replace_if function
replace_if(arr, arr + n, IsOdd, new_val);

// new array after using std::replace
cout << "New Array:";
for (int i = 0; i < n; i++)
    cout << ' ' << arr[i];
cout << '\n';

return 0;

}

`

Output

Original Array: 1 2 3 4 5 6 7 8 9 10 New Array: 0 2 0 4 0 6 0 8 0 10

Also you can add any kind of function in std::replace_if that can only have one argument only.