# C++ Algorithm Library - copy_if() Function

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## Description

The C++ function std::algorithm::copy_if() copies a range of elements to a new location if predicate returns true for value.

## Declaration

Following is the declaration for std::algorithm::copy_if() function form std::algorithm header.

### C++11

```template <class InputIterator, class OutputIterator, class UnaryPredicate>
OutputIterator copy_if(InputIterator first,InputIterator last,
OutputIterator result,UnaryPredicate pred);
```

## Parameters

• first − Input iterators to the initial positions of the searched sequence.

• last − Input iterators to the final positions of the searched sequence.

• result − Output iterator to the initial position in the new sequence.

• pred − Unary predicate which takes an argument and return a bool value.

## Return value

Returns an iterator pointing to the element that follows the last element written in the result sequence.

## Exceptions

Throws an exception if either element assignment or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

## Time complexity

Linear in the distance between first to last.

## Example

The following example shows the usage of std::algorithm::copy_if() function.

```#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

bool predicate(int n) {
return (n %2 != 0);
}

int main(void) {
vector<int> v1 = {1, 2, 3, 4, 5};
vector<int> v2(3);

copy_if(v1.begin(), v1.end(), v2.begin(), predicate);

cout << "Following are the Odd numbers from vector" << endl;

for (auto it = v2.begin(); it != v2.end(); ++it)
cout << *it << endl;

return 0;
}
```

Let us compile and run the above program, this will produce the following result −

```Following are the Odd numbers from vector
1
3
5
```
algorithm.htm