std::find_if , std::find_if_not in C++

std :: find_if and std :: find_if_not are algorithm functions in C++ Standard Library in <algorithm> header file. These functions provide an efficient way to search for an element in a container using a predicate function.

std :: find_if

This function returns an iterator to the first element in the range [first, last) for which pred(Unary Function) returns true. If no such element is found, the function returns last.

Syntax

InputIterator find_if (InputIterator first, InputIterator last, UnaryPredicate pred);

Parameters

• first, last: range which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
• pred: Unary function that accepts an element in the range as an argument and returns a value in boolean.

Return Value

• This function returns an iterator to the first element in the range [first, last) for which pred(function) returns true. If no such element is found, the function returns last.

Example

The following C++ program illustrates the use of the find_if() function.

C++

// CPP program to illustrate std::find_if
#include <bits/stdc++.h>
using namespace std;

// Returns true if argument is odd
bool IsOdd(int i) { return i % 2; }

// Driver code
int main()
{
    vector<int> vec{ 10, 25, 40, 55 };

    // Iterator to store the position of element found
    vector<int>::iterator it;

    // std::find_if
    it = find_if(vec.begin(), vec.end(), IsOdd);

    cout << "The first odd value is " << *it << '\n';

    return 0;
}

Output

The first odd value is 25

std :: find_if_not

This function returns an iterator to the first element in the range [first, last) for which pred(Unary Function) returns false. If no such element is found, the function returns last.

Syntax

InputIterator find_if_not (InputIterator first, InputIterator last, UnaryPredicate pred);

Parameters

• first, last: range which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.
• pred: Unary function that accepts an element in the range as an argument and returns a value in boolean.

Return value

• This function returns an iterator to the first element in the range [first, last) for which pred(function) returns false.

Example

The following C++ program illustrates the use of the find_if_not() function.

C++

// CPP program to illustrate std::find_if_not
#include <bits/stdc++.h>
using namespace std;
// Returns true if argument is odd

bool IsOdd(int i) { return i % 2; }

// Driver code
int main()
{
    vector<int> vec{ 10, 25, 40, 55 };

    // Iterator to store the position of element found
    vector<int>::iterator it;

    // std::find_if_not
    it = find_if_not(vec.begin(), vec.end(), IsOdd);

    cout << "The first non-odd(or even) value is " << *it
         << '\n';

    return 0;
}

Output

The first non-odd(or even) value is 10