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C++ Algorithm Library - find_if() Function

The C++ function std::algorithm::find_if() finds the first occurrence of the element that satisfies the condition. It uses unary predicate to specify condition.

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

template <class InputIterator, class UnaryPredicate>
InputIterator find_if (InputIterator first, InputIterator last, 
   UnaryPredicate pred);

• first − Input iterator to the initial position.

• last − Final iterator to the final position.

• pred − A unary predicate which accepts one argument and returns bool.

returns an iterator to the first element in the range (first,last) for which unary predicate returns true. If no such element is found, the function returns last.

Throws exception if either predicate or an operation on an iterator throws exception.

Please note that invalid parameters cause undefined behavior.

Linear i.e. O(n)

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

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

using namespace std;

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

int main(void) {
   vector<int> v = {10, 2, 3, 4, 5};
   auto it = find_if(v.begin(), v.end(), unary_pre);

   if (it != end(v))
      cout << "First even number is " << *it << endl;

   v = {1};

   it = find_if(v.begin(), v.end(), unary_pre);

   if (it == end(v))
      cout << "Only odd elements present in the sequence." << endl;

   return 0;
}

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

First even number is 10
Only odd elements present in the sequence.

algorithm.htm

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