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find_if

Locates the position of the first occurrence of an element in a range that satisfies a specified condition.

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

first

An input iterator addressing the position of the first element in the range to be searched.

last

An input iterator addressing the position one past the final element in the range to be searched.

pred

User-defined predicate function object or lambda expression that defines the condition to be satisfied by the element being searched for. A predicate takes single argument and returns true (satisfied) or false (not satisfied). The signature of pred must effectively be bool pred(const T& arg);, where T is a type to which InputIterator can be implicitly converted when dereferenced. The const keyword is shown only to illustrate that the function object or lambda should not modify the argument.

An input iterator that refers to the first element in the range that satisfies the condition specified by the predicate (the predicate results in true). If no element is found to satisfy the predicate, returns last.

This template function is a generalization of the algorithm find, replacing the predicate "equals a specific value" with any predicate. For the logical opposite (find the first element that does not satisfy the predicate), see find_if_not.

// cl.exe /W4 /nologo /EHsc /MTd
#include <vector>
#include <algorithm>
#include <iostream>
#include <string>

using namespace std;

template <typename S> void print(const S& s) {
    for (const auto& p : s) {
        cout << "(" << p << ") ";
    }
    cout << endl;
}

// Test std::find()
template <class InputIterator, class T>
void find_print_result(InputIterator first, InputIterator last, const T& value) {

    // call <algorithm> std::find()
    auto p = find(first, last, value);

    cout << "value " << value;
    if (p == last) {
        cout << " not found." << endl;
    } else {
        cout << " found." << endl;
    }
}

// Test std::find_if()
template <class InputIterator, class Predicate>
void find_if_print_result(InputIterator first, InputIterator last,
    Predicate Pred, const string& Str) {

    // call <algorithm> std::find_if()
    auto p = find_if(first, last, Pred);

    if (p == last) {
        cout << Str << " not found." << endl;
    } else {
        cout << "first " << Str << " found: " << *p << endl;
    }
}

// Function to use as the UnaryPredicate argument to find_if() in this example
bool is_odd_int(int i) {
    return ((i % 2) != 0);
}

int main()
{
    // Test using a plain old array.
    const int x[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    cout << "array x[] contents: ";
    print(x);
    // Using non-member std::begin()/std::end() to get input iterators for the plain old array.
    cout << "Test std::find() with array..." << endl;
    find_print_result(begin(x), end(x), 10);
    find_print_result(begin(x), end(x), 42);
    cout << "Test std::find_if() with array..." << endl;
    find_if_print_result(begin(x), end(x), is_odd_int, "odd integer"); // function name
    find_if_print_result(begin(x), end(x), // lambda
        [](int i){ return ((i % 2) == 0); }, "even integer");

    // Test using a vector.
    vector<int> v;
    for (int i = 0; i < 10; ++i) {
        v.push_back((i + 1) * 10);
    }
    cout << endl << "vector v contents: ";
    print(v);
    cout << "Test std::find() with vector..." << endl;
    find_print_result(v.begin(), v.end(), 20);
    find_print_result(v.begin(), v.end(), 12);
    cout << "Test std::find_if() with vector..." << endl;
    find_if_print_result(v.begin(), v.end(), is_odd_int, "odd integer");
    find_if_print_result(v.begin(), v.end(), // lambda
        [](int i){ return ((i % 2) == 0); }, "even integer");
}

array x[] contents: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Test std::find() with array...
value 10 found.
value 42 not found.
Test std::find_if() with array...
first odd integer found: 1
first even integer found: 2

vector v contents: (10) (20) (30) (40) (50) (60) (70) (80) (90) (100)
Test std::find() with vector...
value 20 found.
value 12 not found.
Test std::find_if() with vector...
odd integer not found.
first even integer found: 10

Header: <algorithm>

Namespace: std

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