C++ algorithm std::find Function Template

C++ <algorithm> std::find

The std::find function template in C++ is part of the <algorithm> header and is used to search for the first occurrence of a specified value within a range defined by two iterators. If the value is found, it returns an iterator pointing to the first occurrence; otherwise, it returns the end iterator.

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Syntax of std::find

template <class InputIterator, class T>
InputIterator find(InputIterator first, InputIterator last, const T& val);

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Parameters of std::find

Parameter	Description
first	An input iterator to the initial position in a sequence.
last	An input iterator to the final position in a sequence (one past the last element).
val	The value to search for in the range.

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Return Value of std::find

An iterator to the first element in the range that compares equal to val. If no such element is found, the function returns last.

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Exceptions for std::find

Throws if any of the element comparisons or operations on the iterators throw exceptions. Note that invalid parameters cause undefined behavior.

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Examples for algorithm find function

Example 1: Using std::find with an Array

In this example, we use std::find to search for a value in an array.

Program

#include <iostream>
#include <algorithm> // For std::find

int main() {
    int myints[] = {10, 20, 30, 40};
    int* p;

    p = std::find(myints, myints + 4, 30);
    if (p != myints + 4)
        std::cout << "Element found in myints: " << *p << '\n';
    else
        std::cout << "Element not found in myints\n";

    return 0;
}

Output

Element found in myints: 30

Explanation

1. We include the necessary headers: <iostream> for input/output operations and <algorithm> for the std::find function.
2. An array myints is initialized with the values {10, 20, 30, 40}.
3. We use std::find to search for the value 30 in the array. The function searches in the range [myints, myints + 4).
4. If the value is found, p points to the element, and we print its value. Otherwise, we print that the element was not found.

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Example 2: Using std::find with a Vector

This example demonstrates how to use std::find to search for a value in a std::vector.

Program

#include <iostream>
#include <vector>
#include <algorithm> // For std::find

int main() {
    std::vector<int> myvector = {10, 20, 30, 40};
    auto it = std::find(myvector.begin(), myvector.end(), 30);

    if (it != myvector.end())
        std::cout << "Element found in myvector: " << *it << '\n';
    else
        std::cout << "Element not found in myvector\n";

    return 0;
}

Output

Element found in myvector: 30

Explanation

1. We include the necessary headers: <iostream> for input/output operations, <vector> for the std::vector container, and <algorithm> for the std::find function.
2. A vector myvector is initialized with the values {10, 20, 30, 40}.
3. We use std::find to search for the value 30 in the vector. The function searches in the range [myvector.begin(), myvector.end()).
4. If the value is found, it points to the element, and we print its value. Otherwise, we print that the element was not found.

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Example 3: Using std::find with a List

In this example, we use std::find to search for a value in a std::list.

Program

#include <iostream>
#include <list>
#include <algorithm> // For std::find

int main() {
    std::list<int> mylist = {10, 20, 30, 40};
    auto it = std::find(mylist.begin(), mylist.end(), 25);

    if (it != mylist.end())
        std::cout << "Element found in mylist: " << *it << '\n';
    else
        std::cout << "Element not found in mylist\n";

    return 0;
}

Output

Element not found in mylist

Explanation

1. We include the necessary headers: <iostream> for input/output operations, <list> for the std::list container, and <algorithm> for the std::find function.
2. A list mylist is initialized with the values {10, 20, 30, 40}.
3. We use std::find to search for the value 25 in the list. The function searches in the range [mylist.begin(), mylist.end()).
4. Since the value 25 is not present in the list, it is equal to mylist.end().
5. The program prints “Element not found in mylist” because the value was not found.

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Examples for Exceptions Thrown by find function

The std::find function can throw exceptions in two primary scenarios:

• If the comparison between the elements throws an exception.
• If an operation on the iterators (such as dereferencing or incrementing) throws an exception.

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Example 1: Exception Thrown by Element Comparison

This example demonstrates a case where the comparison operation throws an exception during execution.

Program

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

class FaultyComparator {
public:
    bool operator()(int a, int b) const {
        if (a == -1 || b == -1) {
            throw std::runtime_error("Invalid value encountered during comparison.");
        }
        return a == b;
    }
};

int main() {
    std::vector numbers = {-1, 10, 20, 30};

    try {
        auto it = std::find_if(numbers.begin(), numbers.end(), [](int n) {
            if (n == -1) {
                throw std::runtime_error("Faulty value found during comparison.");
            }
            return n == 20;
        });
        if (it != numbers.end()) {
            std::cout << "Element found: " << *it << std::endl;
        } else {
            std::cout << "Element not found." << std::endl;
        }
    } catch (const std::runtime_error& e) {
        std::cout << "Exception caught: " << e.what() << std::endl;
    }

    return 0;
}

Output

Exception caught: Faulty value found during comparison.

Explanation

1. The program defines a lambda function inside std::find_if that throws an exception if the value -1 is encountered during the comparison.
2. A vector numbers is initialized, containing a faulty value -1.
3. The std::find_if function applies the lambda to each element. When it encounters the faulty value, the lambda throws an exception.
4. The exception is caught in the try-catch block, and an error message is printed.

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Example 2: Exception Thrown by an Iterator

This example demonstrates a case where a custom iterator throws an exception during iteration.

Program

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

class FaultyIterator {
public:
    using iterator_category = std::input_iterator_tag;
    using value_type = int;
    using difference_type = std::ptrdiff_t;
    using pointer = const int*;
    using reference = const int&;

    FaultyIterator(const int* ptr, bool fail_after = false)
        : ptr(ptr), fail_after(fail_after), count(0) {}

    reference operator*() const {
        if (fail_after && count >= 2) {
            throw std::runtime_error("Iterator error during dereference.");
        }
        return *ptr;
    }

    FaultyIterator& operator++() {
        ++ptr;
        ++count;
        return *this;
    }

    bool operator!=(const FaultyIterator& other) const {
        return ptr != other.ptr;
    }

private:
    const int* ptr;
    bool fail_after;
    int count;
};

int main() {
    int arr[] = {1, 2, 3};

    try {
        auto it = std::find(
            FaultyIterator(arr, true), 
            FaultyIterator(arr + 3), 
            3
        );

        if (it != FaultyIterator(arr + 3)) {
            std::cout << "Element found: " << *it << std::endl;
        } else {
            std::cout << "Element not found." << std::endl;
        }
    } catch (const std::runtime_error& e) {
        std::cout << "Exception caught: " << e.what() << std::endl;
    }

    return 0;
}

Output

Exception caught: Iterator error during dereference.

Explanation

1. The program defines a custom iterator FaultyIterator that throws a std::runtime_error exception after accessing two elements.
2. An array arr is initialized with three elements.
3. The std::find function searches for the value 3 in the range. The custom iterator throws an exception during the third access.
4. The exception is caught in the try-catch block, and an error message is printed.