C++ algorithm std::for_each Function Template

C++ <algorithm> std::for_each

The std::for_each function template in C++ applies a specified function to each element in a given range. It is part of the C++ Standard Library’s <algorithm> header and is useful for performing operations on each element of a container without explicitly writing a loop.

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Syntax of std::for_each

template <class InputIterator, class Function>
Function for_each(InputIterator first, InputIterator last, Function fn);

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Parameters of std::for_each

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).
fn	A unary function that accepts an element in the range as an argument. The function can modify the elements if the input iterator points to a modifiable sequence.

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Return Value of std::for_each

Returns the function object fn after it has been applied to each element in the range [first, last). This allows fn to maintain state that can be examined after the algorithm completes.

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Exceptions for std::for_each

Throws if the application of fn to an element throws an exception, or if any operation on the iterators throws. Note that invalid arguments cause undefined behavior.

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Examples for for_each Function

Example 1: Using std::for_each to Print Elements

In this example, we use std::for_each to print each element of a vector.

Program

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

void print_element(int n) {
    std::cout << n << " ";
}

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    std::for_each(numbers.begin(), numbers.end(), print_element);
    std::cout << std::endl;

    return 0;
}

Output

1 2 3 4 5 

Explanation

1. We include the necessary headers: <iostream> for input/output operations, <vector> for the std::vector container, and <algorithm> for the std::for_each function.
2. We define a function print_element that takes an integer and prints it followed by a space.
3. We create a vector numbers containing the integers 1 through 5.
4. We use std::for_each to apply print_element to each element in the vector.
5. After the loop, we print a newline character to ensure the output is properly formatted.

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Example 2: Using std::for_each with a Lambda Function

This example demonstrates how to use std::for_each with a lambda function to increment each element in a vector.

Program

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

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};

    std::for_each(numbers.begin(), numbers.end(), [](int &n) {
        n += 1;
    });

    for (int n : numbers) {
        std::cout << n << " ";
    }
    std::cout << std::endl;

    return 0;
}

Output

2 3 4 5 6 

Explanation

1. We include the necessary headers: <iostream> for input/output operations, <vector> for the std::vector container, and <algorithm> for the std::for_each function.
2. We create a vector numbers containing the integers 1 through 5.
3. We use std::for_each with a lambda function that takes an integer reference and increments it by 1.
4. We then use a range-based for loop to print each element in the modified vector.
5. After the loop, we print a newline character to ensure the output is properly formatted.

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Example 3: Using std::for_each to Accumulate a Sum

In this example, we use std::for_each to calculate the sum of elements in a vector by capturing a local variable in a lambda function.

Program

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

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5};
    int sum = 0;

    std::for_each(numbers.begin(), numbers.end(), [&sum](int n) {
        sum += n;
    });

    std::cout << "Sum: " << sum << std::endl;

    return 0;
}

Output

Sum: 15

Explanation

1. We include the necessary headers: <iostream> for input/output operations, <vector> for the std::vector container, and <algorithm> for the std::for_each function.
2. We create a vector numbers containing the integers 1 through 5.
3. We declare an integer variable sum and initialize it to 0. This variable will store the sum of all elements in the vector.
4. We use std::for_each with a lambda function that captures sum by reference ([&sum]). The lambda function takes each element of the vector as input and adds it to sum.
5. After the std::for_each call, sum contains the total sum of all elements in the vector.
6. We print the value of sum to the console using std::cout.

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Examples for Exception Handling in std::for_each

The std::for_each function can throw exceptions if the provided function object (fn) or any operation on the iterators throws an exception. Here are examples demonstrating these scenarios:

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Example 1: Function Object Throws an Exception

This example demonstrates a case where the function object passed to std::for_each throws an exception during execution.

Program

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

void faulty_function(int n) {
    if (n < 0) {
        throw std::runtime_error("Negative number encountered.");
    }
    std::cout << n << " ";
}

int main() {
    std::vector numbers = {1, 2, -3, 4, 5};

    try {
        std::for_each(numbers.begin(), numbers.end(), faulty_function);
    } catch (const std::runtime_error& e) {
        std::cout << "\nException caught: " << e.what() << std::endl;
    }

    return 0;
}

Output

1 2 
Exception caught: Negative number encountered.

Explanation

1. The program defines a function faulty_function that throws a std::runtime_error if a negative number is encountered.
2. A vector numbers is initialized, including a negative number.
3. The std::for_each function applies faulty_function to each element in the vector. When it encounters the negative number, the function 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: Iterator Throws an Exception

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.");
        }
        return *ptr;
    }

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

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

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

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

void print_element(int n) {
    std::cout << n << " ";
}

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

    try {
        std::for_each(
            FaultyIterator(arr, true), 
            FaultyIterator(arr + 3), 
            print_element
        );
    } catch (const std::runtime_error& e) {
        std::cout << "\nException caught: " << e.what() << std::endl;
    }

    return 0;
}

Output

1 2 
Exception caught: Iterator error.

Explanation

1. The program defines a custom iterator FaultyIterator that throws a std::runtime_error after iterating over two elements.
2. An array arr is initialized with three elements.
3. The std::for_each function applies the print_element function to the elements. The custom iterator throws an exception during iteration.
4. The exception is caught in the try-catch block, and an error message is printed.

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