for_each loop in C++

Apart from the generic looping techniques, such as “for, while and do-while”, C++ in its language also allows us to use another functionality which solves the same purpose termed “for-each” loops. This loop accepts a function which executes over each of the container elements. This loop is defined in the header file “algorithm”: #include<algorithm>, and hence has to be included for successful operation of this loop.

• It is versatile, i.e.  Can work with any container.
• It reduces chances of errors one can commit using generic for loop
• It makes code more readable
• for_each loops improve overall performance of code

Syntax:

for_each (InputIterator start_iter, InputIterator last_iter, Function fnc)

start_iter : The beginning position 
from where function operations has to be executed.
last_iter : The ending position 
till where function has to be executed.
fnc/obj_fnc : The 3rd argument is a function or 
an object function which operation would be applied to each element. 

CPP

// C++ code to demonstrate the
// working of for_each loop

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

// helper function 1
void printx2(int a)
{
    cout << a * 2 << " ";
}

// helper function 2
// object type function
struct Class2
{
    void operator() (int a)
    {
        cout << a * 3 << " ";
    }
} ob1;


int main()
{

    // initializing array
    int arr[5] = { 1, 5, 2, 4, 3 };

    cout << "Using Arrays:" << endl;

    // printing array using for_each
    // using function
    cout << "Multiple of 2 of elements are : ";
    for_each(arr, arr + 5, printx2);

    cout << endl;

    // printing array using for_each
    // using object function
    cout << "Multiple of 3 of elements are : ";
    for_each(arr, arr + 5, ob1);

    cout << endl;

    // initializing vector
    vector<int> arr1 = { 4, 5, 8, 3, 1 };

    cout << "Using Vectors:" << endl;


    // printing array using for_each
    // using function
    cout << "Multiple of 2 of elements are : ";
    for_each(arr1.begin(), arr1.end(), printx2);

    cout << endl;

    // printing array using for_each
    // using object function
    cout << "Multiple of 3 of elements are : ";
    for_each(arr1.begin(), arr1.end(), ob1);

    cout << endl;

}

Output

Using Arrays:
Multiple of 2 of elements are : 2 10 4 8 6 
Multiple of 3 of elements are : 3 15 6 12 9 
Using Vectors:
Multiple of 2 of elements are : 8 10 16 6 2 
Multiple of 3 of elements are : 12 15 24 9 3 

Exceptions and for_each:

In the cases of exceptions, if the function throws an exception or if any of the operations on iterators throws an exception, for_each loop will also throw an exception and break/terminate the loop. 

Note: 

• Invalid arguments may leads to Undefined behavior.
• For_each can not work with pointers of an array (An array pointer do not know its size, for_each loops will not work with arrays without knowing the size of an array)

CPP

// C++ code to demonstrate the working
// of for_each with Exception

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;

// Helper function 1
void printx2(int a)
{
    cout << a * 2 << " ";
    if ( a % 2 == 0)
    {
        throw a;
    }

}

// Helper function 2
// object type function
struct Class2
{
    void operator() (int a)
    {
        cout << a * 3 << " ";
        if ( a % 2 == 0)
        {
            throw a;

        }
    }
} ob1;


int main()
{

    // Initializing array
    int arr[5] = { 1, 5, 2, 4, 3 };

    cout << "Using Array" << endl;

    // Printing Exception using for_each
    // using function
    try
    {
        for_each(arr, arr + 5, printx2);
    }
    catch(int i)
    {
        cout << "\nThe Exception element is : " << i ;
    }
    cout << endl;

    // Printing Exception using for_each
    // using object function
    try
    {
        for_each(arr, arr + 5, ob1);
    }
    catch(int i)
    {
        cout << "\nThe Exception element is : " << i ;
    }

    // Initializing vector
    vector<int> arr1 = { 1, 3, 6, 5, 1 };

    cout << "\nUsing Vector" << endl;

    // Printing Exception using for_each
    // using function
    try
    {
        for_each(arr1.begin(), arr1.end(), printx2);
    }
    catch(int i)
    {
        cout << "\nThe Exception element is : " << i ;
    }
    cout << endl;

    // printing Exception using for_each
    // using object function
    try
    {
        for_each(arr1.begin(), arr1.end(), ob1);
    }
    catch(int i)
    {
        cout << "\nThe Exception element is : " << i ;
    }
}

Output

Using Array
2 10 4 
The Exception element is : 2
3 15 6 
The Exception element is : 2
Using Vector
2 6 12 
The Exception element is : 6
3 9 18 
The Exception element is : 6

Using Lambdas:

With the introduction of lambda functions, this can be easily used to make the whole thing inline which is very compact and useful for people looking for using functional programming.

C++

#include <bits/stdc++.h>
#include <iostream>
using namespace std;

int main()
{

    vector<int> vec{ 1, 2, 3, 4, 5 };

    // this increases all the values in the vector by 1;
    for_each(vec.begin(), vec.end(), [](int& a) { a++; });

    // this prints all the values in the vector;
    for_each(vec.begin(), vec.end(),
             [](int a) { cout << a << " " << endl; });

    return 0;
}

Output

2 
3 
4 
5 
6 

This article is contributed by Astha Tyagi. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. 
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