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.
// 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;
} |
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)
// 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 ;
}
} |
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.
#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;
} |
2 3 4 5 6