Range-based for loop in C++

In this topic, we will discuss the range-based for loop in the C++ programming language. The C++ language introduced a new concept of the range-based for loop in C++11 and later versions, which is much better than the regular For loop. A range-based for loop does not require large coding to implement for loop iteration. It is a sequential iterator that iterated each element of the container over a range (from beginning to end).

Syntax

range_declaration: It is used to declare a variable whose type is the same as the types of the collected elements represented by the range_expression or reference to that type.
range_expression: It defines an expression that represents the suitable sequence of elements.
loop statement: It defines the body of the range-based for loop that contains one or more statements to be repeatedly executed till the end of the range- expression.

Note: If we don't know the data type of the container elements, we can use the auto keyword that automatically identifies the data type of the range_expression.

Program to print each element of the array using-range based for loop

Let's consider an example to print the int and double array using the range-based for loop in C++.

program.cpp

#include <iostream>
using namespace std;
int main ()
{
int arr1 [5] = { 10, 20, 30, 40, 50};
double darr [5] = { 2.4, 4.5, 1.5, 3.5, 4.0 };

// use range based for loop
for ( const auto &var : arr1 )
{
cout << var << " " ;
}
// use auto keyword to automatically specify the data type of darr container. 
for ( const auto &var : darr )
{
cout << var << " " ;
}
return 0;
}

Output

10 20 30 40 50
2.4 4.5 1.5 3.5 4.0

Program to demonstrate the vector in range based for loop

Let's write a simple program to implement the vector in range based for loop.

Program2.cpp

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

int main()
{
	int x; // declare integer variable
	// declare vector variable
	vector <int> vect = {5, 10 , 25, 20, 25};
	
	// display vector elements
	for ( int x : vect)
	{
		cout << x << " ";
	}
	return 0;
}

Output

5 10 25 20 25

Program to print the arrays using Range based for loop in C++ with reference

Let's consider an example to print the array elements using range based for loop in C++.

Program3.cpp

#include <iostream>
#include <array>
 #include <cstdlib>
 using namespace std;

 int main(){
 array<int, 7> data = {1, 3, -2, 4, 6, 7, 9};
 cout << " Before updating the elements: " << endl;
 for (int x : data){
 cout << x << " ";
 }
 // pass the references
 for (int &itemRef : data){
 itemRef *= 3;
 }
 cout << endl << " After modification of the elements: " << endl;
 for (int x : data){
 cout << x << " ";
 }
 cout << endl;
 return 0;
 }

Output

Before updating the elements:
1 3 -2 4 6 7 9
 After modification of the elements:
3 9 -6 12 18 21 27

Nested range-based for loop

When a loop is defined inside the body of another loop, the loop is called a nested for loop. Similarly, when we define a range in a loop inside another range-based loop, the technique is known as a nested range-based for loop.

Syntax:

for ( int x : range_expression) // outer loop
{
for ( int y : range_expression) // inner loop
{
// statement to be executed
}
// statement to be executed
}

In the above syntax, we define one range-based for loop inside another loop. Here we call inner and outer range-based for loop in C++.

Program to print the nested range-based for loop in C++

Consider an example to demonstrate the nested range based for loop in C++ programming language.

Range.cpp

#include <iostream>
using namespace std;
int main ()
{
int arr1[4]  = { 0, 1, 2, 3 };
int arr2[5] = { 1, 2, 3, 4, 5 };
// use nested range based for loop
for ( int x : arr1 )
{
// declare nested loop
for ( int y : arr2 )
{
cout << " x = " << x << " and j = " << y << endl;
}
}
return 0;
}

Output

x = 0 and j = 1
 x = 0 and j = 2
 x = 0 and j = 3
 x = 0 and j = 4
 x = 0 and j = 5
 x = 1 and j = 1
 x = 1 and j = 2
 x = 1 and j = 3
 x = 1 and j = 4
 x = 1 and j = 5
 x = 2 and j = 1
 x = 2 and j = 2
 x = 2 and j = 3
 x = 2 and j = 4
 x = 2 and j = 5
 x = 3 and j = 1
 x = 3 and j = 2
 x = 3 and j = 3
 x = 3 and j = 4
 x = 3 and j = 5

What is the difference between traditional for loop and range-based for loop?

A traditional for loop is used to repeatedly execute the block of code till the specified condition is true. A traditional for loop has three parameters, initialization of the variable, specify the condition, and the last one is counter that is incremented by one if the condition remains true.

Syntax:

for ( variable_initialization; specify_condition; updated_counter)
{
// statement to be executed;
}

Range-based loop

On the other hand, we have a new range-based for loop available in the C++ 11 and later version. It has two parameters, range declaration, and the range_ expression. It is also used to repeatedly execute the block of code over a range.

Syntax

for ( range_declaration : range_ expression )
{
loop _statement;
// statement to be executed;
} 

The range_declaration is used to declare the type of variable related to the range_expression (container). The range_expression: It is just like a container that holds the same types of elements in a sequential manner. The loop_statement defines the statement which is executed inside for loop.

Advantages of the range-based for loop

It is easy to use, and its syntax is also simple.
A range-based for loop does not require the calculation of the number of elements in a containers
It recognizes the starting and ending elements of the containers.
We can easily modify the size and elements of the container.
It does not create any copy of the elements.
It is much faster than the traditional for loop.
It usually uses the auto keyword to recognize the data type of the container elements.