C++ For Loop

C++ Loops

In computer programming, loops are used to repeat a block of code. Loops in Programming come into use when we need to repeatedly a block of statements.
For example, let's say we want to show a message 100 times. Then instead of writing the print statement 100 times, we can use a loop.
That was just a simple example; we can achieve much more efficiency and sophistication in our programs by making effective use of loops.

In programming, a loop is a sequence of instructions that is repeated until a certain condition is reached.

Types of Loops

There are 3 types of loops in C++:
1) for loop
2) while loop
3) do...while loop

This tutorial focuses on C++ for loop. We will learn about the other type of loops in the upcoming tutorials.

C++ for loop

• For loop is a Entry Controlled Loop or we can say repetition control structure.
• It allows us to write a loop that is executed a specific number of times.
• The loop enables us to perform 'n' number of steps together in one line.
• For loop is used to execute a set of statement repeatedly until a particular condition is satisfied.

The syntax of for-loop is:

for (initialization; condition; update) {
    // body of-loop 
}

Here,

• initialization - initializes variables and is executed only once
• condition - if true, the body of for loop is executed
  if false, the for loop is terminated
• update - updates the value of initialized variables and again checks the condition

To learn more about conditions, check out our tutorial on C++ Relational and Logical Operators.

Flowchart of for Loop in C++

Example 1: Print Numbers From 1 to 5

#include <iostream>     
using namespace std;
      
int main() {
        for (int i = 1; i <= 5; ++i) {
        cout << i << " ";
    }
    return 0;
}

Output

1 2 3 4 5

Here is how this program works

Example 2: Display a text 5 times

// C++ Program to display a "Hello World!" 5 times using For loop
      
#include <iostream>
      
using namespace std;
      
int main() {
    for (int i = 1; i <= 5; ++i) {
        cout <<  "Hello World! " << endl;
    }
    return 0;
}

Output

Hello World!
Hello World!
Hello World!
Hello World!
Hello World!

Here is how this program works

Example 3: Find the sum of first n Natural Numbers

// C++ program to find the sum of first n natural numbers using For Loop
// positive integers such as 1,2,3,...n are known as natural numbers
      
#include <iostream>

using namespace std;
      
int main() {
    int num, sum;
    sum = 0;
      
    cout << "Enter a positive integer: ";
    cin >> num;
      
    for (int count = 1; count <= num; ++count) {
        sum += count;
    }
      
    cout << "Sum = " << sum << endl;
      
    return 0;
}

Output

Enter a positive integer: 5
Sum = 15

In the above example, we have two variables num and sum. The sum variable is assigned with 0 and the num variable is assigned with the value provided by the user.

Note that we have used a for loop.

for(int count = 1; count <= num; ++count)

Here,

• int count = 1: initializes the count variable
• count <= num: runs the loop as long as count is less than or equal to num
• ++count: increase the count variable by 1 in each iteration

When count becomes 6, the condition is false and sum will be equal to 0 + 1 + 2 + ... + 5.

Nested For Loop

• A for loop within another for loop is called Nested For loop

The syntax of nested for loop is:

for (initialization; condition; update) {
        for (initialization; condition; update) {
                // body of inner for-loop 
        }
    // body of outer for-loop 
}

Example 4: Program to display a triangular pattern

// C++ program to display a triangular pattern 
// with 5 rows
                                            
#include <iostream>
using namespace std;
                                            
int main() {
                                            
   int rows = 5;
                                            
   for (int i = 1; i <= rows; ++i) {
      for (int j = 1; j <= i; ++j) {
         cout << "*  ";
      }
      cout << endl;
   }
    return 0;
}

Output

*    
*  *   
*  *  *  
*  *  *  * 
*  *  *  *  *

In this program, the outer loop iterates from 1 to rows.

The inner loop iterates from 1 to i. Inside the inner loop, we print the character '*'.

• Note: There is no rule that a loop must be nested inside its own type. In fact, there can be any type of loop nested inside any type and to any level.
• If you don't understand above example, We recommended you to know little more about Nested Loop.

Ranged Based for Loop

In C++11, a new range-based for loop was introduced to work with collections such as arrays and vectors. Its syntax is:

for (variable : collection) {
    // body of loop
}

Here, for every value in the collection, the for loop is executed and the value is assigned to the variable.

Example 5: Range Based for Loop

#include <iostream>
      
using namespace std;
      
int main() {
        
    int num_array[] = {1, 2, 3, 4, 5};
        
    for (int n : num_array) {
        cout << n << " ";
    }
    return 0;
}

Output

1 2 3 4 5

In the above program, we have declared and initialized an int array named num_array. It has 5 items.

Here, we have used a range-based for loop to access all the items in the array.

If you didn't understand the above example, don't worry we will discuss about Array in later Tutorials.

C++ Infinite for loop

If the condition in a for loop is always true, it runs forever (until memory is full). For example,

// infinite for loop
for(int i = 1; i > 0; i++) {
// block of code
}

In the above program, the condition is always true which will then run the code for infinite times.

Check out these examples to learn more:

• C++ Program to Calculate Sum of Natural Numbers
• C++ Program to Find Factorial
• C++ Program to Generate Multiplication Table

Next Tutorial

We hope that this tutorial helped you develop better understanding of the concept of For Loop in C++.

Keep Learning : )

In the next tutorial, you'll learn about C++ ranged-for Loop.