C++ Exercises: Find the sum of the series 1 +11 + 111 + 1111 + .. n terms

Last update on April 07 2025 13:01:26 (UTC/GMT +8 hours)

26. Sum of the Series [1 + 11 + 111 + ...]

Write a program in C++ to find the sum of the series 1 +11 + 111 + 1111 + .. n terms.

Visual Presentation:

Sample Solution:-

C++ Code :

#include <iostream> // Including input-output stream header

using namespace std; // Using standard namespace to avoid writing std::

int main() // Start of the main function
{
    int n, i, sum = 0; // Declaration of integer variables 'n' (number of terms), 'i' (loop counter), 'sum' (to store the sum)
    int t = 1; // Initializing variable 't' to 1

    cout << "\n\n Display the sum of the series 1 + 11 + 111 + 1111 + .. n terms:\n"; // Displaying a message on the console
    cout << "-------------------------------------------------------------------\n";
    cout << " Input number of terms: "; // Prompting the user to input the number of terms
    cin >> n; // Reading the input value as the number of terms

    for (i = 1; i <= n; i++) // Loop to generate the series and calculate the sum
    {
        cout << t << " "; // Displaying the current term of the series
        if (i < n) // Checking if it's not the last term to print '+'
        {
            cout << "+ "; // Displaying '+' to separate terms if it's not the last term
        }
        sum = sum + t; // Adding the current term to the sum
        t = (t * 10) + 1; // Generating the next term of the series by appending '1' to the current term
    }

    cout << "\n The sum of the series is: " << sum << endl; // Displaying the final sum of the series
}

Sample Output:

 Display the sum of the series 1 +11 + 111 + 1111 + .. n terms:        
-------------------------------------------------------------------    
 Input number of terms: 5                                              
1 + 11 + 111 + 1111 + 11111                                            
 The sum of the series is: 12345  

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C++ program to generate a series where each term is formed by repeating the digit 1, and compute the sum for n terms.
• Write a C++ program that reads a number n and prints the series 1, 11, 111, ... up to n terms, then outputs the total sum.
• Write a C++ program to construct numbers made entirely of 1's with increasing lengths and add them together.
• Write a C++ program that forms the series of repeated 1's for n terms and calculates their cumulative sum.

C++ Code Editor:

Click to Open Editor

Contribute your code and comments through Disqus.

Previous: Write a program in C++ to find the sum of the series [ x - x^3 + x^5 + ......].
Next: Write a program in C++ to display the first n terms of Fibonacci series.