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

Last update on March 18 2025 13:35:38 (UTC/GMT +8 hours)

26. Sum of Series 1 + 11 + 111 + …

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

This C program calculates the sum of a series where each term is a number composed of repeated digits '1' (i.e., 1, 11, 111, 1111, etc.). The user inputs the number of terms n, and the program then computes and sums these terms, displaying the result. This involves generating each term based on its position and summing them up sequentially.

Visual Presentation:

Calculate the sum of the series 1 +11 + 111 + 1111 + .. n terms

Sample Solution:

C Code:

#include <stdio.h> // Include the standard input/output header file.

void main()
{
  int n, i;  // Declare variables for loop control and input.
  long sum = 0; // Initialize a variable to hold the sum of the series.
  long int t = 1; // Initialize a variable to generate the series.

  printf("Input the number of terms : "); // Prompt the user for input.
  scanf("%d", &n); // Read the value of 'n' from the user.

  for(i = 1; i <= n; i++) // Loop to generate and sum the series.
  {
     printf("%ld  ", t); // Print the current term of the series.

      if (i < n)
      {
          printf("+ "); // Print addition sign between terms (except the last term).
      }

     sum = sum + t; // Add the current term to the running sum.
     t = (t * 10) + 1; // Generate the next term of the series.
  }

  printf("\nThe Sum is : %ld\n", sum); // Print the final sum of the series.
}

Output:

Input the number of terms : 5                                                                                 
1 + 11 + 111 + 1111 + 11111                                                                                  
The Sum is : 12345

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to generate the series 1, 11, 111, … for n terms and compute their product.
Write a C program to display the series using string concatenation and then compute the total sum.
Write a C program to generate the series and then calculate the average of its terms.
Write a C program to display the series and then reverse each term before summing them.

C Programming Code Editor:

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