C Exercises: Display the sum of the series [ 9 + 99 + 999 + 9999 ...]

Last update on March 18 2025 13:35:41 (UTC/GMT +8 hours)

21. Sum of Series [9 + 99 + 999 + …]

Write a program in C to display the sum of the series [ 9 + 99 + 999 + 9999 ...].

This C program calculates and displays the sum of a series where each term is formed by repeating the digit 9 (e.g., 9, 99, 999, 9999, ...). The program prompts the user for the number of terms (n) and then computes the sum using a loop that generates each term and adds it to the total sum. The final sum is then printed to the console.

Visual Presentation:

Sample Solution:

C Code:

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

void main()
{
    long int n, i, t = 9; // Declare variables to store input, control loop indices, and temporary value.

    int sum = 0; // Initialize a variable to store the sum.

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

    for (i = 1; i <= n; i++) // Loop for the number of terms.
    {
        sum += t; // Add 't' to the sum.
        printf("%ld   ", t); // Print the current value of 't'.
        t = t * 10 + 9; // Update 't' for the next iteration.
    }

    printf("\nThe sum of the series = %d \n", sum); // Print the sum of the series.
}

Output:

Input the number or terms :5                                                                                  
9   99   999   9999   99999                                                                                   
The sum of the series = 111105  

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C program to generate the series 9, 99, 999, ... for n terms and compute their product.
• Write a C program to display the series 9, 99, 999, ... using string manipulation and then sum the series.
• Write a C program to calculate the sum of the series using arithmetic progression logic for digit repetition.
• Write a C program to generate the series and compute the average of the generated terms.

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