---
title: "Check triangular number"
subject: "Computer Programming"
chapter: 5
key: "semester_1/computer_programming:triangular-number"
source: https://www.bctengineer.com/q/semester_1/computer_programming/triangular-number
site: BCT Engineering
---

# Check triangular number

Computer Programming · Chapter 5: Control statements

- Asked in: 2072 Chaitra, 6 marks
- Video: [C Program To Check Triangular Number Or Not (Hindi)](https://www.youtube.com/watch?v=Sc5e45Upiwk) — Computer Revival
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_1/computer_programming/triangular-number
- Whole chapter: https://www.bctengineer.com/md/semester_1/computer_programming/chapter-5

## Question

Write a program to check whether a given integer is a triangular number or not.
(Any number is a triangular number if the sum of natural number from 1 to any number i is exactly equal to that number. For e.g 1,3,6,10,15 etc are triangular number as, 1+2=3, 1+2+3=6, 1+2+3+4=10, 1+2+3+4+5=15)

## Short answer

Add natural numbers 1,2,3,... to sum until sum >= n. If sum equals n and n>0, it's triangular; else not.

## Answer

A triangular number is the sum 1 + 2 + ... + i for some i. The program keeps adding 1, 2, 3, ... to `sum` until `sum` reaches or passes the given number. If it stops at exactly `n`, the number is triangular.

```c
#include <stdio.h>
int main(void)
{
    int n, i = 1, sum = 0;
    printf("Enter an integer: ");
    scanf("%d", &n);
    while (sum < n)
    {
        sum += i;
        i++;
    }
    if (sum == n && n > 0)
        printf("%d is a triangular number\n", n);
    else
        printf("%d is not a triangular number\n", n);
    return 0;
}
```
Output for 15:
```text
Enter an integer: 15 is a triangular number
```

Check: 1+2+3+4+5 = 15. For 14 the sums go 1, 3, 6, 10, 15 (more than 14), so 14 is not a triangular number.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A triangular number is?
   A. Square
   B. Sum 1+2+...+i for some i
   C. Product
   D. Prime
   Answer: B. By definition. (Key point: Definition)

2. What does the while loop do?
   A. Subtracts
   B. Doubles
   C. Adds 1,2,3,... to sum until sum >= n
   D. Divides
   Answer: C. Stops at or past n. (Key point: Loop)

3. When is n triangular?
   A. sum > n
   B. i == n
   C. n even
   D. Loop stops with sum exactly equal to n
   Answer: D. Equality means it is a sum. (Key point: Decision)

4. Why is 14 not triangular?
   A. Sums go 10 then 15, skipping 14
   B. 14 is even
   C. 14 is composite
   D. 14>10
   Answer: A. 14 is never reached exactly. (Key point: 14)

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