Thread: Triangular number

  1. #1
    Registered User
    Join Date
    Jun 2019

    Triangular number

    Triangular number is the amount of point that can fill equilateral triangle. For example number 6 is triangular number because all sides of triangle has the same amount of point. The problem for this task is to solve equation T(n) = n * (n + 1) / 2. t(n) is given we are searching n. My code below
    #include <stdbool.h>
    bool is_triangular(int t) {
    unsigned long long int delta = 1+8*t;  
    double x=(-1+sqrt(delta))/2.0;
          return true;
    return false;

    It gives me proer solutions except t==458000245 and for t==453351216. I don't know where is the problem.

  2. #2
    Registered User
    Join Date
    Feb 2019
    int is 32 bits long (UINT_MAX = 4294967295)
    long long is 64 bits long
    double is 53. Try:

    bool is_triangular( unsigned long long t )
      unsigned long long delta = 1ULL+8ULL*t;
      long double x = (-1.0L + sqrtl(delta)) / 2.0L;
      return floorl( x ) == x;

  3. #3
    Registered User
    Join Date
    Jun 2019

    Problem solved

    Thank you for answer

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