Thread: Rounding & DOUBLE type

Ive written the program below to determine types of triangle from their side lengths. The problem is that it cannot seem to determine when its a right angled triangle. It gives either acute (all angles less than 90 degrees) or obtuse (one angle greater than 90 degrees).

The side lengths 3, 4 and 5 will give one interior angle of exactly 90 degrees, yet the program produces 89.99999 etc. Im sure its a rounding problem, but can anyone help me rewrite the program to cure it?

Code:

#include <stdio.h>
#include <math.h>

int main()
{
 char another='Y';
 double a,b,c,A,B,C;

 printf("This is a program to categorise triangles by side length  and angle size\n");

 while(another == 'Y' || another == 'y')
  {
   printf("Enter 3 real numbers for each side of the triangle\n");
   scanf("%lf%lf%lf",&a,&b,&c);
   if(a>0 && b>0 && c>0)
   {
    A = (b*b + c*c - a*a)/(2*b*c); /* calculates the value of cosA */
    B = (a*a + c*c - b*b)/(2*a*c);
    C = (a*a + b*b - c*c)/(2*a*b);

    /* checks triangles are possible */
    if((A>-1 && A<1) && (B>-1 && B<1) && (C>-1 && C<1))	
			{
    printf("The triangle you have input has side lengths %lf, %lf, %lf\n\n",a,b,c);
	
    printf("Angle A = %lf radians",acos(A));
    printf(" or %lf degrees\n",acos(A)*(180/3.1415927));
    printf("Angle B = %lf radians",acos(B));
    printf(" or %lf degrees\n",acos(B)*(180/3.1415927));
    printf("Angle C = %lf radians",acos(C));
    printf(" or %lf degrees\n",acos(C)*(180/3.1415927));
    
    if(a==b && b==c && c==a)printf("\nTriangle is equilateral");
    if(a!=b && b!=c && c!=a)printf("\nTriangle is scalene");	
    if((a==b && c!=a) || (a==c && a!=b) || (b==c && b!=a))printf("\nTriangle is isoceles");
    if(acos(A)==acos(0)||acos(B)==acos(0)||acos(C)==acos(0))printf(" and right-angled\n\n");
    if(acos(A)>acos(0)||acos(B)>acos(0)||acos(C)>acos(0))printf(" and obtuse\n\n");
    else if(acos(A)<acos(0)&&acos(B)<acos(0)&&acos(C)<acos(0))printf(" and acute\n\n");
   }
   else printf("Triangle is impossible!\n\n");
   }
   else printf("Input is invalid, lengths must be real numbers\n\n");

  printf("Would you like to categorise another triangle?(Y or N)");
  another=getchar();
  while (another !='Y'&&another !='y'&&another !='n'&&another !='N')
  another=getchar();
  }
 printf("\nThis program is now ending\n\n");
 return 0;
}

the type of triangle in regards to its side lengths works fine, its the type of triangle in regards to its angle sizes that isnt working.

In the code i have if an angle = 90 then it outputs "right angled", but the program is working out an angle that SHOULD be 90 exactly as 89.99999. Is that because of rounding errors or because of the amount of decimal places i used Pi as? Is there a C function or operator for Pi?

>printf(" or %lf degrees\n",acos(A)*(180/3.1415927));
>printf(" or %lf degrees\n",acos(B)*(180/3.1415927));
>printf(" or %lf degrees\n",acos(C)*(180/3.1415927));

Try:
printf(" or %lf degrees\n",acos(A)*(180./M_PI));
printf(" or %lf degrees\n",acos(B)*(180./M_PI));
printf(" or %lf degrees\n",acos(C)*(180./M_PI));

Originally posted by quzah
> void main()

ARRRRG. Jesus, does everyone do this??

Quzah.

Hehe, could be a trend!