CODEWITHSUNDEEP
c
Muhammad Nazmi
Muhammad Nazmi

Reputation: 43

Validating a triangle in C

I'm new in C, so I try to create a program that calculate the area of triangle as a start.

Calculating the area is easy when the triangle exists, however the validation of straight line is working partially.

Example:

A(0,-4) B(1,0) C(4,12) does not produce straight line error.

but A(4,12) B(1,0) C(0,-4) produce straight line error.

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

double square(double num){
    return (num*num);
}

int main()
{

    double x[5],y[5],a,b,c,angle,area;

    printf("Hello there! Calculating the area of triangle.\n");

    printf("Enter a coordinate A :\n");
    scanf("%lf,%lf",&x[0],&y[0]);

    printf("Enter another coordinate B :\n");
    scanf("%lf,%lf",&x[1],&y[1]);

    printf("Enter another coordinate C :\n");
    scanf("%lf,%lf",&x[2],&y[2]);

    // AB as base (a) , c is opposite side 

    a = sqrt( square((x[0]-x[1])) + square((y[0]-y[1])) );
    b = sqrt( square((x[0]-x[2])) + square((y[0]-y[2])) );
    c = sqrt( square(x[1]-x[2]) + square((y[1]-y[2])) );

    double num = (square(a)+square(b)-square(c))/(2*a*b);
    angle = acos(num);
    area = .5*a*b*sin(angle);

   //printf("%lf %lf %lf %lf %lf \n",a,b,c,num,angle);

    if (num == 1 || num ==-1){
        printf("That's a straight line.");
    }else{
        printf("Area of triangle is %lf\n",area);
    }

    return 0;

}

Upvotes: 2

Views: 434

Answers (2)

doca
doca

Reputation: 1558

The problem in your code is double num = (square(a)+square(b)-square(c))/(2*a*b); gets evaluated to a number slightly larger than 1 in some cases. This can happen with floating point computations. In your case you can safely add if (num > 1) num = 1; after that line since cosine equation always will give you a value larger than 0 and less than 1 for triangles.

However there's a problem if two points overlap and a or b becomes zero. You will have to check for that and handle it as a special case if such input is expected by your code. (If two points overlap then they are collinear anyways. You can check the overlap by checking if any of a,b,c are zero)

Upvotes: 2

jxh
jxh

Reputation: 70502

You can use a different test, and perhaps a different area formula. If you use Heron's formula, then once you have the lengths of the sides a, b, and c, you can compute:

double p = (a + b + c)/2;
double area = sqrt(p*(p-a)*(p-b)*(p-c));

You can detect if the triangle is valid by checking that p is greater than each of the sides.

double p = (a + b + c)/2;
if ((p > a) && (p > b) && (p > c)) {
    double area = sqrt(p*(p-a)*(p-b)*(p-c));
    printf("Area of triangle is %lf\n", area);
} else {
    printf("I don't consider that a triangle.\n");
}

Try it online!

Upvotes: 3

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