//* Using functions - revisited to demo structures *
#include <stdio.h>
#include <math.h>

typedef struct POINT {  // a point on plains
    double x, y;
} POINT;

double sqr (double x);
double DIST(const POINT *a, const POINT *b);
double AREA(double a, double b, double c);

int main() {
    POINT p, q, r;
    double d1, d2, d3, area;

    printf("Please enter the coordinates of the triangle vertices and press ENTER.\n");
    scanf("%lf %lf %lf %lf %lf %lf", &p.x, &p.y, &q.x, &q.y, &r.x, &r.y);
    
    d1=DIST(&p,&q);
    d2=DIST(&p,&r);
    d3=DIST(&q,&r);
    area=AREA(d1, d2, d3);
    area=AREA(d1, d2, d3);

    printf("The area of the triangle is %lf\n",  area);
    return(0);
}

double sqr (double x) {
    return(x*x);
}

double DIST(const POINT *a, const POINT *b) {
    return( sqrt( sqr(a->x - b->x) + sqr(a->y - b->y) ) );
    // OR
    // return( sqrt( pow( fabs(a->x - b->x),2)+pow( fabs(a->y - b->y),2) ) );
    // warning: C library requires that the base in pow is >0 !!!
}

double AREA(double a, double b, double c) {
    // Hero's formula for the triangle area
    double p=(a+b+c)/2.0;
    double ar=sqrt(p*(p-a)*(p-b)*(p-c));
    return(ar);
}