c - I'm losing the type of my object at some point -

// construct array of length max_points. // populate array given points , initialize number points being handled. // print given points user. struct point points[max_points]; int numpoints = readpoints(points); printf("set of points: \n"); displaypoints(points, numpoints);  // construct array represent hull max possible points (numpoints). struct point hull[numpoints]; struct point pointonhull = leftmostpoint(points, numpoints); struct point endpoint; int e = 0;  // perform jarvis march. {     hull[e] = pointonhull;     endpoint = points[0];     ( int = 1; < numpoints; i++ )         if ( (endpoint == pointonhull) || (ccw(hull[e], endpoint, points[i]) > 0) )             endpoint = points[i];     ++e; } while ( endpoint != hull[0] );        // we've looped beginning.  // print mapped hull points. printf("convex hull: \n"); displaypoints(hull, numpoints); 

that's code jarvis march (gift wrapping) program solve convex hull. gcc raising errors on both of struct point comparisons (endpoint == pointonhull , endpoint != hull[0]). i'm brand new c, , 1 of first projects i'm doing play around language. can me see i'm messing up?

the specific errors:

jarvis.c:31:19: error: invalid operands binary == (have 'struct point' , 'struct point')

jarvis.c:34:21: error: invalid operands binary != (have 'struct point' , 'struct point')

there no comparison operator structs in c. have define own comparison function , use in place of ==. try this:

bool equalpoints(const struct point p1, const struct point p2) {     return (p1.x == p2.x) && (p1.y == p2.y); }  if (equalpoints(endpoint, pointonhull)) { /* code here */ } 

or if structure more complex (though, not case):

bool equalpoints(const struct point *p1, const struct point *p2) {     /*       * in case structure more complex here goes      * more complex comparison code      */     return (p1->x == p2->x) && (p1->y == p2->y); }  if (equalpoints(&endpoint, &pointonhull) { /* code here */ } 

the latter won't copy whole point structure pass equalpoints, pass pointer (reference type) instead. const important because won't let accidentally modify points when want compare them.