我想得到 C 中点数组的凸包。我有一个结构：
struct points{
int i;
int x;
int y;
};
其中i是标签，x和y是坐标。
我创建了一个结构数组。然后我把它排序为增加 x 值和增加 y=值。现在，我制作了一个作为堆栈实现的链表。当 3 点不右转时，我会弹出重点。然后推送数组中的下一个点。但我不能正确地做到这一点。
这是我的代码。
struct node* top = NULL;
for (a = 0; a <= sizeof(pt); a++){
for (b = a + 1; b <= sizeof(pt); b++){
for (j= b+1; j <= sizeof(pt); j++){
if(((pt[b].x - pt[a].x)*(pt[j].y - pt[a].y) - (pt[b].y - pt[a].y)*(pt[j].x - pt[a].x)) > 0){
top = pop(top, &pt[b].i);
}
else {
top = push(top, pt[j].i);
}
}
}
}
pt 是我的结构数组的名称。请帮忙。
请看一下这个，它计算平面中点的凸包
#include <math.h>
#define TRUE 1
#define FALSE 0
#define PI 3.1415926 /* ratio of circumference to diameter */
#define EPSILON 0.000001 /* a quantity small enough to be zero */
typedef int bool;
typedef struct {
double a; /* x-coefficient */
double b; /* y-coefficient */
double c; /* constant term */
} line;
#define DIMENSION 2 /* dimension of points */
#define X 0 /* x-coordinate index */
#define Y 1 /* y-coordinate index */
typedef double point[DIMENSION];
#define MAXPOLY 200 /* maximum number of points in a polygon */
typedef struct {
int n; /* number of points in polygon */
point p[MAXPOLY]; /* array of points in polygon */
} polygon;
typedef struct {
point p1,p2; /* endpoints of line segment */
} segment;
typedef point triangle[3]; /* triangle datatype */
typedef struct {
int n; /* number of triangles in triangulation */
int t[MAXPOLY][3]; /* indicies of vertices in triangulation */
} triangulation;
typedef struct {
point c; /* center of circle */
double r; /* radius of circle */
} circle;
/* Comparison macros */
#define max(A, B) ((A) > (B) ? (A) : (B))
#define min(A, B) ((A) < (B) ? (A) : (B))
points_to_line(point p1, point p2, line *l)
{
if (p1[X] == p2[X]) {
l->a = 1;
l->b = 0;
l->c = -p1[X];
} else {
l->b = 1;
l->a = -(p1[Y]-p2[Y])/(p1[X]-p2[X]);
l->c = -(l->a * p1[X]) - (l->b * p1[Y]);
}
}
point_and_slope_to_line(point p, double m, line *l)
{
l->a = -m;
l->b = 1;
l->c = -((l->a*p[X]) + (l->b*p[Y]));
}
bool parallelQ(line l1, line l2)
{
return ( (fabs(l1.a-l2.a) <= EPSILON) &&
(fabs(l1.b-l2.b) <= EPSILON) );
}
bool same_lineQ(line l1, line l2)
{
return ( parallelQ(l1,l2) && (fabs(l1.c-l2.c) <= EPSILON) );
}
intersection_point(line l1, line l2, point p)
{
if (same_lineQ(l1,l2)) {
printf("Warning: Identical lines, all points intersect.n");
p[X] = p[Y] = 0.0;
return;
}
if (parallelQ(l1,l2) == TRUE) {
printf("Error: Distinct parallel lines do not intersect.n");
return;
}
p[X] = (l2.b*l1.c - l1.b*l2.c) / (l2.a*l1.b - l1.a*l2.b);
if (fabs(l1.b) > EPSILON) /* test for vertical line */
p[Y] = - (l1.a * (p[X]) + l1.c) / l1.b;
else
p[Y] = - (l2.a * (p[X]) + l2.c) / l2.b;
}
closest_point(point p_in, line l, point p_c)
{
line perp; /* perpendicular to l through (x,y) */
if (fabs(l.b) <= EPSILON) { /* vertical line */
p_c[X] = -(l.c);
p_c[Y] = p_in[Y];
return;
}
if (fabs(l.a) <= EPSILON) { /* horizontal line */
p_c[X] = p_in[X];
p_c[Y] = -(l.c);
return;
}
point_and_slope_to_line(p_in,1/l.a,&perp); /* non-degenerate line */
/*printf("perpendicular bisector "); print_line(perp);*/
intersection_point(l,perp,p_c);
/*printf("closest point "); print_point(p_c);*/
}
double distance(point a, point b)
{
int i; /* counter */
double d=0.0; /* accumulated distance */
for (i=0; i<DIMENSION; i++)
d = d + (a[i]-b[i]) * (a[i]-b[i]);
return( sqrt(d) );
}
/***********************************************************************/
copy_point(point a, point b)
{
int i; /* counter */
for (i=0; i<DIMENSION; i++) b[i] = a[i];
}
swap_point(point a, point b)
{
point c; /* temporary point */
copy_point(a,c);
copy_point(b,a);
copy_point(c,b);
}
points_to_segment(point a, point b, segment *s)
{
copy_point(a,s->p1);
copy_point(b,s->p2);
}
segment_to_points(segment s, point p1, point p2)
{
copy_point(s.p1,p1);
copy_point(s.p2,p2);
}
bool point_in_box(point p, point b1, point b2)
{
return( (p[X] >= min(b1[X],b2[X])) && (p[X] <= max(b1[X],b2[X]))
&& (p[Y] >= min(b1[Y],b2[Y])) && (p[Y] <= max(b1[Y],b2[Y])) );
}
bool segments_intersect(segment s1, segment s2)
{
line l1,l2; /* lines containing the input segments */
point p; /* intersection point */
points_to_line(s1.p1,s1.p2,&l1);
points_to_line(s2.p1,s2.p2,&l2);
if (same_lineQ(l1,l2)) /* overlapping or disjoint segments */
return( point_in_box(s1.p1,s2.p1,s2.p2) ||
point_in_box(s1.p2,s2.p1,s2.p2) ||
point_in_box(s2.p1,s1.p1,s1.p2) ||
point_in_box(s2.p2,s1.p1,s1.p2) );
if (parallelQ(l1,l2)) return(FALSE);
intersection_point(l1,l2,p);
return( point_in_box(p,s1.p1,s1.p2) && point_in_box(p,s2.p1,s2.p2) );
}
double signed_triangle_area(point a, point b, point c)
{
return( (a[X]*b[Y] - a[Y]*b[X] + a[Y]*c[X]
- a[X]*c[Y] + b[X]*c[Y] - c[X]*b[Y]) / 2.0 );
}
double triangle_area(point a, point b, point c)
{
return( fabs(signed_triangle_area(a,b,c)) );
}
bool ccw(point a, point b, point c)
{
double signed_triangle_area();
return (signed_triangle_area(a,b,c) > EPSILON);
}
bool cw(point a, point b, point c)
{
double signed_triangle_area();
return (signed_triangle_area(a,b,c) < - EPSILON);
}
bool collinear(point a, point b, point c)
{
double signed_triangle_area();
return (fabs(signed_triangle_area(a,b,c)) <= EPSILON);
}
print_points(point p[], int n)
{
int i; /* counter */
for (i=0; i<n; i++)
printf("(%lf,%lf)n",p[i][X],p[i][Y]);
}
print_polygon(polygon *p)
{
int i; /* counter */
for (i=0; i<p->n; i++)
printf("(%lf,%lf)n",p->p[i][X],p->p[i][Y]);
}
print_point(point p)
{
printf("%7.3lf %7.3lfn",p[X],p[Y]);
}
print_line(line l)
{
printf("(a=%7.3lf,b=%7.3lf,c=%7.3lf)n",l.a,l.b,l.c);
}
print_segment(segment s)
{
printf("segment: ");
print_point(s.p1);
print_point(s.p2);
}
point first_point; /* first hull point */
convex_hull(point in[], int n, polygon *hull)
{
int i; /* input counter */
int top; /* current hull size */
bool smaller_angle();
if (n <= 3) { /* all points on hull! */
for (i=0; i<n; i++)
copy_point(in[i],hull->p[i]);
hull->n = n;
return;
}
sort_and_remove_duplicates(in,&n);
copy_point(in[0],&first_point);
qsort(&in[1], n-1, sizeof(point), smaller_angle);
copy_point(first_point,hull->p[0]);
copy_point(in[1],hull->p[1]);
copy_point(first_point,in[n]); /* sentinel to avoid special case */
top = 1;
i = 2;
while (i <= n) {
if (!ccw(hull->p[top-1], hull->p[top], in[i]))
top = top-1; /* top not on hull */
else {
top = top+1;
copy_point(in[i],hull->p[top]);
i = i+1;
}
}
hull->n = top;
}
sort_and_remove_duplicates(point in[], int *n)
{
int i; /* counter */
int oldn; /* number of points before deletion */
int hole; /* index marked for potential deletion */
bool leftlower();
qsort(in, *n, sizeof(point), leftlower);
oldn = *n;
hole = 1;
for (i=1; i<oldn; i++) {
if ((in[hole-1][X] == in[i][X]) && (in[hole-1][Y] == in[i][Y]))
(*n)--;
else {
copy_point(in[i],in[hole]);
hole = hole + 1;
}
}
copy_point(in[oldn-1],in[hole]);
}
main(){
point in[MAXPOLY]; /* input points */
polygon hull; /* convex hull */
int n; /* number of points */
int i; /* counter */
scanf("%d",&n);
for (i=0; i<n; i++)
scanf("%lf %lf",&in[i][X],&in[i][Y]);
convex_hull(in,n,&hull);
print_polygon(&hull);
}
bool leftlower(point *p1, point *p2)
{
if ((*p1)[X] < (*p2)[X]) return (-1);
if ((*p1)[X] > (*p2)[X]) return (1);
if ((*p1)[Y] < (*p2)[Y]) return (-1);
if ((*p1)[Y] > (*p2)[Y]) return (1);
return(0);
}
/*
bool leftlower(point *p1, point *p2)
{
if (fabs((*p1)[X] - (*p2)[X]) > EPSILON) {
if ((*p1)[X] < (*p2)[X]) return (-1);
if ((*p1)[X] > (*p2)[X]) return (1);
}
if (fabs((*p1)[Y] - (*p2)[Y]) > EPSILON) {
if ((*p1)[Y] < (*p2)[Y]) return (-1);
if ((*p1)[Y] > (*p2)[Y]) return (1);
}
return(0);
}
*/
bool smaller_angle(point *p1, point *p2)
{
if (collinear(first_point,*p1,*p2)) {
if (distance(first_point,*p1) <= distance(first_point,*p2))
return(-1);
else
return(1);
}
if (ccw(first_point,*p1,*p2))
return(-1);
else
return(1);
}