#include <stdio.h>
#include <string.h>

void swap(double points[20][2],int i, int j){
    double tmp[2];
    memcpy(tmp, points+i, sizeof(*points));
    memcpy(points+i, points+j, sizeof(*points));
    memcpy(points+j, tmp, sizeof(*points));
} 

int main(void) {
	double points[20][2]={{1,2},{2,3},{3,4},{4,5}};
	swap(points, 0, 1);
	for (int i = 0 ; i != 4 ; i++) {
		printf("%f %f\n", points[i][0], points[i][1]);
	}
	return 0;
}