seamlessthree.c

/* Compile with:
make LDLIBS='-lm' CFLAGS="-g -Wall -std=gnu11" seamlessthree
*/
#include <stdio.h>
#include <math.h>

typedef struct point {
   double x, y;
} point;

typedef struct {
    union {
        struct {
           double x, y;
        };
        point p2;
    };
    double z;
} threepoint;

double length (point p){
    return sqrt(p.x*p.x + p.y*p.y);
}

double threelength (threepoint p){
    return sqrt(p.x*p.x + p.y*p.y + p.z*p.z);
}

int main(){
    threepoint p = {.x=3, .y=0, .z=4};
    printf("p is %g units from the origin\n", threelength(p));
    double xylength = length(p.p2);
    printf("Its projection onto the XY plane is %g units from the origin\n", xylength);
}