I want to print my output in which only x coordinate is printed out.
1. printf("sphere{\n");
2. printf("\t<%g %g %g>,\n", V3ARGS(ell->v));
3. printf("//<\n %g \n// %g %g>\n }", V3ARGS(ell->a));
In third line of code, after 1st variable I put '//' because its a sign of comments i.e. I am printing 2nd and 3rd variable as comment in output file. Is there any way in which I can print out only 'x' value i.e. 1st variable?
Here is some code that could be representative of what you're using, plus macros to extract one argument at a time:
#include <stdio.h>
#define V3ARGS(v) (v).x, (v).y, (v).z
#define ARG_1(x, y, z) (x)
#define ARG_2(x, y, z) (y)
#define ARG_3(x, y, z) (z)
#define ARG1(v) ARG_1(v)
#define ARG2(v) ARG_2(v)
#define ARG3(v) ARG_3(v)
typedef struct Point_3D
{
double x;
double y;
double z;
} Point_3D;;
typedef struct DoublePoint
{
Point_3D a;
Point_3D v;
} DoublePoint;
int main(void)
{
DoublePoint d = { { 1.1, 2.2, 3.3 }, { 4.4, 5.5, 6.6 } };
DoublePoint *ell = &d;
printf("sphere{\n");
printf("\t<%g %g %g>,\n", V3ARGS(ell->v));
printf("//<\n %g \n// %g %g>\n }\n", V3ARGS(ell->a));
printf("a.x = %g\n", ARG1(V3ARGS(ell->a)));
printf("a.y = %g\n", ARG2(V3ARGS(ell->a)));
printf("a.z = %g\n", ARG3(V3ARGS(ell->a)));
printf("v.x = %g\n", ARG1(V3ARGS(ell->v)));
printf("v.y = %g\n", ARG2(V3ARGS(ell->v)));
printf("v.z = %g\n", ARG3(V3ARGS(ell->v)));
return 0;
}
The trick here is the two levels of macro, ARG1 and ARG_1. Note that ARG1() takes a single argument, expected to be an invocation of V3ARGS, and then calls ARG_1(). This trick expands the macro argument into the three comma-separated arguments expected by ARG_1(), which then 'returns' just the first of those arguments.
The output from the code shown (where the first output is the code you showed in the question with a newline at the end of the output):
sphere{
<4.4 5.5 6.6>,
//<
1.1
// 2.2 3.3>
}
a.x = 1.1
a.y = 2.2
a.z = 3.3
v.x = 4.4
v.y = 5.5
v.z = 6.6
As @JonathanLeffler pointed out, whatever your V3ARGS(...) is, it's probably a #define macro that resolves to a comma list like x, y, z. This means you are essentially asking what format string only prints x if you said:
printf("some format string here", x, y, z);
You're in luck, as it so happens that it is legal to pass fewer format specifiers than you pass arguments:
Passing too many arguments to printf
So if you only want the first argument, then only use one specifier:
printf("%g", x, y, z);
It gets trickier if you wanted just the second, or just the third. Because there's no generalized way to skip values; the format specifiers are consumed in order (though there is a POSIX extension to access by number):
Is there a "null" printf code that doesn't print anything, used to skip a parameter?
So if you got into a case like this and wanted to just print z, and couldn't modify the macro, this should work in ANSI C99:
typedef struct {
double x,
double y,
double z
} Coordinate;
Coordinate temp;
temp = (Coordinate){ V3ARGS(ell->a) };
printf("%g", temp.z);
That takes advantage of the fact that structure fields can be initialized by a comma delimited list in that style of assignment.
But you might be better served by looking at the definition of V3ARGS. Because it may well be doing something like:
#define V3ARGS(foo) (foo).x, (foo).y, (foo).z
In which case you could just say something like:
printf("%g", ell->a.z);
