Array index

space_age.h

#ifndef SPACE_AGE_H
#define SPACE_AGE_H

#include <stdint.h>

typedef enum planet {
   MERCURY,
   VENUS,
   EARTH,
   MARS,
   JUPITER,
   SATURN,
   URANUS,
   NEPTUNE,
} planet_t;

float age(planet_t planet, int64_t seconds);

#endif

space_age.c

#include "space_age.h"

static const int EARTH_YEAR_SECONDS = 31557600;
static const double RATIOS [] = {
    0.2408467, 0.61519726, 1.0, 1.8808158, // inner planets
    11.862615, 29.447498, 84.016846, 164.79132 // outer planets
};

float age(planet_t planet, int64_t seconds){
    return planet >= 0 && planet <= 7 ?
        seconds / EARTH_YEAR_SECONDS / RATIOS[planet] : -1;
}

In the header is defined an enum with values 0 through 7 for the planets MERCURY through NEPTUNE.

In the source file an array of the planet orbit ratios is defined, with the index of each ratio corresponding with the planet's value in the enum. It also defines the earth year in seconds as a const. The const value is given a meaningful name instead of using 31557600 as a magic number.

The age function takes an argument of that enum along with the seconds.

It uses a ternary operator operator to test if the planet value is in the valid range. If so, it divides the seconds by the earth year seconds and divides the result by the orbit ratio in the ratio array at the index determined by the planet value. The result of all that is returned by function.

If the planet value is not in the valid range, then the ternary operator returns -1 for an error value, which is returned by the function.