C program, struct and functions help

[jam12]

Hello, i want to write a function in the form of "float estM( Planet *body);" to calculate radius^3/period^2 for the planet earth, where i have an array of values for radius and period of different planets.
Here is the program so far, i am getting a -NaN error:

#include <stdio.h>
#include <math.h>

typedef struct pt {
char *name;
float period; /* orbital period in years */
float radius; /* orbital radius in ast units */
} Planet;

main() {

float estM( Planet *body); /* Have been told that the function is of this form to calculate radius^3/period^2 for earth*/

Planet Earth;
Planet plist[6];
int n, i;
Planet *body;
body=&Earth;

char nameMe[]="Mercury", nameV[]="Venus", nameE[]="Earth", nameMa[]="Mars", nameJ[]="Jupiter", nameS[]="Saturn";


plist[0].name = nameMe;
plist[0].period = 0.241;
plist[0].radius = 0.387;

plist[1].name = nameV;
plist[1].period = 0.615;
plist[1].radius = 0.723;

plist[2].name = nameE; /*Here is info for Earth */
plist[2].period = 1.0;
plist[2].radius = 1.0;

plist[3].name = nameMa;
plist[3].period = 1.881;
plist[3].radius = 1.524;

plist[4].name = nameJ;
plist[4].period = 11.86;
plist[4].radius = 5.203;

plist[5].name = nameS;
plist[5].period = 29.46;
plist[5].radius = 9.540;


n=6;
printf("Planet name\t period\t radius\n");

for (i=0;i<n;i++) {

printf("%s\t\t %.3f\t %.3f\n",plist.name, plist.period, plist.radius);

}

printf("%f\n",estM(body)); /* I want to calculate radius^3/period^2 for Earth */


}

float estM( Planet *body) { /*function to caculate radius^3/period^2 for Earth */

float m;
Planet Earth;
body=&Earth;
m=pow((*body).radius,3)/pow((*body).period,2);
return m;
}

 

[Filip Larsen]

Hello, i want to write a function in the form of "float estM( Planet *body);" to calculate radius^3/period^2 for the planet earth, where i have an array of values for radius and period of different planets.
Here is the program so far, i am getting a -NaN error:

float estM( Planet *body) {		/*function to caculate radius^3/period^2 for Earth */

float m;
Planet Earth;
body=&Earth;
m=pow((*body).radius,3)/pow((*body).period,2);
return m;
}

Looks like you accidentally left a non-initialized Earth structure in which you point the body pointer to (non-initialized floating point numbers are very likely to give a NaN). You should probably leave out the Earth variable, like

float estM( Planet *body) {		/*function to caculate radius^3/period^2 for Earth */
  return pow(body->radius,3)/pow(body->period,2);
}

where I have also replaced the (*pointer).member notation with pointer->member. You should then call this function with something sensible, like

printf("%f\n",estM(plist[2])); /* I want to calculate radius^3/period^2 for Earth */

Note, that in your code, the Earth variable in main is again non-initialized and will contain garbage, so either use plist[2] directly to refer to the values for Earth (as I did in the above snippet), or declare, say, Earth as "Planet *earth = plist[2];" after the plist declaration and then use "estM(earth)" later. If you want to call estM in your loop you will probably want to call with "estM(plist)".

 

[jam12]

Thanks just did it now, the code i used is:
in main:

Planet plist[6];
Planet *earth;
int n, i;
earth=&plist[2];

Here i didnt know that i could use a pointer to point to a member in the array, but it makes sense since all the members in the array are of type Planet.

then my function definition is just:

float estM( Planet *body) { /*function to caculate radius^3/period^2 for Earth */

return pow(body->radius,3)/pow(body->period,2);
}

Thanks again