changing values of a struct
 

Hello,

i'm still trying to learn C, and I have a new question ;).

I want to declare a struct, and use a function to manipulate the entries. If I change the values within a function it's ok; however, when i access the members outside the function, they're not defined anymore.

How do i change the values in the struct such that a different function can use it as well? I thought using a pointer to struct would be 'good enough', but apparently it isn't ;).

Thanks in advance,

#include <stdio.h>

#include <stdlib.h> // for drand48

#include <math.h>



struct atom{

        double x,y,z;

        double vx,vy,vz;

        double fx,fy,fz;

        int id;

};



void initialpos(struct atom *name, int N);



const int N=100;



int main()

{

        struct atom *patom[100];

        

        int i;

        for (i = 0; i < N; i++)

        {

                patom[i]=(struct atom *)malloc(sizeof(struct atom));

        }

        

        for (i = 0; i < N; i++)

        {

                if (patom[i]==NULL)

                {

                        printf("out of MEM\n");

                        return 1;

                }

        }

        

        /* give atoms their initial position */

        initialpos(patom[0],N);

        

        for (i = 0; i < N; i++)

        {

                printf("pos. of %d: %f\t %f\t %f\t \n", i, patom[i]->x, patom[i]->y, patom[i]->z);

        }

        

        

        return 0;

}



void initialpos(struct atom *name, int N)

{        

        int i;

        

        for (i = 0; i < N; i++)

        {

                name[i].x=drand48()*20;

                name[i].y=drand48()*20;

                name[i].z=drand48()*20;

                printf("pos of %d: %f\t %f\t %f\t \n", i, name[i].x, name[i].y, name[i].z);

        }

}

Re: changing values of a struct
 

Taking this as an example:
name[i].x=drand48()*20;
You are assigning to a field of a structure, but that is not a field of a structure. It is a pointer to a field of a structure. It can be done in two ways:
(*name[i]).x = drand48() * 20; or
name[i]->x = drand48() * 20;
Same thing is going on with the others and even with the parameters in the printf();

Re: changing values of a struct
 

I tried changing the function like that as well; but it still doesn't work.
Maybe I'm silly, but I just don't see it.

(and thanks for helping)

 void initialpos(struct atom *name, int N)

{    

    int i;

    

    for (i = 0; i < N; i++)

    {

        name[i]->x=drand48()*20;

        name[i]->y=drand48()*20;

        name[i]->z=drand48()*20;

        printf("pos of %d: %f\t %f\t %f\t \n", i, name[i].x, name[i].y, name[i].z);

    }

}

Re: changing values of a struct
 

>but it still doesn't work.
What's not working?. What's supposed to happen and it isn't?.
You forgot to change the arguments also in the printf().

Re: changing values of a struct
 

Now it's like this:

void initialpos(struct atom *name, int N)

{        

        int i;

        

        for (i = 0; i < N; i++)

        {

                name[i]->x=drand48()*20;

                name[i]->y=drand48()*20;

                name[i]->z=drand48()*20;

                printf("pos of %d: %f\t %f\t %f\t \n", i, name[i]->x, name[i]->y, name[i]->z);

        }

}

(i forgot the printf function the previous time).
Now it's complaining about 'invalid type argument of '->' '
I'll explain what i want to do.
In main() a the function initialpos() is called, to give a particle some initial position. But this position is needed later on as well, so i want to keep the values too when the function is finished.
When i did it like in the opening post, the values are written to the struct, but after the function is fnished, the elements are gone.

Hope you understand what I mean. Thanks for the help so far,

Re: changing values of a struct
 

Needs some changes:
initialpos(&patom[0],N); instead of initialpos(patom[0],N); the & is needed if you want to pass it the address of a structure.
void initialpos(struct atom *name[], int N) instead of void initialpos(struct atom *name, int N) *name[] is the beginning of the array of structures.

Re: changing values of a struct
 

Thanks! it's working now. It makes sense: before I only gave one element as an argument, but instead I had to give an array with starting address &patom[0]. Now all variables in the struct are kept (because the array in the argument).
I'd never think of that! (even though it makes sense now). Thanks a lot! Now I can finally go to bed.. it's 2 AM here ;).
(If something doesn't work, I keep wondering how to solve it... and then I can't sleep. )
Thanks again!