Put a struct into an array element in C

Question

I have some code like (I've simplified it a bit):

# define NUMBER_OF_PARTICLES 1

typedef struct {
    Axes velocity;      // struct with x,y,z floats
} Particle;

Particle * array_of_particles;

Particle create(Particle p) {
    p.velocity.x = 0.0f;
    p.velocity.y = 0.0f;
    p.velocity.z = 0.0f;

    return p;
}

void create_particles() {
    array_of_particles = (Particle *) malloc(sizeof(Particle) * NUMBER_OF_PARTICLES);

    int p;      
    for (p = 0; p < NUMBER_OF_PARTICLES; p++) { 
        Particle current_particle = array_of_particles[p];
        array_of_particles[p] = create(current_particle);
    }
}

Hopefully you can see that I am trying to make the array element at index p be the struct of the current_particle. I think I am misunderstanding how to do this as it returns 0 when I print array_of_particles[p]. Could someone guide me as to the correct way of achieving this?

@unwind oh I had noticed this when looking at examples online, but it hadn't really clicked at the difference between the forms. Thanks for pointing it out. — dgBP
– dgBP, Commented Dec 5, 2013 at 12:20

AndersK · Accepted Answer · 2013-12-05 11:54:13Z

4

try

void create_particles() 
{
  array_of_particles 
    = (Particle *) malloc(sizeof(Particle)*NUMBER_OF_PARTICLES);

  int p;      
  for (p = 0; p < NUMBER_OF_PARTICLES; p++) { 
    Particle* current_particle = array_of_particles + p;
    create(current_particle);
  }
}

and change

void create(Particle* p) 
{
  p->velocity.x = 0.0f;
  p->velocity.y = 0.0f;
  p->velocity.z = 0.0f;
}

what you did was to pass a copy of the argument to the function so the changes never left the function.

there is also no need to return the Particle and then copy it, you already are passing the struct to the function so you can modify it by using the argument 'p'.

edited Dec 5, 2013 at 11:54

answered Dec 5, 2013 at 11:41

AndersK

36.1k6 gold badges63 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

dgBP Over a year ago

ahhh so I reference the particle and then create pointers to it. Cool, thanks!

dgBP Over a year ago

I get this error: error: incompatible types when initializing type ‘struct Particle *’ using type ‘Particle’ at this line: Particle* current_particle = array_of_particles[p];

Premsuraj Over a year ago

Try this, Particle current_particle = array_of_particles[p]; create(&current_particle);

dgBP Over a year ago

@claptrap can you explain how the array_of_particles + p works?

Mikkel K. Over a year ago

It's the address of entry p in your array (base address plus p times the size of a Particle because it's a Particle*). Another way of writing it would be &array_of_particles[p].

|

Collectives™ on Stack Overflow

Put a struct into an array element in C

1 Answer 1

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related