How to access an object instance which is inside an object array?

Question

I am trying to access an object which is a member of an object array, is that possible ?

I have declared a structure named Particle, and initialized an object array of "Particle" about 40 particles,now I need to access each particle, for ex: particle.Gbest any one can help ??

here is my code:

struct particle
{
    double[] position = new double[100];
    double Gbest, Lbest;
    double Pconst = 0.5;
}

object[] swarm = new object[swarm_size];

for (int i = 0; i < swarm_size; i++)
{
    swarm[i] = new particle();
}

You don't have an array of Particle You have an array of object. — Rex Morgan
– Rex Morgan, Commented Jul 27, 2011 at 16:16
Instead of object[] you should be using particle[].. beyond that I'm not sure what you're asking? — Shane Courtrille
– Shane Courtrille, Commented Jul 27, 2011 at 16:17

Jon Skeet · Accepted Answer · 2011-07-27 16:17:56Z

This code is invalid to start with:

struct particle
{
    double[] position = new double[100];
}

You can't specify variable initializers for instance variables in structs.

However, accessing data within another object or value is easy - if it's accessible. In this case your fields are private and you haven't provided any access methods or properties, so you won't be able to get at them "from the outside" without more code.

Here's some modified code:

public class Particle
{
    private readonly double[] positions = new double[100];

    // TODO: Rename these to something useful
    public double Gbest { get; private set; }
    private double Lbest;
    private double Pconst = 0.5;

    public Particle(int g)
    {
        Gbest = g; // Or whatever
    }
}


List<Particle> swarm = new List<Particle>();
for (int i = 0; i < swarmSize; i++)
{
    swarm.Add(new Particle(i));
}

double total = 0;
foreach (Particle particle in swarm)
{
    total += particle.Gbest;
}

Now this isn't doing anything particularly useful, because it's not clear what you're trying to do - but I would suggest you get an introductory book on C#. Stack Overflow is great for specific questions, but I think you're early enough on your journey into C# that a good book or tutorial would help you more.

Yuck · Accepted Answer · 2011-07-28 12:11:36Z

1

var swarm = new particle[swarm_size];

for (int i = 0; i < swarm_size; i++)
{
    swarm[i] = new particle();
}

To access properties of the array you could walk it like this:

for (int i = 0; i < swarm_size; i++)
{
     Console.WriteLine(swarm[i].Gbest);
     Console.WriteLine(swarm[i].Lbest);
}

EDIT: swarm declaration was incorrect.

edited Jul 28, 2011 at 12:11

answered Jul 27, 2011 at 16:20

Yuck

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4 Comments

musefan Over a year ago

I don't think that works. The compiler does not no that your object[] contain particle types. Perhaps you made a mistake with declaring swarm as an object array?

Yuck Over a year ago

@musefan: In my sample the swarm is defined as new particle[swarm_size], which gives the compiler enough information about the data contained within.

musefan Over a year ago

well my compiler (using VS 2010 Net 4.0) didn't like it. Only recognised "object" properties

Yuck Over a year ago

@musefan: I see what you mean, I missed that the first time around. Thanks!

Steve Morgan · Accepted Answer · 2011-07-27 16:18:49Z

0

If you know the index of the particle, you can simply use:

particle part = (particle)swarm[i];
// do stuff with part

If you don't know the index, you can iterate over them:

foreach( particle part in swam )
{
    // do stuff with part
}

Or you can use Linq-to-Objects to do more sophisticated stuff, but I think you need to start with the simple stuff...

By the way, if you can, declare swarm as:

particle[] swarm = new particle[swarm_size];

and get the benefit of type-safety.

answered Jul 27, 2011 at 16:18

Steve Morgan

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Comments

Shane Courtrille · Accepted Answer · 2011-07-27 16:19:14Z

0

struct particle
    {
        public double[] position = new double[100];
        public double Gbest, Lbest;
        public double Pconst = 0.5;
    }

     particle[] swarm = new particle[swarm_size];
         for (int i = 0; i < swarm_size; i++)
           {
            swarm[i] = new particle();
           }

    foreach(particle p in swarm)
    {
       // do magic here
    }

answered Jul 27, 2011 at 16:19

Shane Courtrille

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Comments

musefan · Accepted Answer · 2011-07-27 16:23:52Z

0

You can either change your array type to Particle and access it with index like...

particle[] swarm = new particle[swarm_size];
particle firstParticle = swarm[0];

or you can cast the object back to a particle...

particle firstParticle = (particle)swarm[0];

I would recommend the first option thou for performance reasons.

answered Jul 27, 2011 at 16:23

musefan

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