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1. Setting the Stage

1.1. Ruby

Alice: So here we are, ready to begin writing a toy model to simulate a dense stellar system. We have decided to begin with a very simple integrator, since we will use the material in a course for students with little or no prior background in differential equations and numerical methods.

Bob: That pretty much defines the first integrator to discuss: one based on the forward Euler integration scheme. To take one step, you add to each variable its derivative multiplied by the value of the time step. In other words, you just step forward by incrementing each variable by its derivative, as specified by the differential equation.

Alice: The only way to make that sentence clear to someone with little experience in this area is to give an example. So let's code it up.

Bob: We decided to do this in Ruby, but neither of us have any background in the language. To get started, I had a look at an introductory book, yesterday evening. It is called Programming Ruby by Dave Thomas and Andy Hunt, a.k.a. the Pragmatic Programmers. I found it quite useful, and it is also well written.

Alice: I'll have a look at it then as well. Did you find enough to get started?

Bob: I guess so. Let's try and see. Before getting to gravity and equations of motion, how about building a simple skeleton first, something that just reads in the data and prints them out, without doing any integrating?

Alice: But before doing that, we need to decide upon a data format. Let's not get too fancy, for now. How about just listing for each particle the seven numbers on one line: mass, position, and velocity, with the latter two each being a three-dimensional vector, and therefore with three components each?

Bob: Well, in that case, let's start with an even simpler problem, with reading and writing the data for a single particle. For an object oriented language like Ruby, that suggests that we create a class Body for a particle in an N-body system.

Alice: Can you remind me what a class is?

Bob: I thought you were going to tell me, while pointing out how important they are for your obsession with modular programming!

Alice: Well, yes, I certainly know the general idea, but I must admit, I haven't really worked with object oriented languages very much. At first I myself was stuck with some existing big codes that were written in rather arcane styles. Later, when I got to supervise my own students and postdocs, I would have liked to let them get a better start. However, I realized that they had to work within rather strict time limits, within which to learn everything: the background science, the idea of doing independent research, learning from your mistakes, and so on.

The main problem for my students has been that there is hardly any literature that is both interesting for astrophysicists and inspiring in terms of a really modern programming attitude. When students are pressed for time and eager to learn their own field, they are not likely to spend a long time delving into books on computer science, which will strike them as equally arcane, for different reasons, as the astrophysical legacy codes.

Given that there was no middle ground, I did not want them to focus too much on computational techniques, because that would have just taken too much time. My hope is, frankly, that our toy model approach will bridge this huge gap, between books that are clear but of very limited use, and codes that are useful but of very limited clarity.

Bob: You seem to find a way to introduce a world wide vision for every small task that you encounter. As for me, I'm happy to just build a toy model, and if students will find it helpful, I'd be happy too. But just to answer your question, defining a class is just a way to bundle a number of variables and functions together. Just like a number of scalar values can be grouped together in one array, which can stand for a physical vector for example, you can group a more heterogeneous bunch of variables together. You do this mainly for bookkeeping reasons, and to keep your program simpler and more robust.

In practice, a good guide to choosing the appropriate class structure for a given problem is to start with the physical structures that occur naturally, but that may not always be the best option, and certainly not the only one.

1.2. A Body Class

Bob: For example, a single particle has as a minimum a mass, a position and velocity. Whenever you deal with a particle, you would like to have all three variables at hand. You can't put them in a single array, because mass is a scalar, and the other two variables are vectors, so you have to come up with a more general form of bundling.

The basic idea of this kind of programming is called object-oriented programming. In many older computer languages, you can pass variables around, where each variable can contain a number or an array of numbers; or you may pass pointers to such variables or arrays. In an object-oriented programming language, you pass bundles of information around: for example all the information pertaining to a single particle, or even to a whole N-body system. This provides convenient handles on the information.

If you look in a computer science book, you will read that the glorious reason for object-oriented programming is the ability to make your life arbitrarily difficult by hiding any and all information within those objects, but I don't particularly care for that aspect.

Alice: I do think encapsulation has its good sides, but we can come back to that later. What exactly is it that you can put inside an object? I guess an object can contain internal variables. Can it contain functions as well?

Bob: To take the specific case of Ruby, a typical class contains both. For a class to be useful, at least you have to be able to create an instance of a class, so you need something like what is called a constructor in C++. In the case of Ruby, like in the case of C++, you have the freedom to define an initializer, through which you can create an instance of a class with your desired values for the internal variable. Or you can choose not to define an initializer, that is fine too.

Note as a matter of terminology that what I have called a function, or what would be called a subroutine in Fortran, is called a method in Ruby. There are other objects in Ruby, besides classes. Sometimes you have a group of functions that are either similar or just work together well, and you may want to pass them around as a bundle. In Ruby, such bundles of functions are called modules. But to get started, it is easier to stick to classes for now.

Alice: Ah, that is nice! Does this mean that we can define an integration algorithm as a module, independent of the particular variables in the classes that define a body or an N-body system? I mean, can you write a leapfrog module that can propagate particles, independently of their type? You could have point particles in either a two-dimensional of a three-dimensional world. Or you could have particles with a finite radius, that stick together when they collide; as long as they are not too close, they could be propagated by the same leapfrog module.

Bob: You really have an interesting way of approaching a problem. We haven't even defined a single particle, and you are already thinking about general modules that are particle independent. I suggest we first implement a particle. I think this is how we can introduce a minimal class for a single particle:

 class Body
   def initialize(mass = 0, pos = [0,0,0], vel = [0,0,0])
     @mass, @pos, @vel = mass, pos, vel

In a few minutes, we can go through the precise meaning of these constructs, but here is the general idea: you can give three arguments in a call to initialize, to specify the mass, position, and velocity. If you don't specify some of these arguments, they will acquire the default values that are given here by all the zeroes in the first line of the definition of initialize. The next line assigns these values to internal variables that have names starting with a @ sign.

Alice: That is remarkably short and simple! In fact, it seems too simple. I am surprised that we do not have to declare the internal variables. In other languages that I am familiar with, it is essential that you tell the computer which memory places to set aside before naming them.

Bob: Ruby is dynamically typed. This means that the type of a variable is determined at run time. In other words the type of a variable is simply the type of the value that is assigned to a variable.

Alice: And you can change the type of that value, whenever you want? Can we try that? I'd like to see the syntax of how you do that.

Bob: Before we do that, just one thing: staring at this amazingly simple class definition makes me realize both how similar it is to what you would write in C++, and how different.

Alice: Indeed, the logical structure of C++ class definitions is very similar.

Bob: But the big difference is that a C++ class definition is quite a bit longer.

Alice: I'm curious how much longer. Do you remember how to write a similar particle class in C++?

Bob: That shouldn't be too hard. Always easiest to look at an existing code. Ah, here I have another C++ code that I wrote a while ago. Okay, now I remember. Of course. Here is how you do it in C++:

 ##include  <iostream>
 using namespace std;
 class body
     double pos[3];
     double vel[3];
     double mass;
     body(double inmass, double inpos[3], double invel[3]){
        mass = inmass;
        for(int i=0;i<3;i++){
            pos[i] = inpos[i];
            vel[i] = invel[i];
     void print(){
        cout << mass <<endl;
        cout << pos[0] <<" "<< pos[1] <<" "<< pos[2] <<endl;
        cout << vel[0] <<" "<< vel[1] <<" "<< vel[2] <<endl;
     double zero3[3]={0.0,0.0,0.0};
     body x =  body(0.0,zero3,zero3);

1.3. The irb Interpreter

Alice: That is quite an impressive difference, between Ruby and C++! But aren't you cheating a bit? That last part, with the main function down below, does not occur in your short and sweet Ruby class definition.

Bob: It doesn't occur there, because you don't need it. All that main does for you is create an object and then printing its internal values. You can let Ruby do that for you without even asking for it.

Here is what you do. The easiest way to work with Ruby is to use the command irb. The acronym stands for interactive Ruby. You invoke it by simply typing irb on the command line. Now as soon as you create an instance of a class in Ruby, the interpreter echoes the content to you, for free!

Alice: I'll try it out. But rather than wrestling with a whole particle, I prefer to start with a single variable, to see how Ruby functions at its most basic level. Also, remember that I asked you whether you can change the type of the value of a variable, whenever you want? I want to see that for myself. One thing at a time!

Let me introduce an identifier id. I will first give it a numerical value, and then I will assign to it a string of characters, to give it a name. Since Ruby is friendly enough not to insist on declaring my variables beforehand, I presume I can just go ahead and use id right away.

    |gravity> irb
    irb(main):001:0> id = 12
    => 12
And as you predicted, a value gets produced magically. But where does that come from?

Bob: Just like in C, variables are not the only things that have values. In fact, every expression has a value. And irb makes life more clear by echoing the value of each line as soon as you enter it.

Alice: I like that. It will make debugging a lot easier. Okay, let me try to change the type of id.

    irb(main):002:0> id = cat
    NameError: undefined local variable or method `cat' for main
        from (irb):2
Bob: Ah, Ruby treats your cat in an equally friendly way as your id, assuming it is itself a name of a variable (or a method), rather than a content that can be assigned to a variable.

Alice: But that line works fine when I write shell scripts. Since Ruby is called a scripting language, I thought it might work here too.

Bob: Each scripting language has different conventions. In your shell case, I bet you have to invoke the value of a variable cat by typing $cat, each time you use it. Ruby has another solution: typing cat echoes the value of cat. When you want to introduce a string consisting of the three letters c, a, and t, you type "cat".

Alice: Here goes:

    irb(main):003:0> id = "cat"
    => "cat"
It worked! And presumably id has now forgotten that it ever was a numerical variable.

Bob: Indeed. And at any time you can ask Ruby what the type of your dynamically typed variable currently is. Any variable is an instance of some class. And the class it belongs to in turn has a method built in, not surprisingly called class, which tells you the type of that class. In Ruby, you invoke a method associated with a variable by writing that variable followed by a period and the method name.

Alice: I find it surprising, if you ask me; I would have expected something like type. But I'll take your word for it.

    irb(main):004:0> id.class
    => String
    irb(main):005:0> id = 12
    => 12
    irb(main):006:0> id.class
    => Fixnum
Hey, that is nice! You can immediately check what is going on. Let's see what happens when I type in the text of the Body class declaration above.

    irb(main):007:0> class Body
    irb(main):008:1>   def initialize(mass = 0, pos = [0,0,0], vel = [0,0,0])
    irb(main):009:2>     @mass, @pos, @vel = mass, pos, vel
    irb(main):010:2>   end
    irb(main):011:1> end
    => nil
I see another nice feature. I had been wondering about the meaning of the :0 after each line number. That must have been the level of nesting of each expression. It goes up by one, each time you enter a block of text that ends with end.

Bob: And since you only give the definition of Body without yet creating any of its instances, there is no value associated with it. Here nil means effectively `undefined'.

Alice: But we have just defined the Body class; why does Ruby claim it is undefined?

Bob: In Ruby, the definition of a class does not return a value, since there is no reasonable answer that can be given to a non-existent question. But since the interpreter wants to echo something, it just returns, `nil' as meaning an undefined value, or literally nothing. And this has nothing to do with the definition of a class. This may sound more complicated than it is, but it is quite logical.

Alice: I see, yes, that makes sense. So the class Body has only one function, starting with def and ending with the inner end, correct?

Bob: Indeed. And the last end is the end of the class definition, that starts with class Body. Note the grammatical rule that the name of a class such as Body always starts with a capital letter. The names of normal variables, in contrast, start with a lower case letter: we have three such variables, mass, pos, and vel. All three are given here as possible parameters to the initialization function initialize.
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