Re: [Boost-users] equation (really expression) idea

From: james.jones_at_[hidden]
Date: 2006-04-05 14:28:46

From: "Stephen Gross" <sgross_at_[hidden]>
> >1. Types are not template-ized. I don't want everything to be a double.
> This is an interesting point. My goal so far has been to prioritize 
> simplicity above all. The purpose of the toolkit is for letting people write 
> arithmetic equations easily. In light of that, I think sticking with doubles 
> makes sense.

So if I want to use a rational class, or a higher-precision double provided by a library, or a fixed-precision class, or a boolean type (a la Excel: if(x<y,x,y)) I'm out of luck? I appreciate simplicity of use, but you can still accomplish this by letting double be the default type (which makes sense, I think). Then non-power-users get double all the time, and power users can still do some hacking.

> Well, this is an interesting idea but it runs contrary to the metaphor I 
> want. In the above example, the variables 'x' and 'y' work as global 
> variables. I want the user to have to pass x and y directly to the equation.

I agree - which is why I wrote the bit about an evaluate function that takes positional arguments. Maybe named arguments are even better, though. I hadn't thought of that.

> If you had global-level variables, then it would be difficult to make the 
> above example work properly (passing z on as x and y when f2 invokes f1)..

Please don't call them globals - they aren't. They're in the same scope as the expression.

However, as to your particular issue, this isn't a problem:

variable x, y, z;
expression expr1 = x + y;
expression expr2 = z + expr1;

A variable is just a special form of an expression, after all.

As to mapping, it would be nice to be able to write code like:

double v = expr1(x=5,y=10);
double w = expr2(x=2,y=3,z=8);

I should think the Boost parameter library could handle this.

Actually, maybe you don't even need the Boost parameter library. Just define operator() on expression to ignore its arguments (then x=5, y=10 sets the values of x and y, and expr1 is then evaluated in that context).

- James