Re: Can I define an array of structures depending on input?

[Axel Freyn <axel-freyn at gmx dot de>]

Hi anna,

On Sat, Jan 29, 2011 at 01:26:46PM +0200, Anna Sidera wrote:
> Hello,
> 
> I have a program that uses some input parameters. One of these parameters, for example my_input_parameter, takes values 1, 2, 3 and 4. I want to define an array in which each element is a structure. But I want the structure to depend on the value of my_input_parameter. For example:
> 
> struct my_structure {
>    <things that depend on the value of my_input_parameter>
> };
> struct my_structure *my_array = (struct my_structure *)malloc(10000*sizeof(struct my_structure));
> 
> Can you tell me any way to do this? 
Well, the problem is: the decision, what your structure looks like
exactly, has to be taken at compile-time -- the compiler has to know it.
The input-parameter is only known at runtime -- and then it's to late to
change the structure. So if I understand correctly, your goal is
impossible as far as I know in compiled languages like C, but would only
be possible in interpreted languages like python (there, you can change
data-types during runtime).
What you can however do:

1) Fix the input-parameter already at compile-time, and compile four
different programs (for my_input_parameter=1,2,3 and 4). Then you could
write something like this:
struct my_structure{
#if my_input_parameter == 1
...
#endif
#if my_input_parameter == 2
...
#endif
#if my_input_parameter == 3
...
#endif
#if my_input_parameter == 4
...
#endif
};
and compile the code with e.g. "gcc -Dmy_input_parameter=2"
That will give you best runtime performance -- and inÑtead of passing
the input parameter, you have to call the appropriate program.
Of course, another possibility here would be to link these four cases
into a single program -- and the call the appropriate routine depending
on your input parameter.

2) you can use C++ and virtual functions (however, this might be bad for
the runtime-performance, depending on how you call the code. Calling a
virtual function needs more time than "normal" functions (and in general
they can't be inlined) ). Defining an abstract base-class, which only
contains your interface:
class my_structure{
  public:
    virtual double get_element() = 0;
}
then you can again create 4 independent classes for your 4 parameters:
class my_structure_1{		// for my_input_parameter == 1
  public:
    double get_element(){ ....}
};
class my_structure_2{		// for my_input_parameter == 2
  public:
    double get_element(){ ....}
};
...
3) you can do something analogous in C by using a structure with
function pointers, which you define to behave differently for
my_input_parameter == 1,2,3 or 4.


Axel