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digitalmars.D - 2 simple Q.

reply "Bent Rasmussen" <exo bent-rasmussen.info> writes:
Disclaimer: This is newbieish. First question. I'd like to do the following.

float[3] Torus(float[2] P) {
    ...
}

I can't do that, so what's the "next best thing". I don't mind classes but
it would be nice to be able to prototype it with just arrays in a safe way.

Second question. Is it possible to use templates to parameterize a function
with values. Something like

template Tori(a : float, c : float)
{
    float[3] Torus(float[2] P)
    {
        ...
    }
}

Or should I use a higher-order function with delegates for that?

Bonus question :) - Where can I find a good webresource to read about
templates in general and preferably specifically in D.

Regards,
Bent Rasmussen

(accidentally posted into another subject before :P)
May 14 2004
next sibling parent Stewart Gordon <smjg_1998 yahoo.com> writes:
Bent Rasmussen wrote:


 Disclaimer: This is newbieish. First question. I'd like to do the following.
 
 float[3] Torus(float[2] P) {
     ...
 }
 
 I can't do that, so what's the "next best thing". I don't mind classes but
 it would be nice to be able to prototype it with just arrays in a safe way.


You mean the compiler is giving errors you haven't bothered telling us?

I'm not sure if fixed arrays are allowed as return types.  But dynamic 
arrays are, so you could use one of them.


 Second question. Is it possible to use templates to parameterize a function
 with values. Something like
 
 template Tori(a : float, c : float)


<snip>

Not with that syntax, which would mean a template specialisation for a 
and c being types, and float being chosen for both.

	template Tori(float a, float c)


 Or should I use a higher-order function with delegates for that?


Probably.  In order to use a template, the parameter values have to be 
given at compile time.  If you need to be able to specify them at 
runtime, you'll need a higher-order function.


 Bonus question :) - Where can I find a good webresource to read about
 templates in general and preferably specifically in D.


http://www.digitalmars.com/d/template.html

Stewart.

-- 
My e-mail is valid but not my primary mailbox, aside from its being the 
unfortunate victim of intensive mail-bombing at the moment.  Please keep 
replies on the 'group where everyone may benefit.
May 14 2004
prev sibling parent reply J C Calvarese <jcc7 cox.net> writes:
Bent Rasmussen wrote:

 Disclaimer: This is newbieish. First question. I'd like to do the following.
 
 float[3] Torus(float[2] P) {
     ...
 }
 
 I can't do that, so what's the "next best thing". I don't mind classes but
 it would be nice to be able to prototype it with just arrays in a safe way.


As you noted, the static array could be wrapped in a class. Also, you 
could wrapped the static array in a struct. A third option is to use a 
dynamic array instead of using a static array at all. I'd try the 
dynamic array first.

Some example code:

int[] someFunction() /* dynamic array */
{
     int[] i;

     i.length = 5;
     i[0] = 1;
     i[1] = 1;
     i[2] = 2;
     i[3] = 3;
     i[4] = 5;
     return i;
}


struct fa_int5  /* struct wrapper */
{
     int[5] i; /* static array */
}


fa_int5 someFunction2()
{
     fa_int5 k;

     k.i[0] = 1;
     k.i[1] = 1;
     k.i[2] = 2;
     k.i[3] = 3;
     k.i[4] = 5;
     return k;
}


void main()
{
     int[] j = someFunction();
     fa_int5 m;

     m = someFunction2();
     int[5] n = m.i;
}



 
 Second question. Is it possible to use templates to parameterize a function
 with values. Something like
 
 template Tori(a : float, c : float)
 {
     float[3] Torus(float[2] P)
     {
         ...
     }
 }
 
 Or should I use a higher-order function with delegates for that?
 
 Bonus question :) - Where can I find a good webresource to read about
 templates in general and preferably specifically in D.


I'm relunctant to call it "good", but I think it's better than nothing:
http://www.dsource.org/tutorials/index.php?show_topic=Templates

(If I understood templates better, I'd probably post more examples.)


 
 Regards,
 Bent Rasmussen
 
 (accidentally posted into another subject before :P)



-- 
Justin (a/k/a jcc7)
http://jcc_7.tripod.com/d/
May 14 2004
parent reply "Bent Rasmussen" <exo bent-rasmussen.info> writes:
 As you noted, the static array could be wrapped in a class. Also, you
 could wrapped the static array in a struct. A third option is to use a
 dynamic array instead of using a static array at all. I'd try the
 dynamic array first.


[...]

I see. For this case though the number of elements is so low that it seems
most straightforward to use variables for each element.


 I'm relunctant to call it "good", but I think it's better than nothing:
 http://www.dsource.org/tutorials/index.php?show_topic=Templates


Thanks. Templates appear to be a much more powerful tool than I
expected.

The template problem I had was solved by using syntax given in the first
reply. But it did give me another problem; while this compiles

    template TTorus(int a, int c)
    {
        float[] Torus(float[2] P)
        {
            float[3] Q;
            Q[0] = (c + a*cos(P[1]))*cos(P[0]);
            Q[1] = (c + a*cos(P[1]))*sin(P[0]);
            Q[2] = a*sin(P[1]);
            return Q;
        }
    }

    alias TTorus!(1,2) Torus;

this does not

    template TTorus(float a, float c)
    {
        float[] Torus(float[2] P)
        {
           float[3] Q;
           Q[0] = (c + a*cos(P[1]))*cos(P[0]);
           Q[1] = (c + a*cos(P[1]))*sin(P[0]);
           Q[2] = a*sin(P[1]);
            return Q;
       }
    }

    alias TTorus!(1.0f,2.0f) Torus;

The compiler says

    Error: integral type expected for value-parameter, not float.

So this can't be done with templates, although the reason escapes me.

Regards,
Bent Rasmussen
May 15 2004
parent Ben Hinkle <bhinkle4 juno.com> writes:
Bent Rasmussen wrote:


 As you noted, the static array could be wrapped in a class. Also, you
 could wrapped the static array in a struct. A third option is to use a
 dynamic array instead of using a static array at all. I'd try the
 dynamic array first.


 [...]
 
 I see. For this case though the number of elements is so low that it seems
 most straightforward to use variables for each element.
 

 I'm relunctant to call it "good", but I think it's better than nothing:
 http://www.dsource.org/tutorials/index.php?show_topic=Templates


 
 Thanks. Templates appear to be a much more powerful tool than I
 expected.
 
 The template problem I had was solved by using syntax given in the first
 reply. But it did give me another problem; while this compiles
 
     template TTorus(int a, int c)
     {
         float[] Torus(float[2] P)
         {
             float[3] Q;
             Q[0] = (c + a*cos(P[1]))*cos(P[0]);
             Q[1] = (c + a*cos(P[1]))*sin(P[0]);
             Q[2] = a*sin(P[1]);
             return Q;
         }
     }
 
     alias TTorus!(1,2) Torus;
 
 this does not
 
     template TTorus(float a, float c)
     {
         float[] Torus(float[2] P)
         {
            float[3] Q;
            Q[0] = (c + a*cos(P[1]))*cos(P[0]);
            Q[1] = (c + a*cos(P[1]))*sin(P[0]);
            Q[2] = a*sin(P[1]);
             return Q;
        }
     }
 
     alias TTorus!(1.0f,2.0f) Torus;
 
 The compiler says
 
     Error: integral type expected for value-parameter, not float.
 
 So this can't be done with templates, although the reason escapes me.
 
 Regards,
 Bent Rasmussen


The template parameters have to be converted to strings for name mangling.
This is easy for integral types but floating point has roundoff error and
messy string representations. Plus if non-integral types are allowed then
the name mangler would have to start encoding the type of the parameter to
distinguish between, for example, _TTorus_1 and _TTorus_1f. So basically I
think it is to keep the name mangling and template matching simple.
May 15 2004