Re: finite summation - how to call a function from parameter stack

[Ethan M. <merritt@u.washington.edu>]

On Tuesday, January 11, 2011 09:00:19 am mw...@gm... wrote:
> Hi,
> 
> I want to add finite summation to gnuplot, that is a function sum(a, b, x, f), where a, b (int) are the summation boundary, x (double) is a varialbe, f (string) is the name of the function to eveluate a term. Example usage:
> 
> f(n, x) = sin(n*x)
> plot sum(1, 10, x, "f")

That pair of commands does not seem to be compatible with your  description.
How is the code to know there is a second parameter to f()?  And where would
it get this second parameter from?


> I am stuck at how to call f from sum (see call(f) below) and would appreciate your help very much. (I assume f is the string representing the name of the function to call.)
> 
> -    for (i=a; i<=b; ++i) {
> -        // calculate f_i(x);
> -        push(Gcomplex(&tmp, x.real, x.imag)); // todo check the order
> -        push(Ginteger(&tmp, i));
> -
> -        //call(f); // assume f is the name (string) of the function to call

There is no existing routine that does either call-by-name or 
retrieve-user-function-by-name; you would have to write one.
Something like (untested):

struct udft_entry *
get_udf_by_name(char *key)
{
    struct udft_entry **udf_ptr = &first_udf;

    while (*udf_ptr) {
        if (!strcmp(key, (*udf_ptr)->udf_name))
            return (*udf_ptr);
        udf_ptr = &((*udf_ptr)->next_udf);
    }

    return NULL;
}

It would belong in eval.c next to add_udv_by_name().

Then you would do something like:

    udft_entry *udf = get_udf_by_name( f );
    if (udf)
	execute_at(udf->at);
    else
	int_error(NOCARET, "No user-defined function %s",f);


> -
> -        pop(&tmp);
> -        ret.real += real(&tmp);
> -        ret.imag += imag(&tmp);
> -    }
> 
> Thank you very much for your time and effort.
> 
> --- specfun.c   2011-01-11 17:40:19.000000000 +0100
> +++ specfun.c.orig  2011-01-11 17:37:27.000000000 +0100
> @@ -1933,44 +1933,3 @@
>      push(Gcomplex(&a, x, 0.0));
>  }
>  
> -void
> -f_sum(union argument *arg)
> -{
> -    struct value tmp;
> -    int a, b;
> -    struct cmplx x, ret, result;
> -    int i;
> -
> -    (void) arg;                        /* avoid -Wunused warning */
> -
> -    // pop x
> -    pop(&tmp);
> -    x.real = real(&tmp);
> -    x.imag = imag(&tmp);
> -
> -    // pop b
> -    if (pop(&tmp)->type != INTGR)
> -        int_warn(NO_CARET, "expecting an integer as second argument to sum");
> -    b = tmp.v.int_val;
> -
> -    // pop a
> -    if (pop(&tmp)->type != INTGR)
> -        int_warn(NO_CARET, "expecting an integer as first argument to sum");
> -    a = tmp.v.int_val;
> -    
> -    ret.real = 0;
> -    ret.imag = 0;
> -    for (i=a; i<=b; ++i) {
> -        // calculate f_i(x);
> -        push(Gcomplex(&tmp, x.real, x.imag)); // todo check the order
> -        push(Ginteger(&tmp, i));
> -
> -        //call(f);
> -
> -        pop(&tmp);
> -        ret.real += real(&tmp);
> -        ret.imag += imag(&tmp);
> -    }
> -
> -    push(Gcomplex(&tmp, ret.real, ret.imag));
> -}
> 
> --- eval.c  2011-01-11 16:25:01.000000000 +0100
> +++ eval.c.orig 2011-01-11 17:38:00.000000000 +0100
> @@ -176,8 +176,6 @@
>      {"atanh",  f_atanh},
>      {"lambertw",  f_lambertw}, /* HBB, from G.Kuhnle 20001107 */
>  
> -    {"sum",  f_sum}, /* summation */
> -
>      {"column",  f_column}, /* for using */
>      {"valid",  f_valid},   /* for using */
>      {"timecolumn",  f_timecolumn}, /* for using */
> --- specfun.h   2011-01-11 16:22:05.000000000 +0100
> +++ specfun.h.orig  2011-01-11 17:37:21.000000000 +0100
> @@ -61,6 +61,4 @@
>  void f_inverse_erf __PROTO((union argument *x));
>  void f_lambertw __PROTO((union argument *x));
>  
> -void f_sum __PROTO((union argument *x));
> -
>  #endif /* GNUPLOT_SPECFUN_H */
> 
>