Passing Method Name as Argument

[Steve Emsley]

Hi,

I need to take my utilitarian knowledge of Objective C syntax up a
notch ...

Anyone who has used Numerical Methods Runge Kutta with recognize this
function prototype:

void rk4(double y[], double dydt[], int n, double h, double yout[],
        void (*derivs)(int, double [], double []))

The function derivs returns the derivatives of the n equations given
the initial values stored in y. The Runge Kutta advances the solution
for a step h and returns the answer in yout.

This routine is general since the function that calculates the
derivatives is passed as an argument.

I've been using this C version in my model but have now bit the bullet
and written the objectiveC methods:

-rk: (double *)y : (double *)dydt : (int)n : (float)h : (double *)yout
       : ?? ;

My problem is that I cannot work ur how to pass this method the
method: -derivs: (int) x : (double *) y : (double *) dydt

Naively I tried passing it as: (id) derivs and all combinations, such
as (id) (derivs: (int) x : (double *) y : (double *) dydx), which is
analogous to the C function being passed as a pointer to the function.

This doesn't work - many compiler objections!
After trawling my cache of mailing list advice and the ObjC manual I
tried to pass it using a selector:

-rk: (double *)y : (double *)dydt : (int)n : (float)h : (double *)yout
   : (SEL) deriv;

with the method call:

  [self rk: x : initial : 5 : step : result : @selector(derivs3:::)]

This doesn't work either:

warning: `ModelClass' does not respond to `deriv:::'

error: ModelClass (instance)
ModelClass does not recognize deriv:::
IOT trap/Abort

I've tried all the combinations I can think of without success. I then
added the method call:

  [self perform:@selector(deriv:::)withObject:2 :yt :dyt];

within the rk method leading to the error:

warning: `ModelClass' does not respond to `perform:withObject:::'

I'm lost ... could someone explain how I should perform this task. It
appears that arguments to selectors can only be type id. My arrays are
standard  C arrays rather than swarm objects. I was thinking that I
would have to cast the arguments to type (id). The calls within the rk
method are, for example:

  [self deriv: 3 : yt : dym];

How would I pass a double array to the selector.

Thanks in advance.

P.S. Not that my model is sensitive to imprecision in the 10th
sig. fig. does anyone have an explanation for this:

Using Objective C Runge Kutta
 5.9584421922662844e-06  1.7028652804204555e-13
Using standard C Runge Kutta
 5.9584421935048042e-06  1.7028652818251797e-13

The derivs routine is the same for both. In fact the only difference
is the form of the function header between C and ObjC. This precision
error doesn't propagate (at least I've checked for 24*365*10
iterations and the results from both methods are still the same except
in the 10th sig. fig). On my machine DBL_DIG = 16:    

printf(" %25.20e  %25.20e \n", (float) 1.e-18, (double) 1.e-18);
gives 1.00000004581370496574e-18  1.00000000000000007154e-18   

I can live with that (even when calculating the amount of nutrient
uptake to a single phytoplankton cell in 15 minutes!!) but am curious
as to why C and ObjC lead to different results.

Steve
-- 
---------------------------------------------------------------------
 Steve Emsley                 Ecosystems Analysis & Management Group
 address@hidden              University of Warwick, England
 http://www.oikos.warwick.ac.uk/~sme/           (+44) (0)1598 753648
---------------------------------------------------------------------

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