Structure or array for S-function input, output and parameters?

Hi,

What is faster and what do you recommend?

I have a C S-function with 14 inputs. 1. Right now I am using a single port and I see the input as an array. I have 14 #define in order to help selecting the right input: for example #define U_IDX_FLOW_WATER 0 and then I select it as u[U_IDX_FLOW_WATER]. It helps understanding the code better than using a plain number.

2. I just came across passing structures to S-functions and, I guess, I could select my input like u->flow_water and forget about the hardcoding that I did before.

Is there a performance penalty when using approach 2?

Thanks, Remus.

Answer 1

[Kaustubha Govind]

I can't imagine there being a major difference in Simulink performance between #1 and #2 - they are equivalent to writing two C functions - one that takes several function arguments and the other takes one structure argument with each field represent arguments from the first function. So it boils down to more of a C question - is one more efficient that the other in C?

However, from my experience, using structure/bus signals to combine similar signals is good modeling practice.

[Remus Mihail Prunescu]

What about the state and derivatives vector? Can you get rid of indexing those and use structs instead?

My goal was to get rid of indexing vectors with numbers (because they are not human readable especially when the number of states gets to more than 10 and inserting a new state in the middle of the vector becomes annoying). I guess I could do it myself by tranforming the vectors to structs and vice-versa but has matlab built-in functions for it? ssGetdX(S) and ssGetContStates(S) return only real_T*.

[Remus Mihail Prunescu]

Actually it's easy to do:

typedef struct {
 real_T mass;
 real_T a_t;
 real_T b_t;
 real_T c_t;
} PX;

....

PX *x = (PX*)ssGetContStates(S);
PX *dx = (PX*)ssGetdX(S);

:D. Thanks!

[Kaustubha Govind]

Structure types are only supported for inputs, output and parameters, not states. But looks like you've found a solution anyway. :)