How do I write a MATLAB file S-function when the output width depends on the input width and input value?

I want to write a MATLAB file S-function that allows dynamic input and output width; besides, the output width depends on the input width and input value. For example, my mdlInitializeSizes routine contains the following code fragment:

sizes.NumOutputs     = -1;

sizes.NumInputs      = -1;

sizes.DirFeedthrough = 1;

and my mdlOutputs routine looks like the following:

function sys=mdlOutputs(t,x,u)

if (length(u) - 1) > 4 %% Input width larger than 4

      if u(1) == 1   

          y = [1;u(2:5)];  %% Assign first four input elements to output

      else   

          y = [0;u(2:length(u))];  %% Assign all input elements to output

      end

else  %% Input width less than or equal to 4

      y = [0;u(2:length(u))];  %% Assign all input elements to output

end

sys = y;

However, the S-function does not work when the output width is different from the input width. For example, when the input is [1, 2, 3, 4, 5, 6], the output is supposed to be [1, 2, 3, 4, 5]; but I receive the following error message:

 Output returned by S-function 'modified_dac' in block 'customer_dec/S-Function' 

 during flag=3 call must be a real vector of length 6

Answer 1

[MathWorks Support Team]

In order to make the output width dependent on input width and input value in a MATLAB file S-function, you can use the GET_PARAM function in the mdlInitializeSizes routine to define dynamic input and output width. An example, which consists of the my_decode.mdl and winnie_dec.m files, is available at the bottom of the page.

Instead of setting sizes.NumOutputs and sizes.NumInputs to -1, the following code fragment in the mdlInitializeSizes routine of the winnie_dec.m file also achieves dynamic input and output width:

L = length(str2num(get_param('my_decode/Constant','value')));

%%get the length of the vector in the constant block.

if (L - 1) > 4  %%Input width larger than 4

      in = str2num(get_param('my_decode/Constant','value'));

      %%Get the value of the input vector.

      in1 = in(1);%%Get the first element value of the input vector.

      if in1 == 1  %%First element is one

           out_width = 5;

      else

           out_width = L; 

      end

else  %% Less than four inputs

      out_width = L; 

end

sizes.NumContStates  = 0;

sizes.NumDiscStates  = 0;

sizes.NumOutputs     = out_width;

sizes.NumInputs      = L;

sizes.DirFeedthrough = 1;

In this example, input and output widths can be different and both are dynamic. For example, running the model with an input of vector [1, 2, 3, 4, 5, 6] will yield [1, 2, 3, 4, 5].