Error - 'All inputs must be constant' in MATLAB Coder

I am trying to convert the function poly2trellis into C using MATLAB Coder. The problem arise when I check the issues in the Coder App and it displays the error that ' all inputs must be constant'.

function [ trellis ] = con_poly2trellis(k) %#codegen
generator_matrix=[0 0];

if k==3
  generator_matrix=[5 7];
else
  generator_matrix=[32 35];
end

trellis = poly2trellis(k,generator_matrix);

end

Does it mean that I can generate code for a single specific input and not based on any value passed for 'k'?

What if a generic function is required which can accept any value of 'k' and I can include multiple if-else case for generator_matrix?

Please help and suggest.

Answer 1

[Walter Roberson]

if k==3
  trellis = poly2trellis(3,[5,7]);
elseif k==4

And so on.

[Puneet Khurana]

Thanks for the answer. I tried this and checked for the input k=4. But it is now producing error at k=3 Generator describes code of less than specified constraint length.

The coder checks each and every mention of the poly2trellis function for all the cases irrespective of input provided.

if K==2
    trellis = poly2trellis(2,[0 0]);
elseif K==3
    trellis = poly2trellis(3,[5 7]);
elseif K==4
    trellis = poly2trellis(4,[15 17]);
elseif K==5
    trellis = poly2trellis(5,[32 35]);
elseif K==6
    trellis = poly2trellis(5,[53 75]);
elseif K==7
    trellis = poly2trellis(7,[133 171]);
elseif K==8
    trellis = poly2trellis(8,[247 371]);
elseif K==9
    trellis = poly2trellis(9,[1167 1545]);
end

The coder checks for each if else and produces error. I even tried switch case.

[Walter Roberson]

https://www.mathworks.com/matlabcentral/answers/94694-why-do-i-receive-an-error-message-when-i-try-to-implement-a-convolutional-encoder-using-poly2trellis

[Puneet Khurana]

Yes the above link clarifies the usage of poly2trellis. But how to resolve the conversion of multiple if-else calls to poly2trellis by coder.

It seems to execute all function calls irrespective of the value of k.

[Puneet Khurana]

got it, thanks!!

[Walter Roberson]

It does not execute all of the function calls, it compiles all of the function calls down into the specific code needed for that one trellis variety.