Constants and Matlab Coder

Question

Some functions requires the input to be a constant, when run in Matlab Coder. I wish to find a way to declare the input as a constant before it is input as an example for the problematic situation:

  function foo = subsubfunction(x,y)            
      [B,A]=butter(1,x/y);

This will return the error 'All inputs must be constant'

How do I declare x and y as constants so that butter() gets happy? I have tried many solutions and unfortunately not found anything really satisfying. If the command line operation coder.newtype('constant',x) could be used it would simplify everything.

do you really mean constant or just one value and not a vector/array ? you could use the numel function... — Smash
– Smash, Commented Mar 2, 2012 at 15:55
Hello and thanks for the answer! But how do you mean that I could use numel to solve this problem? In which way will that ensure matlab that the input is constant? — Michael Ward
– Michael Ward, Commented Mar 2, 2012 at 16:28

Lokesh A. R. · Accepted Answer · 2014-01-14 15:17:03Z

2

Use coder.const in the function, so that the function butter knows you are passing a constant input. The documentation is available here.

 function foo = subsubfunction(x,y)            
  [B,A]=coder.const(@butter,1,x/y);

Note: You cannot change the value of x/y in the generated code. You could individually change x and y, but not the ratio of the two numbers.

edited Jan 14, 2014 at 15:17

answered Jan 14, 2014 at 14:36

Lokesh A. R.

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2 Comments

Dennis Jaheruddin Over a year ago

The documentation you refer to requires a login, please check whether this one is similar and consider updating the link: mathworks.nl/help/simulink/slref/coder.const.html

jpo38 Over a year ago

Does not work for me, please see: stackoverflow.com/questions/60202957/…

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