Creating and Manipulating Models

Direct Property Referencing

An alternative way to query/modify property values is by structure-like referencing. Recall that LTI objects are basic MATLAB structures except for the additional flag that marks them as TF, ZPK, SS, or FRD objects (see LTI Objects). The field names for LTI objects are the property names, so you can retrieve or modify property values with the structure-like syntax.

• PropertyValue = sys.PropertyName% gets property value
  sys.PropertyName = PropertyValue% sets property value
  

These commands are respectively equivalent to

• PropertyValue = get(sys,'PropertyName')
  set(sys,'PropertyName',PropertyValue)
  

For example, type

• sys = ss(1,2,3,4,'InputName','u');
  sys.a
  

and you get the value of the property "a" for the state-space model sys.

• ans =
       1
  

Similarly,

• sys.a = -1;
  

resets the state transition matrix for sys to -1.

Unlike standard MATLAB structures, you do not need to type the entire field name or use upper-case characters. You only need to type the minimum number of characters sufficient to identify the property name uniquely. Thus either of the commands

• sys.InputName
  sys.inputn
  

produces

• ans = 
  
      'u'
  

Any valid syntax for structures extends to LTI objects. For example, given the TF model

• h = tf(1,[1,0],'variable','p');
  

you can reset the numerator to by typing

• h.num{1} = [1 2];
  

or equivalently, with

• h.num{1}(2) = 2;
  

Accessing Property Values Using get

Additional Insight into LTI Properties