geting started with oop

hi everybody, i got a probally simple question but would be gratefull if anyone could help

i got the following code (that does not work)

classdef Knot
    properties
        x
        y
    end % properties
    methods
        function K = Knot(x,y)
            K.x=x;
            K.y=y;
        end%functions
    end%methods
end %class

classdef Slab<Knot
properties
    Material
    StartKnot
    EndKnoten
end%properties
methods 
    function S = Slab(StartKnot,EndKnot)
        S.StartKnot = StartKnot;
        S.EndKnot = EndKnot;
    end%function
end%methods
end%class

in my mind the subclass Slab should get the properties x and y.

trying to run it gives me this:

>> k1 = Knoten(1,1);
>> k2 = Knoten(2,2);
>> s= Stab(k1)
??? Input argument "x" is undefined.

Error in ==> Knoten>Knoten.Knoten at 8
            K.x=x;

Error in ==> Stab>Stab.Stab at 8
    function S = Stab(StartKnoten)

acctually i wanted to get a Slab class that has the properties of Knot and could be acceses like this:

k=Knot(1,1);kk = Knot(1,2);S = Slab(k1,k2);

Slab.StartKnot.x should deliver the x propertie of Knot

obviously i got something wrong

thankfull for help

Answer 1

[per isakson]

Firstly, you need to be more careful with the names. Should it be Knot or Knoten and Slab or Stab.

Secondly, the constructor of Knot requires values of the two inputs. Add the line:

if nargin == 0, return, end

to Knot

>> clear all, clear classes
>> k1 = Knot(1,1);
>> k2 = Knot(2,2);
>> s  = Slab(k1,k2)
s = 
  Slab

    Properties:
       Material: []
      StartKnot: [1x1 Knot]
        EndKnot: [1x1 Knot]
              x: []
              y: []
    Methods, Superclasses
  >> s.StartKnot.x
  ans =
       1

Thirdly, do you really want Slab both to inherit Knot and have Knot objects as values of properties?

[jeff wu]

thanks alot for your answer, it works now but i dont get why.when i create a StabObject i dont call the knot function? or is this because stab inherist all of the knot?

i want them to be inherited both because x and y could change of an knoten element without the stab knowing about it and otherwise. and i want to be able to call the knotcoordinates within stab.

thanx again

[per isakson]

When you create a Slab object the Knot constructor is called automatically because Slab inherits Knot. Matlab works that way.

I guess you need to read carefully about the difference between "handle" and "value" classes/objects.