Fall 2006AE6382 Design Computing1 OOP Programming in Matlab Review our ASSET class and study how it might be used as a parent for other classes Extend the ASSET class to a subclass of STOCK Consider others like: BONDS & SAVINGS. Learning Objectives We will continue exploring objects and will construct a subclass called STOCK from the parent ASSET class Topics

Fall 2006AE6382 Design Computing2 Creating a Subclass Our ASSET class is of limited use because we will need to use it to describe many different kinds of assets –We could simply keep adding more properties to cover all situations we might encounter… –but this makes for very messy objects with lots of unused or unknown (and therefore dangerous) data –A more elegant way to handle this problem is to create a subclass of ASSET for each type of asset we might work with. Consider a STOCK subclass of ASSET –We will want to include the number of shares and the share price –We’ll also need to update the CurrentValue property in the parent ASSET object Properties: descriptor date current_value ASSET class Inherited Fields: descriptor date current_value STOCK Fields: num_shares share_price STOCK class

Fall 2006AE6382 Design Computing3 Creating the STOCK Subclass STOCK will be a subclass (child) of ASSET Create directory in WORK for class files function s = stock(varargin) % STOCK Stock class constructor % s=stock(descriptor, num_shares, share_price) switch nargin case 0 % no arguments so create default object s.num_shares = 0; s.share_price = 0; a = asset('none',0); % create dummy object to define parent class s = class(s,'stock',a) % create STOCK object as subclass of ASSET case 1 % single argument so make copy if (isa(varargin{1},'stock')) s = varargin{1}; else error('STOCK: Input argument is not a STOCK object') end case 3 s.num_shares = varargin{2}; s.share_price = varargin{3}; current_value = s.num_shares.* s.share_price; a = asset(varargin{1},current_value); % create parent object s = class(s,'stock',a); % create STOCK object as child of parent otherwise error('STOCK: Wrong number of input arguments.') end define the STOCK properties create a “parent” object now create the STOCK “child” object Note: parent object must be created with specified property values

Fall 2006AE6382 Design Computing4 STOCK display Method Just like display() for ASSET function display(s) % DISPLAY(a) displays a STOCK object display(s.asset) % display properties in parent object str = sprintf('Number of shares: %g\nShare Price: %3.2f\n',... s.num_shares, s.share_price); disp(str) First display parent property values Then display subclass property values NOTE: the 3 rd line shows how you access properties in another object ( obj_name.obj_type )

Fall 2006AE6382 Design Computing5 STOCK get Method function val = get(s, prop_name) % GET: Get STOCK properties from specified object if nargin==1 % list object’s properties if only object is given disp('STOCK properties:') disp(' Descriptor, Date, NumberShares, SharePrice, CurrentValue') return end if ~ischar(prop_name), error('GET: prop_name must be string.'), end prop_name = lower(prop_name(isletter(prop_name))); %remove nonletters if (length(prop_name) < 2), error('GET: prop_name must be at least 2 chars.'), end switch prop_name(1:2) case 'nu' val = s.num_shares; case 'sh' val = s.share_price; case 'de' val = get(s.asset,'descriptor'); % call ASSET GET method case 'da' val = get(s.asset,'date'); % call ASSET GET method case 'cu' val = get(s.asset,'current_value'); % call ASSET GET method otherwise error(['GET: ',prop_name,' is not a valid asset property.']); end Display object properties get STOCK properties get parent ASSET properties using its SET method

Fall 2006AE6382 Design Computing6 STOCK set Method Must define which properties can be set and which will be computed Providing error checking is IMPORTANT function s = set(s,varargin) % SET: set STOCK properties and return updated object. % Use: xyz=stock(xyz,'prop_name', value,...); % (Note: you must assign results to same or new object) % stock(xyz) returns list of settable properties. if nargin==1 % list settable properties if only object is given disp('STOCK properties:') disp(' Descriptor, Date, NumberShares, SharePrice, CurrentValue') return end if rem(nargin,2) ~= 1 error('SET: prop_name, values must be provided in pairs.') end Display object properties if only object is given Make sure that proper number of arguments are provided… continued on next slide

Fall 2006AE6382 Design Computing7 STOCK set Method (cont’d) for k=2:2:nargin-1 prop_name = varargin{k-1}; if ~ischar(prop_name), error('SET: prop_name must be string.'), end prop_name = lower(prop_name(isletter(prop_name))); %remove nonletters value = varargin{k}; switch prop_name(1:2) case 'nu' s.num_shares = value; case 'sh' s.share_price = value; case 'de' s.asset = set(s.asset,'descriptor',value); %set parent property otherwise error('SET: invalid prop_name provided.') end % update current value of stock in parent: value = s.num_shares.* s.share_price; s.asset = set(s.asset,'CurrentValue',value,'Date',date); Set properties in STOCK object Must compute STOCK value and then set in parent field Set property in parent ASSET object Note that not all properties can or should be settable!

Fall 2006AE6382 Design Computing8 Examples Here we create a STOCK object with initial values and then use the SET method to update the price. >> s1=stock('Intel',25,15) Descriptor: Intel Date: 03-Dec-2001 Current Value: Number of shares: 25 Share Price: >> set(s1); STOCK properties: Descriptor, Date, NumberShares, SharePrice, CurrentValue >> s1=set(s1,'SharePrice', 20) Descriptor: Intel Date: 03-Dec-2001 Current Value: Number of shares: 25 Share Price: Note that SET reports the settable properties for this calling syntax

Fall 2006AE6382 Design Computing9 Example: Changing Property Values Use set to change a property value REMEMBER: you must reassign the output to the object or the change will not be permanent! >> set(s1,'SharePrice', 30) Descriptor: Intel Date: 03-Dec-2001 Current Value: Number of shares: 25 Share Price: >> display(s1) Descriptor: Intel Date: 03-Dec-2001 Current Value: Number of shares: 25 Share Price: Update the share price It is reported in the automatic display when no semicolon is provided But… if you try a subsequent display(), the price was not changed!

Fall 2006AE6382 Design Computing10 Creating the subsref Method In order to be able to access STOCK fields using Matlab’s array indexing or structure field notation, we must define subsref for this class… function b = subsref(s, index) % SUBSREF Define field name indexing for STOCK objects fc = fieldcount(s.asset); %find out how many fields are in parent object switch index.type case '()' if (index.subs{:} <= fc) b = subsref(s.asset,index); else switch index.subs{:} - fc case 1 b = s.num_shares; case 2 b = s.share_price; otherwise error(['SUBSREF: Index must be in range 1 to ',num2str(fc+2)]) end case '.' switch index.subs case 'num_shares' b = s.num_shares; case 'share_price' b = s.share_price; otherwise b = subsref(s.asset,index); end case '{}' error('SUBSREF: Cell array indexing not supported for STOCK objects.') end Each case defines a different type of indexing: array, field, or cell See Matlab documentation for further explanation of this method

Fall 2006AE6382 Design Computing11 Creating the subsasgn Method In order to be able to assign values to STOCK fields using Matlab’s array indexing or structure field notation, we must define subsasgn for this class… function s = subsasgn(s, index, val) % SUBSASGN Define index assignment for STOCK objects fc = fieldcount(s.asset); % get # fields in parent object switch index.type case '()' if (index.subs{:} <= fc) s.asset = subsasgn(s.asset,index,val); else switch index.subs{:} - fc case 1 s.num_shares = val; case 2 s.share_price = val; otherwise error(['SUBSASGN: Index must be in range 1 to ',num2str(fc+2)]) end case '.' switch index.subs case 'num_shares' s.num_shares = val; case 'share_price' s.share_price = val; otherwise s.asset = subsasgn(s.asset,index,val); end case '{}' error('SUBSASGN: Cell array indexing not supported for asset objects.') end Each case defined a different type of indexing: array, field, or cell See Matlab documentation for further explanation of this method

Fall 2006AE6382 Design Computing12 Example: Using subsref We’ll create a STOCK object and then access its properties using array indexing >> s2=stock('oracle',20,35) Descriptor: oracle Date: 03-Dec-2001 Current Value: Number of shares: 20 Share Price: >> s2(1) ans = oracle >> s2(4) ans = 20 >> s2.share_price ans = 35 >> s2{1} ??? Error using ==> stock/subsref SUBSREF: Cell array indexing not supported for STOCK objects. Access the first property using array indexing The 4-th property is the number of shares You can also use the internal field names (if you know them) Note that we didn’t define cell indexing for our STOCK objects

Fall 2006AE6382 Design Computing13 Other Methods for STOCK While these methods provide powerful capabilities for our new objects, there are some other methods we might want to consider… –Methods to convert the data from one form into another convert between different numeric formats like double and int32 this is more important for a simple class like char or double –Method to convert the data into a string or cell array toString is a common OOP method name for this purpose converting to a cell array may be more useful when an object contains many different fields –We will have to create loadobj and saveobj methods if we want to be able to save our objects to a. mat file along with the rest of the workspace

Fall 2006AE6382 Design Computing14 Where to from Here? With only a little modification we can create an entirely new subclass that is based on our STOCK class –We can do this by specifying that our new class will inherit the structure and methods from a parent or superclass (or from several parent classes). –This is called “inheritance” and is an important feature of OOP. Consider a BOND class –It has an identical structure to a STOCK, –But there will be different properties –We will create new display, get and set methods –We will use the set and get methods from ASSET to modify properties in the parent

Fall 2006AE6382 Design Computing15 Summary… OOP has let us create an entirely new data type in Matlab –this is done by creating a class to define this data type –we must also create all the methods (functions) that can be used with this special data type –the internal details are hidden inside the class What else could we do with these new classes? –A portfolio might contain a mix of STOCK, BOND and SAVINGS assest… How can we keep this together? –AGGREGATION allows us to construct a new class whose contents are objects from other classes –See Matlab reference docs for an example of how to do this.

Fall 2006AE6382 Design Computing16 Summary Action Items Review the lecture Perform the exercises See if you can create the BOND class and get it working like we did for the STOCK class. Consider how this might be extended to other applications Learning Objectives OOP concepts STOCK subclasse Other subclasses Aggregation Summary