MATLAB: Help with understanding a piece of code

classMATLABoop

Hi all!

Could you please help me understand this piece of code here?

classdef Tire<handle
    properties (Constant)
          DEFAULT_DIR = 'C:\Users\Vicente Silva\Desktop\';
    end
      properties
          Name;
          Dir;
      end
      properties
          % here comes the declaration of a bunch of properties, 
          % which are not important for understanding the code itself
      end
      methods
          function tire = Tire(varargin)  
              tire.Name = varargin{1};
              if nargin > 1               
                  tire.Dir = varargin{2};
              else
                  tire.Dir = Tire.DEFAULT_DIR;
              end
              try
                  load([tire.Dir tire.Name '.mat']);
              catch               
                  tire = tire.LoadFromTir([tire.Name '.tir']);
                  tire.Fzo = tire.FNOMIN;
              end
          end
          function tire = LoadFromTir(tire, tir_filename)
              tirfile = fopen([tire.Dir tir_filename], 'r');
              line = fgetl(tirfile);
              while ischar(line)
                  if regexp(line, '^[A-Z]')
                      prop = strsplit(line,{'=' '$'});
                      propname = char(strtrim(prop{1}));                  
                      if isprop(tire, propname)
                          propval = char(strtrim(prop{2}));
                          if ~strncmpi(propval,'''',1)
                              tire.(propname) = str2double(propval);
                          else
                              tire.(propname) = strrep(propval, '''', '');
                          end    
                      end
                  end
                  line = fgetl(tirfile);
              end
              fclose(tirfile);
          end
          function Save(tire)
              save([tire.Dir tire.Name '.mat'], inputname(1));
          end

I have used 'help' command for most of the commands I didn't know, but it didn't help understand much. More importantly, it surely didn't allow me to understand how to keep on writing pieces of code myself! Keep in mind, this is part of a work I'm currently developing with a friend of mine. But he is the computer sciences student, and I'm the mechanical engineering one. I only understand a bit of coding in MATLAB and Simulink. This here is practically russian for me. I have tried asking my friend for more help and clarifications, but it seems he is busy. I really wish I could do all this work on my own, but I'm at a much more superficial level of coding right now. My work is due at the 15th this month, and without any help I won't be able to finish everything. Please, help me!

Best Answer

Which part of it do you not understand? There are a number of different elements in that code and if you already understand some aspects it would be a waste of time someone explaining them in detail! So in relatively little detail:

Properties blocks store class properties that are available to any function in the class, similar to variable in function scope, but referred to with the object scope when used (I assume you have no problem with these)
Methods blocks contain the functions that you can call. In this case all may be called from external to the class, whether that is the intention or not.
The first function is the constructor which takes a variable sized array of inputs and creates the actual object instance. It does no validation of the inputs, but the expected inputs can be worked out easily enough from the code which assigns them to the relevant properties. The body of the constructor appears to try to load data from a .mat file and if it fails it loads from a .tir file instead (which I assume is always valid since it has no error handling on that part).
The second function is the loadFromTir called in the constructor when loading from .mat fails. The body of that uses regular expressions to extract the required information to store in the class properties using dynamic strings for the property names.
The save function should theoertically save the tire name and directory to a mat file, but since inputname appears to be undefined I assume that function will crash.

The following syntax:

tire.(propname)

allows you to use dynamic strings to assign to properties, but it does come with the assumption that ' propname ' is something that evaluates to one of the properties defined in your properties blocks. It is a method I have used to good effect on a few occasions, but can easily become the source of difficult to track down bugs due to its obfuscation of the property names it is actually assigning to. Basically this will load in and assign all those properties you omitted as "not important for understanding the code itself".

The actual code in the load function is mostly just dynamic string manipulation with regular expressions and other string manipulation functions that trim off extra bits of the string to get the correct property names in the strings to then assign as mentioned above.

If you need more understanding of the exact nature of the regexp and string manipulations I will let someone else add that as it is something I battle with occasionally and forget quickly afterwards!! Your tags suggest it is the OOP aspect you are struggling with though.

Related Solutions

MATLAB: Object composition and property updating

You should think about whether the tire needs to be exposed as a public property on the car, or whether it can be private. In any case, one approach for your problem is to use SET methods for properties. An example is below, where the car has a public color, which when set updates the car's bodyColor and its tire's paintColor.

 classdef Car < handle
    properties(Dependent)
        color;
    end
    properties(Access = private)
        tire;
        bodyColor;
    end
    methods        
        function set.color(this, v)
            this.bodyColor = v;
            this.tire.paintColor = v;
        end
        function v = get.color(this)
            v = this.bodyColor;
        end
        function obj = Car
            obj.tire = Tire;
            obj.color = 'black';
        end
    end
 end
 classdef Tire < handle
    properties
        paintColor = 'black'; 
    end
 end

Using that approach, the tire doesn't even need to know what car it is attached to. Now if it is the case that you need to have the tire exist separately from the car, another approach would be to have the tire's color dependent upon the car it is attached to:

 classdef Car < handle
    properties(Dependent)
        color;
    end
    properties(Access = private)
        tire;
        bodyColor;
    end
    methods        
        function set.color(this, v)
            this.bodyColor = v;
        end
        function v = get.color(this)
            v = this.bodyColor;
        end
        function obj = Car(aTire)
            obj.tire = aTire;
            obj.tire.car = obj;
            obj.color = 'black';
        end
    end
 end
 classdef Tire < handle
    properties
        car;
    end
    properties(Dependent, SetAccess = private)
        paintColor = 'black'; 
    end
    methods
        function v = get.paintColor(this)
            v = this.car.color;
        end
    end
 end

MATLAB: Should one use class and enumeration instead of structures? and what effect MATLAB will result after using class and enumeration

I always use classes over structures as they are a lot more different in Matlab than e.g. C++.

A struct is just a christmas tree that you can hang anything off at any time you want. Very nice and convenient, but can be a nightmare for debugging, sharing code amongst others, readability, etc etc because just by looking at a piece of code you have no idea what fields are on a struct at any given moment unless it is entirely defined and limited to the scope you are looking at.

A class defines clearly what its properties are and obviously has the big advantage of wrapping methods up with those too (i.e. like a class in any language).

I would never really compare specifically an enumeration with a struct though. Where I don't use an enumeration I generally use strings instead, I have never considered a struct to be in any way an alternative to an enumeration.

You need to expand on your question though if you want an answer more tailored to what you want. You didn't ask anything at all or explain what you want t o know in the body of your question.

Best Answer

Related Solutions

MATLAB: Object composition and property updating

MATLAB: Should one use class and enumeration instead of structures? and what effect MATLAB will result after using class and enumeration

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