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Comparing Objects in C++

Comparing two objects

You can run a method on an object in C++ (or in any other OO language) to find an attribute of that object - let's say how costly it is (example, how costly it is to jave your group see a film). But you can't really run a method on TWO or more objects, which is what you would need to do if you were looking for the cosylier.

Solution ... call a method on one object and pass the secondf method in as a parameter ... so the call might look like this:

Film * pry_c = Babysitting[0]->seeer(Babysitting[1]);

You'll notice that the call returns an object of the type that you're comaring and not a cost value. If you want to find the most expensive, you can always call the method that tells you the price of the returned object, but if you wrote the code the other way around you wouldn't be able to get the other details of the object (e.g. which film) from the value you got back.

Here's the code of the seeer method:

Film * Film::seeer(Film * that) {
  if (this->see() > that->see()) return this;
  return that;
  }


this is a reserved word in C++, referring to the current object. that is just my choice of name for the other object we're comparing to.

Comparing a whole array of objects

When it comes to comparing a whole lot of objects, there's a different issue - you can't define a mehod with a completely unknown number of paramaters and it would be hard to code anyway. So to find the costlIEST, you are best advised to use a static method ... so the call would look something like this:

Film * pry_c = Film::seeest(Babysitting,3);

And here's the code for that method:

Film * Film::seeest(Film ** options, int owmany) {
  Film * sofar = options[0];
  for (int i=1; i<owmany; i++) {
    sofar = sofar->seeer(options[i]);
    }
  return sofar;
  }


This starts off by taking element zero as the costliest, then compares it (or the costliest so far) to all the other films in the array .. using the "sofar" varaible to keep note of the costliest so far.

In other languages such as Perl, PHP and Python, you can dynically expand a list or array, and you have functions to tell you how long it is. That is NOT the case in C++, so the method in this example is also provided with a count to tell it how many items to check. The alternative technology here would be to have an array that's longer than you need and put a cardinal value (such as a NULL) after the ens of the data, as is done when you make char arrays into strings.

And Finally

This example is demonstrated using a base class of "Film", but in fact the full example that I wrote during the course this week uses classes of type "Cinema" and "Blockbuster" both of which are derived from "Film". This is a really excellent example of the use of inheritance and polymorphism, as there was only one set of comparators to write and I can use them to compare two cinemas, two blockbusters, or one of each.

You can see the full source code of this example here and learn about things like this on our C++ Course.
(written 2008-06-13)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
C235 - C and C based languages - I/O in C++
  [3810] Reading files, and using factories to create vectors of objects from the data in C++ - (2012-07-21)
  [3807] Reading (and writing) files in C++ - (2012-07-18)
  [3390] Printing objects in C++ - (2011-08-13)
  [3252] C++ - unknown array size, unknown object type. Help! - (2011-04-17)
  [3124] C++ - putting the language elements together into a program - (2011-01-08)
  [1478] Some new C programming examples - files, structs, unions etc - (2007-12-19)


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