What's the difference between a method and a function?

Question

Can someone provide a simple explanation of methods vs. functions in OOP context?

Answer 1

Function is the concept mainly belonging to Procedure oriented programming where a function is an an entity which can process data and returns you value

Method is the concept of Object Oriented programming where a method is a member of a class which mostly does processing on the class members.

Answer 2

A function is a mathematical concept. For example:

f(x,y) = sin(x) + cos(y)

says that function f() will return the sin of the first parameter added to the cosine of the second parameter. It's just math. As it happens sin() and cos() are also functions. A function has another property: all calls to a function with the same parameters, should return the same result.

A method, on the other hand, is a function that is related to an object in an object-oriented language. It has one implicit parameter: the object being acted upon (and it's state).

So, if you have an object Z with a method g(x), you might see the following:

Z.g(x) = sin(x) + cos(Z.y)

In this case, the parameter x is passed in, the same as in the function example earlier. However, the parameter to cos() is a value that lives inside the object Z. Z and the data that lives inside it (Z.y) are implicit parameters to Z's g() method.

Answer 3

Methods on a class act on the instance of the class, called the object.

class Example
{
   public int data = 0; // Each instance of Example holds its internal data. This is a "field", or "member variable".

   public void UpdateData() // .. and manipulates it (This is a method by the way)
   {
      data = data + 1;
   }

   public void PrintData() // This is also a method
   {
      Console.WriteLine(data);
   }
}

class Program
{
   public static void Main()
   {
       Example exampleObject1 = new Example();
       Example exampleObject2 = new Example();

       exampleObject1.UpdateData();
       exampleObject1.UpdateData();

       exampleObject2.UpdateData();

       exampleObject1.PrintData(); // Prints "2"
       exampleObject2.PrintData(); // Prints "1"
   }
}

Answer 4

for me: the function of a method and a function is the same if I agree that:

• a function may return a value
• may expect parameters

Just like any piece of code you may have objects you put in and you may have an object that comes as a result. During doing that they might change the state of an object but that would not change their basic functioning for me.

There might be a definition differencing in calling functions of objects or other codes. But isn't that something for a verbal differenciations and that's why people interchange them? The mentions example of computation I would be careful with. because I hire employes to do my calculations:

new Employer().calculateSum( 8, 8 );

By doing it that way I can rely on an employer being responsible for calculations. If he wants more money I free him and let the carbage collector's function of disposing unused employees do the rest and get a new employee.

Even arguing that a method is an objects function and a function is unconnected computation will not help me. The function descriptor itself and ideally the function's documentation will tell me what it needs and what it may return. The rest, like manipulating some object's state is not really transparent to me. I do expect both functions and methods to deliver and manipulate what they claim to without needing to know in detail how they do it. Even a pure computational function might change the console's state or append to a logfile.

Answer 5

In OO world, the two are commonly used to mean the same thing.

From a pure Math and CS perspective, a function will always return the same result when called with the same arguments ( f(x,y) = (x + y) ). A method on the other hand, is typically associated with an instance of a class. Again though, most modern OO languages no longer use the term "function" for the most part. Many static methods can be quite like functions, as they typically have no state (not always true).

Answer 6

Methods are functions of classes. In normal jargon, people interchange method and function all over. Basically you can think of them as the same thing (not sure if global functions are called methods).

http://en.wikipedia.org/wiki/Method_(computer_science)