Parameters, Functions, and Methods From wikipedia

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Parameters, Functions, and Methods From
wikipedia
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Expressions vs. Statements

• Expression
• Generates a new value
• In the shell, shows a result
• If x something makes sense, then something is
  an expression
• More precisely, if x does not become None, then
  something is an expression
• def myFunction() return ____means that
  calling myFunction() is an expression

• Statement
• Executed for side effect
• In the shell, shows no result
• print is kind of a special case, but not really
• If x something doesnt make sense, then
  something is a statement
• More precisely, if x becomes None, then something
  is a statement
• def myFunction() ltno return anywheregt
  means that calling myFunction() is a statement
• If you treat a statement as if it were an
  expression, then the statement gets executed (the
  side-effect occurs), and the statement
  evaluates to None.
• That is, if a statement is put in a context that
  requires an expression, the value None will be
  used.

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Parameter Passing

• Formal parameters
• The names of the parameters in the function (or
  method) definition
• Actual parameters
• The values passed in to a function (or method)
  invocation
• When you understand the following, you can figure
  out any parameter passing issue
• Parameter passing works asformalParam
  actualParam
• works as copy the box
• Copying for primitive types (int, float, long, )
• Aliasing for reference types (string, list,
  Point, )
• Watch exact description of functions / methods to
  see if its a mutator or not

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Consequences of the Rules on Parameter Passing

• Primitive types (int, long, float, some others)
  cant be changed
• All other mutable types can be changed (class
  instances, lists) using mutators
• So things like move(), and myListi ___
• These mutate will change the parameter
• If you construct a new object, then its a new
  object nothing you do to the new object will
  affect the original

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Methods

• A method is like a function, except that it is
  associated with an object.
• These are methods
• s.capitalize() for a String s
• p.getX() for a Point p
• p.setFill(c) for a Point p
• These are not methods (they are merely
  functions)
• string.capitalize(s)
• abs(n)
• math.sqrt(n)

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Accessors vs. Mutators

• An accessor is a method that accesses data from
  an object
• Ex p.getX()
• Often start with get (style)
• Calling an accessor is always an expression
• A mutator is a method that changes data internal
  to an object
• Ex p.setFill()
• Often start with set (style)
• But not necessarily p.move(dx, dy)
• Calling a mutator is (typically) a statement
• Strictly speaking, accessors and mutators only
  work on objects. That is, they are methods, not
  merely functions.
• objName.methodName()

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Is It A Mutator?

• What do you have to do to make the change happen?
• Nothing extra?
• p.move(5, 5)
• move() IS a mutator
• Use ?
• s s.upper() (p. 96)
• upper IS NOT a mutator
• Often, but not always
• Mutator does not return anything. Its a
  statement.
• Non-mutator returns something. Its an
  expression.

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Other Methods

• Some methods are neither accessors nor mutators,
  or it is less clear.
• The distinction gets less important here
• p.draw(win) ?
• Mutates win, in a sense, but the Point object
  itself doesnt change (?), so it isnt a mutator
  for Point.
• Unless Point maintains a list of windows its
  drawn on, in which case this would mutate Point.
• We only speak of mutators and accessors with
  respect to the object on the left of the dot.

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Writing Functions and Methods

• So far, weve used both methods and functions
• and were just now learning the details of
  writing functions.
• Well learn how to write methods later on.
• Once we do that, well see more of what it means
  to write a mutator, accessor, and other method