CIS162AD C

1
CIS162AD C

• Call-by-Reference Methods
• 06_methods_by_ref.ppt

2
Overview Of Topics

• Review Top-Down Design
• Calling Event Methods
• Review pass-by-value Arguments
• Introduce Void Methods
• Introduce pass-by-reference Arguments

3
Top-Down Design

• A design method where the major task to be
  accomplished is divided into subtasks.
• Each subtask should perform a single well-defined
  task.
• Each subtask may produce some result.
• Treat them as small programs.Input -gt Process -gt
  Output

4
Top-Down Design Implementation

• Top-Down design is implemented using methods.
• A method is a collection of statements that are
  grouped together to perform a specific operation.
• Methods have inputs called arguments.
• Methods can only return one value through the
  return statement.
• A method that will not return a value should have
  its return type defined as void.
• Methods can return more than one value through
  call-by-reference arguments.

5
Calling Event Methods

• One of the reasons to breakup programs into small
  methods is so that code does not duplicated. If
  there is a method that does what we need to do,
  we just call it.
• For example, we can add a menu option to perform
  the same calculations defined for a button (CS7).
• Instead of duplicating the code, we can call the
  button method which has the code defined and
  tested from the menu method. Be sure to pass the
  required arguments. private void
  mnuEditCalculate_Click (object sender,
  System.EventArgs e) btnCalculate_Click(sender,
  e)

6
Local Variables

• Variables are local to the method in which they
  are defined.
• Variables defined in a particular method are
  assigned their own memory and can only be
  referenced in that method.
• Variables defined in calcExtendedPrice( ) are
  assigned their own memory and can only be
  referenced in calcExtendedPrice( ).
• Different methods cannot see or reference each
  others variables.
• They have separate memory allocations even though
  the variable names may be the same.

7
Pass-by-value Reviewed

• Methods with pass-by-value arguments will be
  passing the values of the arguments into the
  methods local variables.
• The method cannot change the values stored in the
  calling method.
• Two different memory locations are used for each
  variable.
• One in the calling method and one in the called
  method.

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Pass-by-value - Example

• private void btnCalculate_Click() int
  intQty decimal decPrice, decExtendedPrice
  intQty int.Parse(txtQuantity.Text) decPrice
  decimal.Parse(txtPrice.Text) decExtendedPrice
  calcExtendedPrice(intQty, decPrice)
• private decimal calcExtendedPrice(int intQty,
  decimal decPrice) decimal decExtended decExt
  ended intQty decPrice return decExtended

9
Values are Passed between Methods
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Methods and Pass-by-Value

• Values of the arguments are passed to the
  methods local variables.
• Many values can be passed to methods, but only
  one value can be returned.
• Enter void and pass-by-reference methods

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Void Methods

• No value returned through return statement.
• Return statement is optional.
• Defined the same way as call-by-value, but
  return type is void.private void
  btnExit_Click (object sender, System.EventArgs
  e) this.Close()

12
Calling Void Methods

• A method that returns a value needs a variable to
  the left of the equal sign.decExtendedPrice
  calcExtendedPrice(intQty, decPrice)
• A call to a void method does not need a variable
  and equal sign on the left.calcExtendedPrice(int
  Qty, decPrice, out decExtendedPrice)

13
Pass-by-Reference

• Addresses of the arguments are passed to the
  methods local variables.
• Many values can be passed to the methods, and
  many values can be returned.
• Use the keyword ref or out to make the argument
  pass-by-reference.
• If you leave ref or out off, the default is
  pass-by-value.

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ref or out

• ref is used for a variable that will be
  initialized before the method call.
• out is used for a variable that might not be
  initialized before the method call, but the
  variable will initialized within the called
  method.

15
Pass-by-reference Example

• private void btnCalculate_Click() int
  intQty decimal decPrice, decExtendedPrice
  intQty int.Parse(txtQuantity.Text) decPrice
  decimal.Parse(txtPrice.Text) calcExtendedPrice
  (intQty, decPrice, out decExtendedPrice)
• private void calcExtendedPrice(int intQty,
  decimal decPrice, out decimal
  decExtended) decExtended intQty
  decPrice return

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Declare Variables and Get Values
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calcExtendedPrice(intQty, decPrice, out
decExtendedPrice) pass-by-reference address of
decExtendedPrice is sent
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Method Uses Local Names of Arguments

• private void btnCalculate_Click() int
  intQty decimal decPrice, decExtendedPrice
  intQty int.Parse(txtQuantity.Text) decPrice
  decimal.Parse(txtPrice.Text) calcExtendedPrice
  (intQty, decPrice, out decExtendedPrice)
• private void calcExtendedPrice(int intQty,
  decimal decPrice, out decimal
  decExtended) decExtended intQty
  decPrice return

19
Perform Calculations decExtended intQty
decPrice
Value is stored at memory location provided
through argument.
20
Returning Multiple Values

• Addresses of the arguments are passed to the
  methods local variables instead of the values
  stored in the arguments.
• The called method can then modify memory
  locations assigned to the calling method.
• When it does modify its memory location, it can
  be considered to have returned a value.
• Code many pass-by-reference (ref or out)
  arguments to return more than one value.

21
Summary of Arguments

• Arguments can be a mixture of pass-by-value and
  pass-by reference.
• If a method should NOT change the value, send it
  as a pass-by-value.
• If values are being returned through the
  arguments, then make the method a void method.

22
Summary

• Pass-by-value Reviewed
• Void Methods
• Pass-by-reference