cis162ad c# call-by-reference methods 06_methods_by_ref.ppt
DESCRIPTION
CIS162AD3 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 -> Process -> OutputTRANSCRIPT
CIS162AD – C#
Call-by-Reference Methods
06_methods_by_ref.ppt
CIS162AD 2
Overview Of Topics Review Top-Down Design Calling Event Methods Review pass-by-value Arguments Introduce Void Methods Introduce pass-by-reference Arguments
CIS162AD 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 -> Process -> Output
CIS162AD 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 it’s
return type defined as void. Methods can “return” more than one value through
call-by-reference arguments.
CIS162AD 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);
}
CIS162AD 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.
CIS162AD 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.
CIS162AD 8
Pass-by-value - Exampleprivate 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;
decExtended = intQty * decPrice;return decExtended;}
CIS162AD 9
Values are Passed between Methods
Procedure Address Variable Value
btnCalculate( ) 101010201030
intQtydecPricedecExtendedPrice
235.5071.00
calcExtendedPrice 104010501060
intQtydecPricedecExtended
235.5071.00
CIS162AD 10
Methods and Pass-by-Value Values of the arguments are passed to the
method’s local variables. Many values can be passed to methods, but
only one value can be returned. Enter void and pass-by-reference methods…
CIS162AD 11
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();
}
CIS162AD 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(intQty, decPrice, out decExtendedPrice);
CIS162AD 13
Pass-by-Reference Addresses of the arguments are passed to the
method’s 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.
CIS162AD 14
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.
CIS162AD 15
Pass-by-reference Exampleprivate 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;}
CIS162AD 16
Declare Variables and Get Values
Procedure Address Variable ValuebtnCalculate 1010
10201030
intQtydecPricedecExtendedPrice
235.500
calcExtendedPrice 104010501060
intQtydecPricedecExtended
000
CIS162AD 17
calcExtendedPrice(intQty, decPrice, out decExtendedPrice) pass-by-reference: address of decExtendedPrice is sent
Procedure Address Variable ValuebtnCalculate 1010
10201030
intQtydecPricedecExtendedPrice
235.500
calcExtendedPrice 104010501060
intQtydecPricedecExtended
235.501030
CIS162AD 18
Method Uses Local Names of Argumentsprivate 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;}
CIS162AD 19
Perform Calculations decExtended = intQty * decPrice;
Procedure Address Variable ValuebtnCalculate 1010
10201030
intQtydecPricedecExtendedPrice
235.5071.00
calcExtendedPrice 104010501060
intQtydecPricedecExtended
235.501030
Value is stored at memory location provided through argument.
CIS162AD 20
Returning Multiple Values Addresses of the arguments are passed to the
method’s 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.
CIS162AD 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.
CIS162AD 22
Summary
Pass-by-value Reviewed Void Methods Pass-by-reference