Subprograms Functions Procedures

Subprograms A subprogram separates the performance of some task from the rest of the program. Benefits: “Divide and conquer“ –Tasks are broken down into pieces, each of which can be programmed separately. This is modularisation. A subprogram can implement a piece of the calculation that is duplicated in different places. –Rather than duplicating the same code you can simply 'call' the subprogram.

Subprogram Statements A subprogram is a section of code with a name. That name can be used as a statement in another part of the program. When the name is encountered, the processing in the other part of the program halts while the named code is executed. This is an example of control coupling.

Subprogram Statements

There are times when the calling unit needs to give information to the subprogram to use in its processing. A parameter list is a list of the identifiers with which the subprogram is to work, along with the data types of each identifier. These are placed in parentheses beside the subprogram name.

Parameters In general, a parameter is any factor that defines a system and determines (or limits) its performance. It is useful, however, to distinguish between those that are used in the definition of the subprogram and those that are used by the calling program.

Parameters The parameters in the declaration of a subprogram are formal parameters. Formal - relating to or involving outward form or structure. The parameters in the statement that calls a subprogram are actual parameters. Actual - being, existing, or acting at the present moment.

Parameters To avoid confusion between formal and actual parameters, and to shorten our references to them, we will adopt a different set of terms…

Parameters Parameters: Identifiers listed in parentheses beside the subprogram declaration; otherwise called formal parameters. Arguments: Identifiers listed in parentheses on the subprogram call; otherwise called actual parameters.

Subprogram Parameters The passing of control and data between subprograms is called coupling. Data coupling can occur in two ways: –by value –by reference

Parameters Value parameter: A parameter that expects a copy of its argument to be passed by the calling unit. Reference parameter: A parameter that expects the address of its argument to be passed by the calling unit.

Subprogram Parameters Coupling parameters ByVal provides the value of the parameter to the subprogram. The subprogram can use this value in calculations, but changes will NOT be passed back. In effect, there’s a one way connection. Coupling parameters ByRef provides the address of the parameter to the subprogram. This creates a two way connection that carries data in both directions. Functions …

Function Subprograms A function is a subprogram that returns a value. We have already used many pre-defined functions, but it is also possible to declare our own.

Function Subprograms The header of a function declaration lists the function name, the parameters that it will receive, and the data type of its result. Private Function functionName (parameter list) _ As resultDataType ‘ function body End Function

Function Subprograms The parameter list may be empty, but the parentheses are required. Private Function functionName () _ As resultDataType ‘ function body End Function

Function Subprograms Private Function functionName _ () As resultDataType function body Return expression End Function A function must include a Return statement to specify which value is to be returned as its result.

Function Subprograms Private Function functionName () As resultDataType function body Return expression End Function The data type of the result is declared in the header. The expression that calculates the Return value must produce data of the specified type.

Parameter Lists The parameter list is very much like a list of declarations for variables used in the body of the function – with some important distinctions. 1.Use the keyword ByVal instead of Dim or Private

Parameter Lists 2.The variable will have already been assigned the value of whatever argument was specified in the function is call. The values of arguments are passed to parameters in the order in which they appear in the list.

Parameter Lists 3.The list is incomplete. Local variables needed for the function to do its job may be declared in the body of the function. The parameter list establishes a set of data connections between the calling routine and the function.

Parameter Lists Private Function Celsius_ (ByVal Fahrenheit As Single)_ As Single Return (Fahrenheit - 32) * 5 / 9 End Function

Function Subprograms Since a function produces a value, it will appear on the right side of an assignment operator. Private Function Celsius_ (ByVal Fahrenheit As Single) As Single Return (Fahrenheit - 32) * 5 / 9 End Function ____________________________________________________ Private Sub cmdGo_Click ( ) Dim degrees As Single degrees = txtInput.Text txtOutput.Text = Celsius (degrees) End Sub Passes CONTROL

Function Subprograms The parameter names in the subprogram declaration are local and need NOT be the same as the call. Private Function Celsius_ (ByVal Fahrenheit As Single) As Single Return (Fahrenheit - 32) * 5 / 9 End Function ____________________________________________________ Private Sub cmdGo_Click ( ) Dim degrees As Single degrees = txtInput.Text txtOutput.Text = Celsius (degrees) End Sub NOT the same

Function Subprograms When the function is called the value of its argument is passed to a formal parameter. Private Function Celsius (Fahrenheit) As Single Return (Fahrenheit - 32) * 5 / 9 End Function ____________________________________________________ Private Sub cmdGo_Click ( ) Dim degrees As Single Degrees = txtInput.Text txtOutput.Text = Celsius (degrees) End Sub Passes data

Function Subprograms When the function is complete the result value is passed back to the calling statement. Private Function Celsius (Fahrenheit) As Single Return (Fahrenheit - 32) * 5 / 9 End Function ____________________________________________________ Private Sub cmdGo_Click ( ) Dim degrees As Single Degrees = txtInput.Text txtOutput.Text = Celsius (degrees) End Sub Passes data Returns result

Function Subprograms A function can have several arguments but should only produce a single value. Private Function loops(first, last) As Integer Dim counter, loops As Integer loops = 0 For counter = first To last loops = loops + 1 Next Return loops End Function Procedures…

Procedure Subprograms A procedure is a subprogram that performs an action. We have already written many procedures that are called by an event, so the syntax is not new: Private Sub ProcedureName(argumentList) ‘ procedure body End Sub

Procedure Subprograms But it is also possible to create procedures that are not associated with control events. Private Sub convert() Dim degrees As Single degrees = txtInput.Text txtOutput.Text = (degrees - 32) * 5 / 9 End Sub must be included

Procedure Subprograms These procedures can be called by name from another procedure. Private Sub convert() Dim degrees As Single degrees = txtInput.Text txtOutput.Text = (degrees - 32) * 5 / 9 End Sub __________________________ Private Sub cmdGo_Click () Call convert End Sub

Procedure Subprograms Note that procedures perform actions, so to invoke them we simply call their name. Private Sub convert() Dim degrees As Single degrees = txtInput.Text txtOutput.Text = (degrees - 32) * 5 / 9 End Sub __________________________ Private Sub cmdGo_Click () Call convert End Sub Passes CONTROL

Procedure Subprograms End Sub passes control back to the calling subprogram. Private Sub convert() Dim degrees As Single degrees = txtInput.Text txtOutput.Text = (degrees - 32) * 5 / 9 End Sub __________________________ Private Sub cmdGo_Click () Call convert End Sub Passes CONTROL Returns CONTROL

Procedure Subprograms Sometimes it is necessary to send data to procedures. This is accomplished with a parameter list. Private Sub convert(ByVal degrees As Single) txtOutput.Text = (degrees - 32) * 5 / 9 End Sub ______________________________________ Private Sub cmdGo_Click () Dim temperature As Single temperature = txtInput.Text Call convert (temperature) End Sub

Procedure Subprograms Data is passed from the Call to the argument. Private Sub convert(ByVal degrees As Single) txtOutput.Text = (degrees - 32) * 5 / 9 End Sub ______________________________________ Private Sub cmdGo_Click () Dim temperature As Single temperature = txtInput.Text Call convert (temperature) End Sub Passes data

Procedure Subprograms Since the subprogram requires the data as input, it is passed ByVal. Private Sub convert(ByVal degrees As Single) txtOutput.Text = (degrees - 32) * 5 / 9 End Sub ______________________________________ Private Sub cmdGo_Click () Dim temperature As Single temperature = txtInput.Text Call convert (temperature) End Sub Passes data

Subprogram Parameters Often a procedure subprogram will both receive data and return data. To return values to the calling routine, parameters must be declared as ByRef.

Subprogram Parameters Private Sub convert (ByVal F As Single,_ ByRef C As Single) C = (F - 32) * 5 / 9 End Sub _______________________________ Private Sub cmdGo_Click() Dim degreesF As Single Dim degreesC As Single degreesF = txtStart.Text Call convert (degreesF, degreesC) txtOut.Text = degreesC End Sub

Subprogram Parameters Notice how multiple arguments are passed from the Call statement: Private Sub convert (ByVal F, ByRef C) _______________________________ Call convert (degreesF, degreesC) Arguments are listed in the same order they appear in the procedure header, separated by commas.

Subprogram Parameters There are 2 things to note in the example: Private Sub convert (ByVal F, ByRef C)

Subprogram Parameters There are 2 things to note in the example: Private Sub convert (ByVal F, ByRef C) 1.More than one parameter can be used. Data is passed according to the ORDER of the parameters. 1

Subprogram Parameters There are 2 things to note in the example: Private Sub convert (ByVal F, ByRef C) 1.More than one parameter can be used. Data is passed according to the ORDER of the parameters. 2.It is not necessary to identify the data type in the parameter list (but is good practice). 1 2

Side Effects Subprograms can produce dangerous side effects. For example: Private Function convert(ByRef F ) As Single Return (F - 32) * 5 / 9 F = F + 50 ‘why? is unimportant End Function _____________________________________ Private Sub cmdGo_Click() Dim degreesF As Single degreesF = txtIn.Text txtOut.Text = convert( degreesF ) txtIn.Text = degreesF End Sub

Side Effects Sub cmdGo_Click() calls the convert function, coupling degreesF with F. Private Function convert(ByRef F ) As Single Return (F - 32) * 5 / 9 F = F + 50 End Function _____________________________________ Private Sub cmdGo_Click() Dim degreesF As Single degreesF = txtIn.Text txtOut.Text = convert( degreesF ) txtIn.Text = degreesF End Sub

Side Effects The function calculates a value which is passed back and assigned to txtOut.Text. Private Function convert(ByRef F ) As Single Return (F - 32) * 5 / 9 F = F + 50 End Function _____________________________________ Private Sub cmdGo_Click() Dim degreesF As Single degreesF = txtIn.Text txtOut.Text = convert( degreesF ) txtIn.Text = degreesF End Sub

Side Effects A new value is assigned to F, which is coupled with degreesF. Private Function convert(ByRef F ) As Single Return (F - 32) * 5 / 9 F = F + 50 End Function _____________________________________ Private Sub cmdGo_Click() Dim degreesF As Single degreesF = txtIn.Text txtOut.Text = convert( degreesF ) txtIn.Text = degreesF End Sub Changes F And changes degreesF

Side Effects For safety, parameters passed to functions should ALWAYS be stipulated as ByVal. Private Function convert (ByVal F ) As Single Return (F - 32) * 5 / 9 F = F + 50 End Function _____________________________________ Private Sub cmdGo_Click() Dim degreesF As Single degreesF = txtIn.Text txtOut.Text = convert( degreesF ) txtIn.Text = degreesF End Sub Changes F But NOT degreesF

Side Effects Of course the same kind of thing can happen with procedure subprograms. The best practice is to declare only those parameters specifically needed to return data as ByRef.