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Programming Fundamentals 5

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Programming Fundamentals

This lecture introduces the following topics:– Procedures

Built-in Functions User-defined Procedures

– Functions– Subroutines

Passing data via Arguments

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Procedures A procedure is a set of instructions that

perform a particular service. (It is a module in structured programming terminology.)

There are 2 generic categories of procedures: built-in (intrinsic) and user-defined.

There are also 3 types of procedures: Functions, Subroutines, and Property procedures.

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Procedures All built-in Visual Basic procedures are of the

function type. User-defined procedures can be either the

function or subroutine type. Functions return a value. Subroutines do not

return a value. Functions and subroutines are capable of

performing the same kind of work. They do it in slightly different ways.

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Procedures

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Procedures versus Methods

Methods are also a type of procedure They perform work for a specific type of object. All methods are procedures, but not all procedures

are methods. Methods differ from procedures in that a method may

only be called from a specific object while generic procedures can be called from any object.

Ex: Debug.write, intX.ToString, strY.Trim

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Built-In Functions

Built-in (intrinsic) functions are used anywhere an expression is valid. For example:datX = Date ’Date’ is an intrinsic function

Some functions require additional information (called arguments).sngX = Sqrt(10)strX = MsgBox(“Prompt”,vbYes,”Error”)sngX = Cos(Rnd)

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Built-In Functions

VB provides many built-in functions.Date - returns current dateFormat - date or number converted to a text stringInputBox - text entered into a dialog box by userMsgBox - text displayed to user at run-timeLen - number of characters in a text stringMid - selected portion of a stringRnd - random numberInStr – find character(s) in a stringVal - string converted to a numberand many more.....

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Built-In Functions Built-in functions are provided by Visual Basic as a

convenience to the programmer. Most could be written as user-defined functions, but

there is no need to do this since they are already written and tested.

You should be familiar with the built-in functions provided by the language to improve personal productivity.

A number of built-in functions are also available as methods in VB .Net.

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Built-In Methods

Objects in VB have methods associated with them.

Methods give VB objects functionality (that is, it allows them to act upon data).

Many VB methods do things that can also be done with functions.

Using methods is preferred because they are compatible with other .NET languages.

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Built-In Methods

VB provides many methods.Number.toString – converts the number to a stringString.toInt32 – converts the string to an integer data typeString.toInt16 – converts the string to a short data typeString.length – returns the length of the string variableString.split – places each string component into an array elementMath.Sqrt(Number) – returns the square root of the numberMyArray.count – returns the number of elements in the arrayMyArray.sort – sorts the elements of an arrayMyArray.reverse – reverses the order of elements in an arrayand many more.....

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User-Defined Procedures

If a built-in function does not exist to perform needed work, you can build your own as a user-defined procedure.

User-defined procedures can be either the function type or the subroutine type.

They can be defined in form modules and code modules.

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User-Defined Functions

Function Syntax:

[Public | Private] [Friend] Function function-name [(args)] [As type][statements][function-name = expression] or [Return expression][Exit Function][statements][function-name = expression]

End Function

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User-Defined Function Example 1

Private Function Divide_Num (byVal sngX as single, byVal sngY as single) as singleIf sngY = 0 then

Return 0Exit Function

End IFReturn (sngX / sngY)

End Function You return a value to the calling routine by assigning

the return value to the function name.

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User-Defined Function Example 2

Public Function Time_Check()as booleanIf TimeValue(Now) < #08:00:00# then

Return vbFalseElse

Return vbTrueEnd IF

End Function Checks the current system time. If its before 8 AM, it

returns false, otherwise it returns true.

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User-Defined Functions

Functions are called by using their name in an expression. The arguments are enclosed in () and are separated by commas.sngX = Divide_Num(15.2, 17.3)

If there are no arguments, you can leave off the ().blnX = System_Status

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User-Defined Functions

User-defined functions have data types. This determines the type of the return value. If not specified, the return type is object.

Since functions return values, they can be used to build up larger (more complex) statements.If Check_It(strInput) then ...

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User-Defined Sub Procedures

There are 2 types of sub procedures: General procedures and Event procedures.

General procedures are invoked by the programmer specifically calling them in the program.

Event procedures are automatically invoked by the system when a specific event occurs. Event procedures have private scope only.

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Sub Procedures

Sub Procedure Syntax:

[Public | Private] [Friend] Sub subroutine-name [(args)] [statements][Exit Sub] or [Return][statements]

End Sub

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Sub Procedure Example 1

Private Sub Compute_Area (byVal sngLen_ as single,byVal sngWidth as single)Dim dblArea as DoubleIf sngLen = 0 or sngWidth = 0 then

Exit SubEnd IfdblArea = sngLen * sngWidthSystem.Console.WriteLine dblArea

End Sub

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Sub Procedure Example 2

Public Sub Beep_Bell ()Dim intX as integerDim intCnt as integerintX = InputBox(“How Many Beeps?”)For intCnt = 1 to intX

BeepNext intCnt

End Sub

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Sub Procedures

General sub procedures are called with the Call statement.Call Compute_Area(11, 21)Call My_SubProcuedure(intX)

Because they do not return a value, subroutines cannot be used to build expressions.

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User-Defined Procedures

Many times you can use either a function or a subroutine procedure to accomplish the same task.

Professional programmers generally use functions for most user-defined procedures because you can test the success of the execution and proceed appropriately.

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Arguments

There are 2 basic methods of passing data to a user-defined procedure: Increase the variable scope or use arguments.

The preferred method is to pass data to the procedure via arguments.

This allow you to better control the data that is used by the procedure.

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Arguments

Passing Arguments to Procedures:– Argument Data Type– Passing Arguments by Value– Passing Arguments by Reference– Optional Arguments– Indefinite Number of Arguments– Named Arguments

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Argument Data Type

All arguments have a data type. If you do not declare a specific type, it is object by default.

Function X (byVal strX as string, byVal objY as object) ...

Sub Y (byVal datZ as date, byVal sngR as single) ...

Function Z (byVal intX as integer, byVal objG) as single ...

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Passing Arguments by Value

A copy of the variable is passed to the procedure. Any changes affect only the copy, not the original. This is the default.

Sub X (ByVal intY as integer)

Function Y (objX, ByVal strY as_ string)as Integer

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Passing Arguments by Reference

The actual variable is passed to the procedure. Any changes to the variable affect the original.

Sub R (byRef intY as integer)

Function Z (byRef objX, byRef blnG as boolean)as Integer

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byRef Arguments

When arguments are passed byRef, any changes made to the value of the variable by the procedure are visible to the entire scope of the variable.

This means that a subroutine can return a value to its calling code if variables are passed ‘byRef’.

Also, this would be somewhat more efficient if the things being passed were large (i.e., objects).

Still better to do this with Functions if you have no overriding reason to use byRef.

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Using Optional Arguments

You can specify that arguments be optional. All optional arguments must supply a default value.

This value cannot be a variable. Once you define one optional argument, all

subsequent arguments must be optional also.Function A (byVal intX as integer, Optional byVal

intY as integer = 2, Optional byVal strZ as string = “test”) ...

To call this function: Answer = A(4,,”string”)

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Using Indefinite Arguments

You can allow an arbitrary number of arguments if the last argument is specified as a ParmArray.

Sub mySum (byRef intSum as integer, _ byVal ParmArray intNums() as integer)

Dim intX as integerFor Each intX in intNums

intSum = intSum + intNums(intX)Next intX

End Sub

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Using Indefinite Arguments

There can only be one ParmArray in a procedure and it must be the last thing defined.

It must be passed byVal. The code in the procedure must treat it as a

one-dimension array. All arguments preceding the ParmArray word

must be required (that is, no Optional arguments allowed).

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Named Arguments

You can specify named arguments by using the := operator in the procedure call.

Function X (byVal strX as string, byVal intX as integer) as Boolean

...

Return_Value = X(intX := 7, strX := “My Name”)

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In-Class Example 1

Write a user-defined function called My_Length to accept a string and return the length of that string to the user.

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Answer #1

Private Function My_Length (byVal strInput as string) as IntegerReturn len(strInput)

End Function

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In-Class Example #2

Modify the previous example to return “a big string” if the string is more than 10 characters long. Return “a small string” if it is 10 or less.

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Answer #2

Private Function My_Length (byVal strInput as string) as StringDim intLen as integerintLen = len(strInput)If intLen > 10 then

Return “a big string”else

Return “a small string”End If

End Function

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In-Class Example #3

Write a user-defined sub procedure called My_Clear to clear the computer screen when called. (Note, the ‘cls’ function does this.) Next, write code to call the procedure.

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Answer #3

Private Sub My_Clear ()cls

End Sub--------------------------Call My_Clear()