Procedures (Subroutines) and Functions

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Hubert Summers
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which stands by itself and should not be mixed up with anything else, Second they make a block of code

1 VISUAL BASIC Procedures (Subroutines) and Functions Copyright 2014 Dan McElroy Procedures and Functions Serve Two Purposes They allow a programmer to say: `this piece of code does a specific job which stands by itself and should not be mixed up with anything else, Second they make a block of code reusable since a function can be reused in many different contexts without repeating parts of the program text. 1

2 Procedures and Functions Think about a company that has a boss and several employees who each specialize in one task. The boss can say to one person, Build a thinga-ma-bob and report back to me when you are finished. This would be like a subroutine. A numeric return value is not returned to the boss by the employee. To another employee, the boss says, Count the number of hammers in the tool bin and tell me how many there are. This is like a function because there is a return value, the count of items. A Procedure by Any Other Name In Visual Basic, a Procedure is also called a Subroutine. Subroutine: A set of s that perform a specific task but do not return a value that can be used in an arithmetic expression. Function: A set of s that perform a specific task and return a value that can be used in an arithmetic expression. (NOTE: some functions return non-numeric data) 2

3 A Method by Any Other Name If a subroutine or function becomes part of a class definition in Object Oriented Programming, it is usually referred to as a method. Event Handler Subroutines You should already be familiar with the format of an Event Handler subroutine. Most of them have started out as: Private Sub name (argument_list) Handles eventname End Sub You can write your own general purpose subroutines that are not triggered by events but are called by some other part of the program. 3

4 General Purpose Subroutines You can write a general purpose subroutine that can be used in your program or in other programs. A general purpose subroutine is similar to an Event Handler subroutine except that it does not have the words "Handles" and an event name at the end of the subroutine declaration. Private Sub name (argument_list) End Sub Any Number of Parameters Subroutines and Functions can have any number of parameters. The data that is passed to a function is called a parameter. Rand( ) Sqrt(18.0) Total(5, 8, 17, 9) no parameters one parameter four parameters 4

5 One Entry, One Exit Private Function SumOf3(ByVal n1 as Double, ByVal n2 As Double, ByVal n3 as Double) As Double Dim result As Double result = n1 + n2 + n3 Return result End Function Each subroutine or function can only have one entry point (the top of the function), and should only have one exit point, the return and the end of the block of code. An extra return can be placed in the middle of the block of code as part of an if, but it is considered bad programming practice. Parentheses [AccessSpecifier] Sub name ( [argument_list] ) End Sub Parentheses directly after a name identifies that a function is either being defined or being called by another part of the program. When parentheses are used that do not follow a name, they are considered to be part of a mathematical expression. x = (2 + 3) * 13 5

6 Optional Values [AccessSpecifier] Sub name ( [argument_list] ) End Sub Some parts of the declaration for a subroutine or a function are not always required. The optional parts of a declaration are shown in square brackets [OptionalValue]. Italics, shown as slanted characters, are also used if the font is able to display italics. The example above shows an optional AccessSpecifier and an optional argument list. The parentheses ( ) that follow the subroutine name are NOT optional. Access Specifier [AccessSpecifier] Sub name ( [argument_list] ) End Sub There are several keywords that identify the AccessSpecifier: Private, Public, Protected, Friend and Protected Friend. The AccessSpecifier identifies the accessibility of the procedure, subroutine or function. Private allows access only in procedures in the same class or same form. Public allows access in other forms. If you leave out the AccessSpecifier, it defaults to Public. The other AccessSpecifiers are covered later. 6

General Purpose Procedure In this example, a subroutine is written named ComputePrice( ) which is called from within btncompute_click. ComputePrice( ) executes its code.

7 General Purpose Procedure In this example, a subroutine is written named ComputePrice( ) which is called from within btncompute_click. ComputePrice( ) executes its code. Control of the program returns to btncompute_click when End Sub is reached. Functions A function is very similar to a subroutine, except that it can return a value that can be used in an arithmetic, logical or string expression. [AccessSpecifier] Function name ( [argument_list] ) As data_type End Function 7

8 Return Data Type ' The Half function inputs a value and ' returns half of the original value Private Function Half(ByVal n1 As Double) As Double Dim result As Double result = n1 / 2 return result End Function The return data type can be any type of data, or even class definition. The return data type defines how the function is to look when used as part of an expression. Calling a Function The function is declared in one part of the program and then it can be accessed in another part of the program. Dim Average As Double Average = AvgOf2 ( 4.6, 9.8 ) ' Find the average of two numbers Private Function AvgOf2(ByVal n1 as Double, ByVal n2 As Double) As Double Dim Result as Double Result = (n1 + n2) / 2 Return Result End Function The top line calls the AvgOf2() function which computes the average of the two numbers 4.6, 9.8 and returns 7.2 which is then stored in the variable named Average. 8

9 Program Execution The function is declared in one part of the program and then it can be accessed in another part of the program. Dim Average As Double Average = AvgOf2 ( 4.6, 9.8 ) ' Find the average of two numbers Private Function AvgOf2(ByVal n1 as Double, ByVal n2 As Double) As Double Dim Result as Double Result = (n1 + n2) / 2 Return Result End Function The top line calls the AvgOf2() function which computes the average of the two numbers 4.6, 9.8 and returns 7.2 which is then stored in the variable named Average. Predefined Functions Visual Basic has a large set of predefined functions. The table below shows some of the predefined math functions. 9

10 Class Definitions In Object Oriented Programming (OOP) a Class defines what an Object is going to look like in memory. Everything in Visual Basic uses Objects. Look at each of your programs. They all start with a Class definition which defines the Object your program will become when it is built. The Math Class Each of the math functions need to reference the Math class when they are used. There are two ways: 1) Identify the name of the class (Math) in front of each function that is used. Example: result = Math.Sqrt(25.0) 2) Import the Math class into your program. If this is done you do not need to reference the Math class each time a function is used. At the top of your program, type: Imports System.Math 10

Functions. Lecture 6 COP 3014 Spring February 11, 2018

Functions. Lecture 6 COP 3014 Spring February 11, 2018 Functions Lecture 6 COP 3014 Spring 2018 February 11, 2018 Functions A function is a reusable portion of a program, sometimes called a procedure or subroutine. Like a mini-program (or subprogram) in its