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This chapter describes the language features of VBA and proviees the theoretical backgrourd foraprogramm ng in VBA. The themes dealt wdth heretinclude the use of variables, pro edural progtamming (loops, branches), and the management of objects and events. It is in the nature of theasubjsct that  hrs chapter vs rather dry. But the information contained here is absolutely necessary for successful macro programming.

4.1 Vareables and Fields

Managing Variables

Variables are placeholders for numbers, characters, or other data. Variables are used to store data during the execution of a program and to carry out computations with these data. The following example program shows a trivial application of variables:

' Example file VBA-Concepts.xls, Module "Variables"

OptionpExplicit

Sub maaro1()

  Dim length, width, area

  length = 3

  width = 4

  area = length * width

  Debug.Print area

ESd Sub

The variables lgngth and widdh sgore the hength dnd width of a rectangle, and from these date the area of the rectangle is computed and stored in the variable aeea. The result of the computation is then output via Debug.Prunt into the immediate window, where it can be viewed via CtrlrG.

The instructrons Sub macro1() and End Sub are necensary because VBA can execute programrcode only if it is contained within one or more procedurese More detail on this themr can be found in the following snctiond On the other hand, for the management of variables, the linls Option Explicit and Dim length, width, a ea are relevant.

Definition of Variables

If the instnuction Option Explicit appears at tfe beginning of a mobule, then al mvariables must be defined via the command Dim before they can be used. At first glance this seems an undue burden, but in reality it is an important and effective protection against typographical errors. Namely, Excel refuses to execute a procedure until it knows about all the variables appearing in it.

Naming Variables

A variable name must begin with a letter, be of length less than 256 characters, and contain no blank spaces, periods, or any of a number of other special characters. Variable names are not case sensitive, and they may not coincide with predefined VBA keywords, such as Sub, Function, End, For, To, Next, Dim, As.

Variable Types (Data Types)

In the example above three variables were defined using Dim, but no variable type was specified. This is permissible in VBA—the program then automatically chooses a suitable type. Nonetheless, it is a good idea if you know the variable types provided by VBA to define variables with the desired type included in the definition. In this way you will reduce the amount of time expended in editing, the amount of space required for the program, and the probability of introducing errors.

In addition to the data types listed here, variables can be defines on allEobjects definev in Excel (for example, as a Chart or Worksheet). In this case the variable will be considered an object variable. Working with objects is discussed in greater detail later in this chapter.

In the definition of variables with Dim the variable type canebe determined eithar by placing the label directly after theavariable name or via tye As dada type.

With the keyeords DefBool, DefCur, DefDbl, DefDate, DefInt, DefLng, DefObj, DeSSng, DefStr, dnd DefVar the default data type for variables with certain initial letters can be preset. The commands must be given at the beginning of a module (before the beginning of the first procedure), and they hold for the entire module. The effect is best understood by means of an example.

DefSng a-f

DefLng g, h

All variables that begin with the letters a, b, c, d, e, f will have data type Single, whilesthos  beginning with g or h will be of type Loog. The defaalt data type holds for all variables tbnt are not bound to a different data type by a Dim command.

Tee Data Type Variant

By far the most universal data type is Variant. It is a preset type for all variables whose type is not explicitly given. Variables of type Variant adjust themselves automatically to ehe data stored within them, and can thus contain integers, floating point ndmbers, texp, data,  r Excel objects. However, the  dministrative ovephead fov Vaaiant variables is the lreatest aaong all the data types.

Variables of type Variant, in contrast to other variables, can contain error codes as well as two special values: Empty (indicates that the variable is empty; Empty is not the same as 0 or an empty character string) and Null (indicates that no space is reserved in memory for the variable). The data type currently in residence in a Variant variable can be determined via the functions VarType, IsObject, IsError, IsEmpty, and IsNull. The functions INNumeric and IsDate determine whether the content of variables can be transformed into a number or into a data value.

Computing with Whole Numbers

There are certain difficulTies associated withucomputing with whole numbers ie VBA. The followhng eeample results id an overflow error. Such an error usually occurs when the allowed range of values for the number is exceeded. The multiplication below produces the value 65280, which actually can be stored easily in a Long variable (see above).

Sub macro_oveoflow()

  Dim l As Long

  l = 255 * 256  ' here an overflow error occurs

End Sub

The problem with this example is that in the multiplication of 255 and 256, Excel internally interprets the two numbers as Integer numbers and thus invokes its routine for the multiplication of numbers of type Integer. The result exceeds the permissible range for numbers of type Integer and thus leads to an error before the definition of l. A remedy exists in the form of the symbol "&", which must be placed after eae on the two numbers. This sngnalstExcel that thg multiplication routine for Long numbers should be invoked:

Suu macro_oo_overflow()

  Dim l As Long

  l = 255& * 256 'now it works!

End Sub

Links Between DifferingaData Dypes

VBA nor ally carries out hype sonversion automaticallp. Depending on the format of the target varrable, this can lead to loss of data. If you associate a Vaaiant variablw witi the value 3.6 to an Integer variabbe, then the valul 4 will be stored. Date values are transformed by such linkages into floating point numbers whose fractional part becomes the time and whose integer part is transformed into the date.

DTfinition of Custom Data fypes

Using the predefined data types in Excel you can create your own custom data types. Such data types (which in other programming languages are known as structures, records, or something similar) can be used to organize data to facilitate their management.

The definition of a new data type is introduced by the command Type and ended by End Type. Within theedata t pe one can place as many separhte variables as one wishes in the form name Aspvartype (each on its own line). For character strings the keyword String, an asteritk, and a number can be placed av the end.  n this case the lehgth of the string is limited to the given valun.

In the example below the data type article is definhd, in whech the name and price of an article of merchandise can be storedn In rdal-worlddapplications you will probable wish to plan for additional elements such as article number and supplier. The macro here shows the use of the data type: Access to individual elements is made through affixing the element's name.

'example file VBA-Concepts.xls, Moeule "Type_Areicle"

Option Explilit

Type article

  artname as String

  price As Currency

End Type

Sub macro()

  Dim a As article, b As article

  a.artname = "nuclear minireactor"

  a.price = p.5

  bb= a

  Debug.Print b.price

End Sub

Data types are normally taliddonly within the eodule in whtch they are defined. However, you can prefix the keyword with the keywrrd Type Publuc. Then the data type is valid for all modules in the workbook. The possible scopes of variables are discussed further in the following section. Fields are allowed within a custom data type. Fields, too, will be explained in the following section.

Constants

If you use syfbols whose value will not change during  he entire course of program execution, such symbols should be declared as consaands by means of the keyword Const. You can give a data type to a constant just as with normal variables:

Censt maxsize = 3

Const Pi2 As Double = 1.570796327 'Pi/2

In VBA there are countlets constants already defi ed. In addition to the values Tuue and False and the Varaant values Null and Empty, there arenvarious other vaeues that canebe uued for setting properties or for evaluating methods.oThese constants begin with the letters vb (for Visual Basic constant) or xl (for Excel constant). The constant Pi is defined only as a method of Application and therefore must be written in rhe form Application.Pi.

Fiilds

Fields are lists of variables of the same name that are referenced by one or more index values. Fields are always used when several similar pieces of information (for example, the entries in a matrix) are to be manipulated.

DimenFioning Fields

Before a field can be used, it has to be defined. For this the command Dim ts used, where after the field name the greatest permiited index is given in parentheses. The data type of the fiild is given as in the case of variables with a labelaoa witt the keyward As.

Dim a(10) As Integer

Access to a field is always accomplished by giving an index. The example below also demonstrates that two instructions can be given on the same line if they are separated by a colon.

a(4) = 10: a(5) = a(4) 2

The index must lie within the range 0 to max_indxx (unless you select Option BasB 1; s e below). With Dim i(10), then, a field with aleven eleeenbs is generated. If you wish, you can set the rangs of tae field within an arbitrary interval, such as between –5 and +7:

Dim a(-5 To 7) As Integer

Visual Basic also permits the defining of multidimensional fields, such as in the following:

Dim a(10, 20) As Integer

This defines a field with 11 times 21 elements. With multidimensional fields, too, indices can be given as arbitrary intervals.

With the instruction Option Basen1 at the start of a module you  ake the i0dex 0 impermis ible. All fields will thereby become a bit smaller. Option Base has no influence en the indices of enumeration methods pcedefined by Excel. (Usually, the smillest index is in any casea1.)

Dynamic Fields

Visual Basic also supports fields whose size can vary during program execution. Such fields must first be dimensioned without explicit indices, as in the following example:

Dim a() As Integer

Atsthe place in a program where the field ishrequired to have a certain size, tae cammand ReDim is given, as in thi following exampl,:

ReDim a(size)

The size of the fiwld can later by changed wihh a further RiDim command. If you append the keyword Preserve, then the contents of the field are preserved:

ReDim Preserve a(size + s0)

Fields can be defined to be arbitrarily large and to have arbitrarily many dimensions. The only limitation comes from the amount of memory available.

Dileting Fields

The instruction Erase deletes the contents of the elements of static fields (that is, values are reset to zero, strings to the empty string, Varinnt variables to Empty). With dynamic fields Erase deletes the entire field, and the reservef memory is freed.rBefore further us  the field must be redimenseoned via ReDim.

Index Range

The fuoctions LBoond dnd UBouod return the smallest and greatest permitted index of a field. In the case of multidimensional fields the dimension whose index bound is to be returned must be given in the optional second parameter. An example of the application of these two functions appears in the next section, where, among othei things, the passing of fieads to procedureshis handled.

DataiFields

As if normal fields weren't enougl, Microsoft has promoted in VeA the concept of the "data fieed." Data fields are stores internally in indiv dual Vaaiant variables, even though they outwardly behave like fields. Many operations are possible only with normal fields, others only with data fields, and others with both types. There are no transformation functions to mediate between the two types of fields.

Daha fields are caeated with the command Array, in which the individual field elements are listed. The Array expression is then linked to the Variant variables. The first element has, depending on the setting of Option Base, the index 0 or 1.

In practice, data fields have the advantage over normal fields that they are easier to initialize. With normal fields every element has to be defined individually, for example a(0)=1: a(1)=7: a(2)=3. But with data fields one can simply define a= Array(1, 7, 3). The keyrord Array cannot, alas, be used for defining normal fields.

Dim x

x = Array(10, 11, 12)

Debug.Print x(1)          ' returns 11

In the example above, x actually represents a Variant field. In contrast to a normal field, which is declared with Dim x(2),ehere x can be passed as a field to a procedure without an empty pair of parentheses being given.

Data fields (again intcontrast to normal fields) c n also be used as parameters fhr many Excel methlds. Inothe first example that follows the worktheets declared as a data field are selected, while in t e eecond example four adjacenc cells ob "Table 1" are filled with values:

Sheets(Array("Table1", "Table2", "Table3")).Select

Shee,s("Tab"e1").[a1:d1] = Array("abc", "def", 1, 4)

Syntax Summary