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When you declare a variable, you should also identify its data type. You're probably already very familiar with data types because you assign data types to table fields. VBA uses the same data types to define a variable.
The most important job of a data type is to ensure the validity of your data. Specifying a data type won't keep you from entering an invalid value, but it will keep you from entering an invalid type. If you omit the data type, VBA applies the Variant data type to your variable it's the most flexible and VBA won't guess at what the data type should be. Table 3.1 compares VBA's many data types.
The Boolean Data Type
Use the Boolean numeric data type to store logical data that contains only two values: on and off, true and value, yes and no, and so on. The keywords True and False are predefined constants and are interchangeable with the values 1 and 0, respectively. To illustrate these keywords, enter the following statements, one at a time in the VBE's Immediate window, as shown in Figure 3.7:
?True = 0 ?True = -1 ?False = 0 ?False = -1 ?True = False
Figure 3.7. True and False equal 1 and 0, respectively.
The Byte Data Type
Byte is VBA's smallest numeric data type and holds a numeric value from 0 to 255. This data type doesn't include any negative values. If you attempt to assign one, VBA returns an error.
The Currency Data Type
Use the Currency numeric data type to store monetary values from 922,337,203,477.5808 to 922,337,203,685,477.5807. A Currency data type results in a scaled value with accuracy to 15 digits to the left of the decimal point and 4 digits to the right. Use this data type to avoid rounding errors when precision is of the utmost importance.
The Date Data Type
The Date data type stores a specially formatted numeric value that represents both the date and time. You don't have to store both the date and time value. The Date data type accepts either the date or the time, or both. Possible values range from January 1, 100 to December 31, 9999.
The Decimal Data Type
The Decimal data type is a subtype of Variant and not a truly separate data type all its own, accommodating values from 79,228,162,514,264,337,593,543,950,335 to 79,228,162,514,264,337,593,543,950,335 if the value contains no decimal places. The data type maintains precision up to 28 decimal places with values from 7.9228162514264337593543950335 to 7.9228162514264337593543950335.
The Double Data Type
Use the Double data type to store precision floating point numbers from 1.79769313486232E308 to 4.94065645841247E-324 or 1.79769313486232E308 to 4.94065645841247E-324.
The Integer Data Type
This is probably the most common data type in use, besides String. Use this data type to store only whole numbers that range from 32,768 to 32,767.
The Long Data Type
The Long data type is also an Integer data type storing only whole numbers, but the range is much larger than the traditional Integer data type. Use Long to store values from 2,147,483,648 to 2,147,486,647.
The Object Data Type
An Object variable is actually a reference to an Access object, such as a form, report, or control. Or, the data type can reference an ActiveX component, or a class object created in a class module.
Class modules are covered briefly in "Introducing the VBA Modules," in Chapter 2 (p. 21) and in more depth in "Introducing Objects," in Chapter 8 (p. 113).
Youll learn more about Object variables in " in Chapter 8, "Understanding Objects" (p. 113).
The Single Data Type
The Single data type stores precision numbers numbers with decimal places or fractional numbers. The data type is similar to Double, but the range is smaller. Use this data type to store values from 3402823E38 to 1.401298E 45 or from 1.401298E 45 to 3.402823E38.
The String Data Type
String is another very common data type; it stores values or numbers, but treats them as text. There are two varieties: fixed and variable. A fixed string can handle from 1 to 65,400 characters. To declare a fixed string, use the Dim statement in the form
Dim variablename As String * stringlength
In contrast, the variable String data type grows and shrinks as required to fit its stored value. By default, all String variables are of this type. To declare this type, use the Dim statement in the form
Dim variablename As String
The Variant Data Type
The Variant data type stores numeric and non-numeric values. This data type is the most flexible of the bunch because it stores very large values of almost any type (matches the Double numeric data type). Use it only when you're uncertain of the data's type or when you're accommodating foreign data and you're not sure of the data type's specifications.
The Variant data type is VBA's default, so the following code interprets varValue as a Variant:
Although the Variant data type is flexible, VBA processes these data types a little slower because it must determine the most accurate data type for the assigned value. However, most likely, you'll never notice the performance hit.
The biggest disadvantage is the data type's lack of readability. By that, we mean that you can't easily determine the appropriate data type by viewing the code, and that can be a problem.
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