Converting Data Types
At times you might find the need to import or link to data from external sources, or you might have to use data differently than the planned purpose. In such cases, the need may arise for you to convert from one data type to another. VBA includes several functions for this purpose. When you use a conversion function, the function returns the converted value but doesn't change the stored value.
→ For more on data types see,"VBA Data Types" p. 28.
This chapter goes over some of the more commonly used conversion functions. You can find a full list by opening the Developers Reference using the VBE Help menu and searching on type conversion functions.
- CBool—Converts a value to a Boolean data type.
- CDate—Converts a value to a Date data type.
- CInt—Converts a value to an Integer data type.
- CStr—Converts a value to a String data type.
- CVar—Converts a value to a Variant data type.
These functions have a simple syntax in common:
where functionname is the name of the function and argument is a value, variable, constant, or expression. The value of the argument is converted to a different data type depending on the function used, so it can be used elsewhere in your application. The value(s) used in the argument remain unchanged. It should be noted that not every data type can be converted to any other data type. The following sections explain the limitations.
Converting to a Boolean Data Type
A Boolean value is either True or False. The False value is either the number or character zero (0). Any other value is considered True. If the argument passed to the CBool function evaluates to a zero, CBool returns a False. If it evaluates to any other value, CBool returns a True. For example; all the following return a True because the arguments all evaluate to a nonzero value:
CBool("1") CBool(1+0) CBool(5) CBool(-50)
Conversely, the following expressions return a False because each argument evaluates to zero:
CBool(0) CBool("0") CBool(15-15)
The argument passed to the CBool function must contain all numeric characters or operators. If you use alphabetic characters you get a type mismatch error. One place where using CBool becomes useful is in conditional statements. For example, you might need to determine whether two values match. In our Inventory application you might need to determine whether you are out of stock on an item. You could use the following expression, which would return a False if the incomings matched the outgoings:
Converting to a Date Data Type
The CDate function converts any valid date/time value to a Date/Time data type. A valid date/time value can be either a number or a string that is formatted as a date or time. CDate determines valid date/time formats according to the regional settings you have chosen in Windows. You can use the following points to understand how dates are converted by CDate:
- If the argument is a numerical value, CDate converts the integer portion of the number according to the number of days since December 30, 1899. If the argument contains a decimal value, it's converted to a time by multiplying the decimal by 24 (for example, .25 would be 6:00 a.m.).
- If the argument is a string value, CDate converts the string if it represents a valid date. For example; "1/16/51", "March 16, 1952", and "6 Jun 84" would all be converted to a date. However, "19740304" would result in a type mismatch error.
- Access recognizes dates from January 1, 100, to December 31, 9999. Dates outside that range result in an error.
- I recommend that you use four-digit years for clarity. However, Access will work with two-digit years. If you enter a year less than 30, Access assumes you want a date in the twenty-first century. If you use a year of 30 or higher, it is assumed to be a twentieth century date.
- Remember that the / is also the division operator and the – is used for subtraction. So, if you enter dates such as 12/3/04 you will get unexpected results. Entering CDATE(12/3/04) returns December 31, 1899, because 12 divided by 3 divided by 4 = 1. So you need to put such dates within quotes.
Converting to an Integer Data Type
The CInt function takes a numeric or string value and converts it to an Integer data type. The argument is required and needs to represent a value within the range of -32,678 to 32,767. If the argument contains a decimal, Access rounds to the next whole number. A value of .5 or higher is rounded up; anything lower is rounded down. Some examples of CInt functions follow:
CInt(10.5) = 11 CInt(25.333) = 25 CInt(10/3) = 3 CInt("1,000") = 1000
The argument must evaluate to a numeric value; otherwise, it returns an error. If the argument evaluates to a value outside the range of the Integer data type, you get an overflow error.
Converting to a String Data Type
The CStr function converts just about every numeric value into a String data type. The required argument can be any variable, constant, expression, or literal value that evaluates to a string.
Converting to a Variant Data Type
As I mentioned in the discussion of VBA data types in Chapter 3, "Using Variables, Constants, and Data Types," the Variant data type is the most flexible because it can accept almost any value. With CVar, you can convert just about any numeric or text string to the Variant data type. With numeric values there is a constraint to the same range for the Double data type.
Converting Null Values
If you try to use a Null value in many expressions, you will probably encounter an error. For example, the following expression results in a runtime error if either of the values contains a Null:
varTotal = ValueA * ValueB
To avoid such errors you can utilize the Nz function to convert the value to a non-Null. The Nz function uses the following syntax:
The Nz function works similarly to an Immediate If (IIF) function. The following expressions are functionally equivalent:
varTotal = IIF(IsNull(ValueA),0,ValueA) * IIF(IsNull(ValueB),0,ValueB) varTotal = Nz(ValueA,0) * Nz(ValueB,0)
The valueifnull is an optional argument; it defaults to 0 or a zero-length string based on the value's data type.