User-Defined functions
Concepts
Microsoft Excel includes a powerful programming language called Visual Basic for Application
(VBA) which enables you to write modules which may be subroutines of functions. A subroutine
performs aprocess such as displaying a dialog box in which the user enters data. A function returns
a value to a cell (or a range) in the same way as a built-in worksheet function. We shall explore
only function coding. If you have experience with any programming language you will be familiar
with many of the topics covered in this chapter. If you are not yet a programmer, VBA is a great
way to begin. The emphasis in this chapter is on how to write functions so we will use simple
examples. Later chapters make use of this skill to code more useful functions. Visual Basic for
Applications is a very broad topic and there are many books devoted to it alone.
Why and when do we use user-defined functions? Just as it is more convenient to use
=sum(a1:a20) rather than =A1+A2+…+A20, a user-defined function may be more convenient
when we repeatedly need to perform a certain type of calculation for which Microsoft Excel has
no built-in function. Once a user-defined function has been correctly coded it may be used in the
same way as a built-in worksheet function.
Before you write a user-defined function, make sure that it is not already provided by Excel. The
built-in functions are more efficient than user-defined functions. After you have written a function
you must test it thoroughly with a wide range of input values.
A simple function
The user-defined function is to calculate the area of a triangle given the length of two sides and the
included angle: Area = 1/2absin(). The function code is as follows.
' Computes area of a triangle given two sides and the included angle
Function Triarea(side1, side2, theta)
Alpha =worksheetFunction.Radian(theta) ' degrees to radians
Triarea = 0.5 * side1 * side2 * VBA.sin(Alpha) ' computes area
End function
Remarks for the codes:
1.
The
complete
syntax
for
referencing
a
worksheet
function
is
Application.WorksheetFunction.FunctionName but the first word may be omitted as it is in our
function. To maintain compatibility with earlier versions you may also use the syntax
Application.FunctionName.
2. The trigonometric Sin function is available in Visual Basic. We have used VBA.sin in the
example in order to call up the list of VBA functions.
3. If a function is available as a VBA function and as a worksheet function, we must use the one
from VBA. You can not use a worksheet function when VBA provides the same function. The
worksheet function SQRT cannnot be used since VBA includes the equivalent SQR function.
Tips : You can include as many user-defined functions in one module as possible. However, ther is
a problem with having more than one function on a single module. If any one of the functions
contains an error then all functions on that module return error values on the worksheet. This can
be confusing, especially for beginners. Ther are other considerations that help you decide whether
to put more than one functions on a single module but these relate to the use of Public and Private
in the function header.
A user-defined Array function
We may require our user-defined function to return more than one value or we may wish to pass a
range of cell values to a function. In this exercise we do both. We will code a function to compute
the real roots of the quadratic equation ax2 + bx + c = 0.
Option base 1
function Quad(a,b,c)
dim temp(3)
d =(b*b)-(4*a*c)
select case d
case is < 0
temp(1) = "no real"
temp(2) = "roots"
temp(3) = ""
case 0
temp(1) = "one root"
temp(2) = -b/(2*a)
temp(3) = ""
case else
temp(1)="Two roots"
temp(2)= (-b + sqr(d)) / (2 * a)
temp(3) = (-b - sqr(d))/ (2*a)
end select
Quad = temp
End function
This function return an array. When you use it, you need select 3 cells first and enter the function
and its arguments, complete the entry with CTRL+SHIFT + ENTER.
The call to the function uses the formula =quad(a1,a2,a3). If you wish to call the function using
=quad(A1:A3), the function could be re-coded as shown below.
function Quad(coeff)
dim temp(3)
d =(coeff(2)^2)-(4*coeff(1)*coeff(3))
select case d
case is < 0
temp(1) = "no real"
temp(2) = "roots"
temp(3) = ""
case 0
temp(1) = "one root"
temp(2) = -coeff(2)/(2*coeff(1))
temp(3) = ""
case else
temp(1)="Two roots"
temp(2)= (-coeff(2) + sqr(d)) / (2 * coeff(1))
temp(3) = (-coeff(2) - sqr(d))/ (2 * coeff(1))
end select
Quad = temp
End function
Inputting an Array
In the last part of the previous exercise we saw how to pass a one-dimensional array to a function.
This could be a group of contiguous cells in a row or in a column. In this exercise, we pass a
two-dimensional array to a function. We pass a block of 12 cells arranged in three rows and four
columns to the function which sums each of the three rows and returns the value of the three sums.
option base 1
'function to return maximum sum of three rows
function MaxRow(data as object)
dim RowSum(3)
for r = 1 to 3
for c = 1 to 4
RowSum(r) = RowSum(r) + Data.cells(r,c)
next c
next r
MaxRow = Application.Max(RowSum)
End function
The code DATA as OBJECT in the argument list of the function references range A3:D5 in the
worksheet formula. In line 4 the array RowSum is created to hold the three values of the sum of
each row. Note the use of DATA.CELLS(r,c) to reference the value of each cell in the range. The
worksheet MAX function is used to find the largest value in the RowSum array.
Improving Paste Function
the function you have written will appear on the paste function dialog box in the user-defined
category but we need to make a small improvement. We will get it and the Formula Palette to
display information about the function.
1 Click the Paste Function tool. Select the User-Defined category and you will see the names of all
the user-defined functions. Select a one you want to add information to. The lower part of the
dialog box mistakenly states Choose the Help button for help on this function and its arguments.
Click on the cancel button.
2 Use the cammand Tools|Macros|Macros to open the Macros dialog box. In the Macro name box
enter the function name you want. Click on the Option button. In the Description box enter a short
description of the functions. Click OK. The description is displayed at the bottom of the MAcro
dialog box.
Some debugging tricks
1 Add MSGBOX function
msgbox "message string"
2. Add DEBUG.PRINT
Debug.Print "message string"
In this case, the information will show in immediate Window. If it is not visible, you should use
the View menu to bring it up.