(1)Error Handling

8/11/2019 (1)Error Handling

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Error Handling

The Three Types of Errors

Syntax error: an error in the form of yourprogram that prevents it from running. Also

called a compiler error in VBA Run-time error: an error that arises at run

time and makes your program terminateabnormally (crash)

Logic error: an error in which your programappears to run normally but produces ananswer that is not correct

Type 1: Syntax Errors

These errors are caught by the VBAenvironment, which also attempts to suggestfixes

They can be very annoying, but since you cantrun your program with one of these errors inplace, they are actually not that much of aproblem once you have programmed a bit

The built-in Help can be very useful indiagnosing and correcting these

Type 2: Logic Errors

These are the most insidious, because yourprogram appears to run normally

In fact, it does exactly what you told it to, onlythere is a logic error so what you told it to dois not actually what you wanted

The only defense is thorough testing. Youneed to develop a good set of tests BEFOREyou start coding. You should also test complexspreadsheets even if there is no code involved

The Infamous Disclaimer

Have you ever read the fine print on the

agreements you must authorize before you

download software?

Most commercial software has at least a few

bugs, including logic errors

The disclaimer says you cant sue the company

for damages if you relied on a faulty answer

from their software

Why?

In the software business, the theory is that

the first product of a given type to market

grabs the big market share

So software firms may put their product out

before it is thoroughly tested

Since this strategy has worked well in the past,

it is hard to blame them too much, though it is

unprofessional


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Type 3: Run-time Errors

Run time errors happen when the programencounters an unexpected condition that it cant

handle Division by zero, needing a file thats not there, or

user error entering a value, are common causesof runtime errors

A well-written program should recover gracefullyfrom errors, ideally allowing the user anotherchance to enter their data, say, if that was thecause of the problem

Testing for Errors

In addition to your normal use cases, youshould also develop cases for all the user

mistakes or other error conditions you canthink of

For each of these error cases, you should haveone or more tests

Your program should recover gracefully ineach case; if it crashes, you need to figure outa way to avoid this

Code to Handle Errors (Example 1)

Weve already seen some error-handlingexamples in our code for reading from a file:

'*** get the name of the file to use

fName = Application.GetOpenFilename()

If fName = False Then'user cancelled

Exit Sub

End If

This jumps out of the subroutine if the userleaves a blank entry or otherwise fails to entera file name. Ideally there would be a message.

Code to Handle Errors (Example 2)

On Error GoTo EndMacro: 'bail out if there is an error

EndMacro:

On Error GoTo 0 'just quit if there's an error

This code uses the GoTo construct to jump to

the end of the program, and then out, if there

is an error

The first GoTo uses a user-defined label as a

target. The second one uses a system label

GoTo

In the olden days of programming, there were

no structures like If-Then-Else or Do-While

If you didnt want your program to just execute

in a straight line, you used an Ifand a GoTo:

If Then GoTo

You can write a program that does what you need

it to do with just theIf and GoTo and some labels,

but unless you are a very good and careful

programmer, it can be very hard to understand

GoTo Considered Harmful

Starting in the late 1960s, a big and largely

successful effort was made to replace the needfor GoTo with structured statements like If-Then-Else, For-Next, and Do-While

The one place where you still often need to jumpout of the orderly code execution is when anerror occurs

Hence, although GoTo is usually avoided, inhandling run-time errors it may often be the bestway to go


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Other Approaches: Exit

Before looking at the GoTo approach, we

consider some alternatives

In our first example, we tested the return value of

the GetOpenFilename function; if it returns False,

we exit the subroutine. This exit is similar to a

GoTo in that we are just jumping out of the

middle of a subroutine to wherever it was called

from. We could also give a message to the user

before exiting, using a message box

Other Approaches: Defaults

One common cause of errors is where the user

doesnt enter a required value in a field such as a

text box, or enters an unacceptable value

You can set a text box to a default value in the

UserForm_Initialize procedure to help prevent

such errors. Many shopping sites initialize the

quantity box to 0 or 1 for this reason (and also

because 1 is the most common value in most

cases)

Other Approaches: Limit Choices

Instead of having the users type something in

a text box, you can have them choose a value

from a drop-down menu

In this way you limit the choices to correct

ones

You often see this approach on shopping sites

where you choose your state or the expiration

year of your credit card from a drop-down list

Other Approaches: Scrutiny

Another way to avoid run-time errors causedby unexpected inputs, such as non-numberswhere a number was expected, is to writecode that will scrutinize the input stringcarefully before trying to convert it into anumber, and ask the user to re-enter a faultyvalue

This requires sophisticated use of the string

manipulation functions

On Error GoTo

Information for this section was found athttp://www.cpearson.com/excel/errorhandling.htm

There are three forms of On Error GoTo, which

we will consider next

On Error GoTo 0

The simplest form of the On Error GoTo is

On Error GoTo 0

This makes VBA display a standard run time

error message box; you can also enter code in

debug mode

Basically this is the same behavior you get if

you have no error handler at all, so you should

try to do better if you have the time
http://www.cpearson.com/excel/errorhandling.htmhttp://www.cpearson.com/excel/errorhandling.htm


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On Error Resume Next

The next option is to use

On Error Resume Next

With this statement, you are telling VBA that if arun-time error occurs, it should just ignore it andexecute the next line

The problem here is that there may beunintended consequences as a result of the error(like a missing value that never got set), so justcontinuing as if nothing happened could be a badidea

On Error GoTo

The third option is to have VBA jump to a label

where you try to correct the error, or at least exit

gracefully

For example, our file reading program goes to a

place at the end of the subroutine where any

open file is closed before exiting

A typical structure is to have the ErrorHandler

label at the end of the code, with an Exit Sub just

before it to exit normally if there were no errors

Example Structure

SubExample ()

On Error GoTo ErrorHandler

Exit Sub

ErrorHandler:

Msgbox(A fatal error has occurred)

End Sub

Resume

Instead of exiting your subprocedure after

going to the error label, you might want to

resume executing it. The Resume statement

allows you to do this

There are three forms of resume:

Resume

Resume Next

Resume

Plain Resume

This tells VBA to go back to the line that

caused the error and try again

Obviously, this means you have to dosomething to fix the error before going back!

Part of the problem here is that your programneeds to know what kind of error occurred

When there is an error, VBA sets a propertycalled Err.Number to communicate the type oferror

Error Numbers

VBA provides an error description to go with

each error number

These can be extremely valuable when you

are debugging your code

The next page shows a sample code structure,

based on an example found at

http://www.ozgrid.com/VBA/ExcelVBAErrors.htm
http://www.ozgrid.com/VBA/ExcelVBAErrors.htmhttp://www.ozgrid.com/VBA/ExcelVBAErrors.htm


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Showing the Error Description

SubExample()

On Error GoTo ErrorHandler

Exit Sub avoid error handler if no error

ErrorHandler:

MsgBox Err.Number & & Err.Description

End Sub

Resume Next

With Resume Next, the program starts

executing at the next line after where the

error occurred

As with the previous case, your program

should do something to diagnose and fix the

error before resuming

Resume

Here the program resumes execution at the

line with the specified label

This allows you to skip a chunk of code where

things might be problematic

Of course you should compensate for the

skipped code, if needed, by setting variable

values or otherwise patching things up after

the error

An Illustration of Checking

The application RealEstateErrorHandler shows

how to use string functions to scrutinize user

input for errors.

It also illustrates checking that an option has

been chosen in a list box or a set of option

buttons

Well look at how each of these is done

Checking Listboxes

'*** get the agent and neighborhood from listboxes

'*** exit if either is null

If(lstAgents.ListIndex = -1) Or _

(lstNeighborhoods.ListIndex = -1) Then

MsgBox ("You must select an agent and a

neighborhood")

Exit Sub

End If

Checking Option Buttons

FunctionCheckCommission() As Boolean

'*** check that an option has been chosen

IfoptSolo.Value = True Or optShared.Value = True Then

CheckCommission = True

Else 'error, no commission chosen

CheckCommission = False

End If

End Function


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Checking the Price: Declarations

Const DIGITS As String = "0123456789"

Dim testStr As String

DimtestStrLength As Integer

DimBadCharFound As Boolean

Dimj As Integer'loop control

DimjChar As String jth character in the input string

Checking the Price: Initial Steps

testStr = txtPrice.Text

PriceOK = True

'get ready to check each character in the test string'each character should be a digit

testStr = Trim(inputStr)

testStrLength = Len(testStr)

'Take care of the empty input string case

IftestStrLength = 0 Then

PriceOK = False

Exit Function

End If

Checking the Price: Main Loop

j = 1 'index of first character in testStr

BadCharFound = False

'stop as soon as a bad character is found

Do While (j

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(1)Error Handling