Errors &

Error Trapping

Types of Errors

•Syntax - violate language rules

•Semantic - violate usage rules of language

•Run-time – occur while program is running

•Logic – incorrect algorithm

Syntax Errors

E.g.If X > 0 And < 10 Then

E.g.me.Cls()

Trapped by CompilerMust be corrected before program will compile

Syntax Errors

E.g.If X > 0 And < 10 Then

E.g.me.Cls()

Trapped by CompilerMust be corrected before program will compile

Incorrectspecification oflogical expression

Misspelled name

Syntax Errors

E.g. Dim x As String

For x = 1 To 10Statements to be repeated 10 times

Next x

Trapped by CompilerMust be corrected before program will compile

Syntax Errors

E.g. Dim x As String

For x = 1 To 10Statements to be repeated 10 times

Next x

Trapped by CompilerMust be corrected before program will compile

For loop controlvariable must beinteger or long

Semantic Errors

E.g. Dim x As Integer, Start As Integer, Finish As Integer

Start = 10Finish = 0For x = Start To Finish

Statements to be repeated 10 timesNext x

Trapped by testing

Semantic Errors

E.g. Dim x As Integer, Start As Integer, Finish As Integer

Start = 10Finish = 0For x = Start To Finish

Statements to be repeated 10 timesNext x

Trapped by testing

Start must be < Finishelse, loop will not run

Run-time Errors (exceptions)

E.g.DC = File.OpenText(“Myfile.text”)

E.g. Dim FileName As String

FileName = txtFileName.TextDC = File.OpenText(FileName)

Trapped by run-time system

Run-time Errors (exceptions)

E.g.DC = File.OpenText(“Myfile.text”)

E.g. Dim FileName As String

FileName = txtFileName.TextDC = File.OpenText(FileName)

Trapped by run-time system

Bad extension

Non-existent pathor filename

Run-time Errors (exceptions)

E.g.Dim Value As Single

Value = txtValue.Text

Run-time Errors (exceptions)

E.g.Dim Value As Single

Value = txtValue.Text

Program will crash if txtValue is empty

Logic ErrorsE.g. to calculate the average of First and Second

Average = First + Second / 2

Program will run but produce incorrect results

Trapped only by testing (recommended)or users noticing incorrect results (unreliable)

Logic Errors

E.g.Average = First + Second / 2

Program will run but produce incorrect results

Trapped only by testing or users noticing incorrect results

Correct statement isAverage = (First + Second) / 2

Logic ErrorsE.g.

Function IsCapital (Letter As Char) BooleanIf Letter > “A” And Letter < “Z” ThenReturn True

ElseReturn False

End IfEnd Function

Program will run but produce incorrect resultsTrapped only by testing or users noticing incorrect results

Logic ErrorsE.g.

Function IsCapital (Letter As Char) BooleanIf Letter > “A” And Letter < “Z” ThenReturn True

ElseReturn False

End IfEnd Function

Program will run but produce incorrect resultsTrapped only by testing or users noticing incorrect results

It will return Falsefor A and Z

Use >= and <=

Logic ErrorsE.g.

Function IsLetter ( Ascii As Char) BooleanIf (Ascii >= “A” And Ascii <= “Z”) And _

(Ascii >= “a” And Ascii <= “z”) ThenReturn True

ElseReturn False

End IfEnd Function

Program will run but produce incorrect resultsTrapped only by testing or users noticing incorrect results

Logic ErrorsE.g.

Function IsLetter ( Ascii As Char) BooleanIf (Ascii >= “A” And Ascii <= “Z”) And _

(Ascii >= “a” And Ascii <= “z”) ThenReturn True

ElseReturn False

End IfEnd Function

Program will run but produce incorrect resultsTrapped only by testing or users noticing incorrect results

Use Or

How to Avoid Software Crashes?i.e. how to catch exceptions (or run-time errors)?

Use validation code

Dim Value As SingleIf txtValue.Text = “” Then

Value = 0Else

Value = txtValue.TextEnd If

Your software will NOT crash

How to Avoid Software Crashes?i.e. how to catch exceptions (or run-time errors)?

Use Try-Catch blockTry

One or more statements that may causeRun-time errors (or exceptions)

Catch variable As exceptionStatements to execute when this exception occurs

Catch variable As exceptionStatements to execute when this exception occurs

End Try

Exception keyword To catch

ArithmeticException errors in a calculation

IO.DirectoryNotFoundException an invalid folder path

IO.FileNotFoundException a non-existent file

FormatException bad parameter values

Exception any error

E.g.Sub OpenInputFileDim DC As StreamReader, FileName As String

TryFileName = txtFilename.TextDC = File.OpenText(FileName)

Catch MyFileError As IO.FileNotFoundExceptionMessageBox.Show (“File not found: “ & FileName)

End Try

End Sub

E.g.Sub DoSomething

TryStatements in proceduredoing something

Catch AnyError As Exception

MessageBox (AnyError.Message)

End TryEnd Sub

Will catch anyerror & showits error message

Your software will NOT crash

E.g.Sub DoSomething

TryStatements in proceduredoing something

Catch ThisError As ?

Catch ThatError As ?

Catch OtherError As ?

Catch AnyError As ?

End TryEnd Sub

You may have as manyCatch statements asyou need, using correctKeyword in place of ?

Use these ideas in your code

Do not let your software crash