MA3696 Lecture 7

Logic &

Error checking LECTURE 7

Logic statements

Select Case

If – statements

Error checking

Yes/No Message Box

SUMMARY

Run specific code based on what the user does

E.g., if they select a specific stock, then only show info about that stock.

We need to use LOGIC

There are two ways to do this:

Select Case

If Then statements

USING LOGIC

Using logic in your code

SELECT CASE

The userform for each of the following 2 examples are in the above excel file on blackboard.

Download this file and follow along with the examples.

LECTURE 7 STUDENT EXAMLE.XLSM

EXAMPLE 1. SELECT CASE

Testing the value of ComboBox1

ComboBox1


ComboBox1 Case Is = “ItemONE”

Is ComboBox1.Value = “ItemONE”


ComboBox1 Case Is = “ItemONE”

Is ComboBox1.Value = “ItemONE”

Yes • Show this

MsgBox • End Select

No Go to the

next Case Is


ComboBox1 Case Is = “ItemONE” Case Is = “ItemTWO”

Does ComboBox1.Value = “ItemTWO”



Does ComboBox1.Value = “ItemTWO”

Yes • Show this

MsgBox • End Select

No Go to

Case Else



None of the Case Is statements are true

Yes Show this MsgBox

Exit the Sub procedure



Give two examples of when Case Else

would be true


ComboBox1

Give two examples of when Case Else

would be true

Case Else Case Else


When will ‘Case Else’ run?


How can we prevent errors from non-numeric

entries?


We test for correct entries rather than

incorrect entries


Just ‘Case’ not ‘Case Is’ for a range like this.


Case Else takes care of every value not between 0-100.

Select Case Name what is being tested

Case Is = Condition 1 … Case Is = Condition 2 … Case Else … End Select

The words in BOLD are VBA key words, so don’t change them.

SELECT CASE. GENERAL FORM.

For example: • Name of control • Name of variable • Name of range




SELECT CASE. GENERAL FORM.

You can have as many conditions as you need




SELECT CASE

An operator: =, >, <, <>




SELECT CASE

Omit if Condition is a range (like example 2).

IF-THEN STATEMENTS

Logic test – Tests IF something is True.

If B is only one line of code, then you can write:

IF – THEN STATEMENTS

If Then

End If

A

B Can be multiple

lines of code

If Then A B

IF A is True, THEN do B

If x < 25 Then y = x + 17

End If

EXAMPLE. IF – THEN.


If x < 25 Then y = x + 17

OR

What happens when 𝑥 ≥ 25?

If x < 25 Then y = x + 17 MsgBox (“y = ” & y)

End If

EXAMPLE. IF – THEN.


Multiple lines of code so we MUST use this syntax

Cannot use one-line syntax

Download Lecture 7 Student Example.xlsm .

Open UserForm1

Code the Next button to test if the value of TextBox1 is <0 (they should enter a value >0).

If it is less than 0, display an error message telling the user what they did wrong. Then exit the sub.

If Then

End If

A

B

EXERCISE #1. LOGIC.

Logic tests – Tests IF something is True.

IF – THEN – ELSE STATEMENTS

If Then

End If

A

B


If Then

Else End If

A

B

C

IF A is True, THEN do B, ELSE do C

If B and C are only one line of code, you can write:

IF – THEN-ELSE STATEMENTS

Can be multiple lines of code

If Then

Else End If

A

B

C


If Then Else A B C

If x < 25 Then y = x + 17

Else y = x - 4

End If

EXAMPLE. IF – THEN – ELSE.

If x < 25 Then y = x + 17 Else y = x - 4

OR


When will y = x – 4?

Use UserForm1 again

Re-write your If-Then statement to test if the textbox is >= 0.

If it is then: Assign the value of the textbox to a variable called numStocks

Output a message stating the number of stocks entered by the user

If it isn’t then: Give an error message and exit the sub.

EXERCISE #2. IF – THEN – ELSE.

If Then

Else End If

A

B

C

We are assuming that the user will only enter a

number and not any other non-numeric value

causing an error.

If x < 25 Then y = x + 17

Else If x > 100 Then y = x - 4

End If End If


When will y = x – 4?

If x < 25 Then y = x + 17

Else If x > 100 Then y = x - 4

End If End If


If x < 25 Then y = x + 17

ElseIf x > 100 Then y = x - 4

End If

IF – THEN-ELSEIF STATEMENTS

IF A is True, THEN do B IF C is True, THEN do D

IF A and C are False, THEN do E

If Then

Elseif Then End If

A

B

D

C

If Then

Elseif Then Else End If

A

B

D

E

C

Optional

Open Module1 and Exercise3()

Re-write the two If-Then statements so that you have one If-Then-Elseif statement


If x < 75 Then y = 5*x

End If

If x > 200 Then y = 2*x

End If


Elseif x > 200 Then y = 2*x

End If

Both are logic statements

If you are only testing one condition, then use an if-then statement.

If you have several conditions, it ’s best to use Select Case.

IF-THEN AND SELECT CASE

Select Case x Case Is < 75 y = 5*x

Case Is > 200 y = 2*x

End Select


Elseif x > 200 Then y = 2*x

Else y = x End If

IF-THEN AND SELECT CASE

Select Case x Case Is < 75 y = 5*x

Case Is > 200 y = 2*x

Case Else y = x

End Select

OR

AND LOGICAL OPERATORS

USING LOGICAL OPERATORS – OR

If x < 25 Then y = x + 1

End If If x > 60 Then

y = x + 1 End If

If x < 25 OR x > 60 Then y = x + 1

End If


If x < 25 Then y = x + 1


y = x + 1 End If

If x < 25 OR x > 60 Then y = x + 1

End If

What if x=5?


If x < 25 Then y = x + 1


y = x + 1 End If

If x < 25 OR x > 60 Then y = x + 1

End If

What if x=35?


If x < 25 Then y = x + 1


y = x + 1 End If

If x < 25 OR x > 60 Then y = x + 1

End If

What if x=100?

USING LOGICAL OPERATORS – AND

If x < 25 Then

If x > 0 Then y = x + 5

End If

End If

If x > 0 AND x < 25 Then y = x + 5

End If

Both must be true

Use UserForm1 again.

Amend your If-Then statement to test if the value of the textbox is between 0 and 100:

If it is then: Assign the value of the textbox to a variable called numStocks

Output a message stating the number of stocks entered by the user

If it isn’t then: Give an error message and exit the sub.


We are assuming that the user will only enter a

number and not any other non-numeric value

causing an error.

Open Module1 and Exercise5()

Rewrite the following.

Decide if you should use AND or OR

EXERCISE #5. USING LOGICAL OPERATORS

ERROR CHECKING

Did they enter a number?

If not, give an error message and exit the sub

If yes, then continue

Did they enter a number ≥ 0 and ≤ 100?



WHAT ‘ERRORS’ SHOULD I CHECK FOR?

We did this already in Exercise #4

Did they enter an integer number?



WHAT ‘ERRORS’ SHOULD I CHECK FOR?

We don’t have to test for everything separately.

We can be more clever and use ‘Else’ to capture most errors.

But, we will need to test if the number is an integer using an if-statement.

We will do this first, then account for the other possible errors.

CHECKING FOR ERRORS USING LOGIC

Testing if a number is an integer or not.

IS THE NUMBER AN INTEGER?

Dim numStocks as Integer

No matter what value is assigned to numStocks, it will

be rounded to an integer value


Dim numStocks as Integer numStocks = TextBox1.Value No matter what value is assigned to numStocks,

it will be rounded to an integer value

What is the value of numStocks?




What is the value of numStocks? numStocks = 8




What is the value of numStocks?




What is the value of numStocks? numStocks = 10


Dim numStocks as Integer numStocks = TextBox1.Value

If TextBox1.Value is integer then numStocks and TextBox1.Value will be the same value.

If TextBox1.Value is not an integer then

numStocks will be an integer and TextBox1.Value will not. Thus, they will not have the same value.


This will be true when TextBox1 is

not an integer

Let’s add this to the code from

Exercise #4


Test for correct numerical values


If they enter a number between 0-100, assign numStocks the

value of the TextBox

If they do not enter a number between 0-100, give an error

message and exit the Sub.


If a number between 0-100 is entered, check if it is an integer

AFTER you assign numStocks the value of TextBox1



Same code using Select Case for the outer logic

statement

Testing if a value is a number.

Use the function called IsNumeric( )

The value of the function is either TRUE or FALSE

If TRUE, then the value of controlName is a number

If FALSE, then the value of controlName is not a number

IS IT A NUMBER?

If IsNumeric(controlName.value) Then

Else

End if

Code to run if controlName.Value is a number

Code to run if controlName.Value is not a number

IsNumeric (controlName.value)

Did they enter a number?


EXAMPLE. IS IT A NUMBER?

If IsNumeric(controlName.value) Then

Else MsgBox(“Your error message”) Exit Sub End if

Code to run if controlName.Value is a number

Use this to confirm user entries or clicks

MSGBOX VBYESNO

If the user enters a value you think may be wrong, then ask them to confirm their response.

Confirm responses using a Yes/No Msgbox

The MsgBox returns a value: “vbYes” or “vbNo”

To save this value, assign it to a variable like this:

answer = MsgBox(“message”, vbYesNo)

IS WHAT THEY ENTERED CORRECT?

MsgBox (“message”, vbYesNo)

The MsgBox returns a value

answer = MsgBox(“Are you sure you don’t want to make an investment?”, vbYesNo)

If they click Yes, then

answer = vbYes

If they click No, then

answer = vbNo

EXAMPLE. YES/NO MSGBOX

EXAMPLE. YES/NO MSGBOX

If the user enters 0 (use a logic statement to figure this out)

then run this code.

Use UserForm1 again. Complete this code which checks for correct and incorrect textbox values:

1. Did they enter a number between 0-100?

2. If so, is it an integer?

3. Have they entered a non-numerical value?

4. Did they enter 0?

We’ve done 1-3 already. Now complete #4 using a vbYesNo MsgBox.

Test your code to make sure it works in every situation.

If your If-statements aren’t working, try stepping through your code.

EXERCISE #6

You are ready to move on when…

LO29: You can read and understand the logic of a Select Case statement. You can also construct a Select Case statement.

LO30: You can read and understand the logic of an If-then statement. You can also construct an If-then statement.

LO31: You can use logic statements to check for errors and invalid entries.

LEARNING OUTCOMES

THE END

MA3696 Lecture 7

Education

x elseif x

x end ifselect case

trueif x

caseif x

end ifif x

logical operators orif

logical operators andif

casejust case