This VBA program will test whether a number entered by the user is a prime number or not. Prime number is a number that cannot be divided by other numbers, it includes the number 2 but exclude 1 and 0 and all the negative numbers. Below is the Excel VBA code for the prime number tester:

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## The Code

Private Sub CommandButton1_Click()

Dim N, D As Single

Dim tag As String

N = Cells(2, 2)

Select Case N

Case Is < 2

MsgBox "It is not a prime number"

Case Is = 2

MsgBox "It is a prime number"

Case Is > 2

D = 2

Do

If N / D = Int(N / D) Then

MsgBox "It is not a prime number"

tag = "Not Prime"

Exit Do

End If

D = D + 1

Loop While D <= N - 1

If tag <> "Not Prime" Then

MsgBox "It is a prime number"

End If

End Select

End Sub

Explanation:

In this program, we use the Select Case ......End Select statement to determine whether a number entered by a user is a prime number or not. For case 1, all numbers that are less than 2 are not prime numbers. In Case 2, if the number is 2, it is a prime number. In the last case, if the number N is more than 2, we divide this number by all the numbers from 3,4,5,6,........up to N-1, if it can be divided by any of these numbers, it is not a prime number, otherwise it is a prime number. We use a Do......Loop While statement to control the program flow. Besides, we used a tag="Not Prime' to identify the number that is not prime, so that when the routine exits the loop, the label will display the correct answer.

## The Interface

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