VBA Code: How would you count even numbers and add them together - excel

Question

I ask the user to put an input number like a "max" and then count and add all the even numbers together. For example: If the user inputs 6 then that would be 2+4+6 = 12
Sub AddupEvenNumbers()
Dim num As Variant
Dim evennum As Variant
Dim sum As Double
Dim str As String
Dim count As Integer
str = "Enter a upper/maximum number "
num = InputBox(str)
evennum = num
If num Mod 2 Then
evennum = num.Value + num
count = count + 1
End If
MsgBox "The sum of even numbers " & vbNewLine & "from 0 to " & num & vbNewLine & "is " & evennum
End Sub

Answer 1

you need to loop using a For loop and step 2:
Sub AddupEvenNumbers()
Dim num As Variant
Dim evennum As Long
Dim sum As Double
Dim str As String
Dim count As Integer
str = "Enter a upper/maximum number "
Do
num = InputBox(str)
If Not IsNumeric(num) Then str = "Must be a number." & vbNewLine & "Enter a upper/maximum number "
Loop While Not IsNumeric(num)
sum = 0
For evennum = 0 To num Step 2
sum = sum + evennum
Next evennum
MsgBox "The sum of even numbers " & vbNewLine & "from 0 to " & num & vbNewLine & "is " & sum
End Sub
Or use Application.InputBox(str,Type:= 1) to force a numeric entry
Sub AddupEvenNumbers()
Dim num As Double
Dim evennum As Long
Dim sum As Double
Dim str As String
Dim count As Integer
str = "Enter a upper/maximum number "
num = Application.InputBox(str, Type:=1)
sum = 0
For evennum = 0 To num Step 2
sum = sum + evennum
Next evennum
MsgBox "The sum of even numbers " & vbNewLine & "from 0 to " & num & vbNewLine & "is " & sum
End Sub

Answer 2

No need for VBA. You can use a formula:
With the entered number in A1:
=SUM(SEQUENCE(A1)*ISEVEN(SEQUENCE(A1)))
In earlier versions of Excel, you can use an array-formula:
=SUM(ISEVEN(ROW(INDEX(A:A,1):INDEX(A:A,A1)))*ROW(INDEX(A:A,1):INDEX(A:A,A1)))
In some earlier versions of Excel, you may need to "confirm" this array-formula by holding down ctrl + shift while hitting enter. If you do this correctly, Excel will place braces {...} around the formula as observed in the formula bar

Answer 3

In answer to your question how to count even numbers less than a number (presumably over zero):
num / 2 + 1
To answer to how to sum these numbers: we know that opposite pairs of these values will sum to the original value (so for 6.... 4,2 = 6. So the number of pairs is half of the count from the previous calculation, multiplied by the num value.
(num / 2 + 1) / 2 * num
Combined with your implied question about handling user errors, see code below for a suggested route I might use. Note I never use mod to test if somethign is even due to the fact that it forces the value to convert to a Long (see my Stack Overflow debut question on that topic... as it WILL error out if number is too big). So I use an int comparison shown below which also rejects decimals.
Sub ExampeEntry()
Dim aResponse As String, aNumberResponse As Long
start:
aResponse = Replace(InputBox("Enter An Even Number"), ",", "") 'or "." if Europe
If aResponse = "" Then
'User cancelled/didn't enter
ElseIf IsNumeric(aResponse) Then
If Int(aResponse / 2) <> aResponse / 2 Then
MsgBox aResponse & " is not an even number..."
GoTo start
Else
'success
aNumberResponse = aResponse
MsgBox (aNumberResponse / 2 + 1)/2 * aNumberResponse
End If
Else
MsgBox "ummm... """ & aResponse & """ isn't a number my friend...", vbCritical, "oh boy..."
GoTo start
End If
End Sub