# User:Eriksiers/Prime numbers

This is something I first wrote several years ago (2002, according the the copyright... I don't remember). It is VBA/Excel (Visual Basic for Applications, Excel-specific) code that calculates the factors of a series of numbers, highlighting prime numbers by turning on the cell's `font.bold`

property. I used Excel because it's especially suited for the tabular nature of the output.

I've placed it here, in its own page, because I feel that it doesn't specifically fit any of the (current) prime number-related tasks (although part of it does demonstrate Primality by Trial Division and Prime decomposition). This could perhaps be listed on the Category:Prime Numbers page, but I'll leave that decision to someone else.

This could be improved by using run-length encoding, assuming your implementation of RLE can handle multiple-character strings. (`2*2*2*11*11`

-> `2^3*11^2`

)

(Note the `Swap`

subroutine. It's included here because VB doesn't provide such a function itself.)

```
' Copyright 2002-2009 Erik Siers
' source available under terms of the GNU GPL, version 2 or
' (at your option) any later version
Sub Swap(ByRef x, ByRef y)
Dim z
z = x
x = y
y = z
End Sub
Sub findPrimes()
' determines prime numbers within a user-entered range
Dim prime As Boolean, neg As Boolean
Dim thisRow As Long, thisNumber As Long, startCol As Long, startRow As Long
Dim m As String, n As String, outP As String
Dim i As Long, j As Long
prime = True ' true until proven false
startCol = ActiveCell.Column
startRow = ActiveCell.Row
thisRow = startRow - 1
m = InputBox("Starting number?", "Enter a whole number")
startVal = Int(Val(m))
n = InputBox("Ending number?", "Enter a whole number")
endVal = Int(Val(n))
If endVal < startVal Then Swap startVal, endVal
For i = startVal To endVal
thisRow = thisRow + 1
outP = ""
Select Case i
Case Is < 0
neg = True
thisNumber = -i
Case Is < 2
outP = Trim$(Str$(i))
neg = False
prime = False
GoTo badNum
Case Else
neg = False
thisNumber = i
End Select
loca:
For j = 2 To Int(thisNumber ^ 0.5)
If thisNumber Mod j = 0 Then
outP = outP & "*" & Trim$(Str$(j))
thisNumber = thisNumber / j
prime = False
GoTo loca
End If
Next
outP = outP & "*" & Trim$(Str$(thisNumber))
If outP = "" Then
outP = Trim$(Str$(i))
Else
outP = Mid$(outP, 2)
End If
prime = (Val(outP) = i)
badNum:
Cells(thisRow, startCol).Formula = Trim(Str(i))
If neg Then outP = "-1*" & outP
Cells(thisRow, startCol + 1).Formula = outP
If prime And Not (neg) Then
Cells(thisRow, startCol + 1).Font.Bold = True
Else
Cells(thisRow, startCol + 1).Font.Bold = False
End If
DoEvents
Next
End Sub
```