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. (
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