Generating Random Strings of Characters

Nancy is trying to get Excel to pick 50 "numbers" that each contain eight random characters. The characters can be either digits or letters (uppercase or lowercase).

If your random numbers were to really be numbers (digits only), then generating them would be easy. All you would need to do is use the RANDBETWEEN function (in the Analysis ToolPak) in this manner:

=RANDBETWEEN(10000000,99999999)

This is not what Nancy wants, however. Her random "numbers" can contain upper- and lowercase letters, as well. This becomes a bit stickier. There are, however, several approaches you can use.

One approach is to put all your possible characters into an individual cell, such as B7:

ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789

Name this cell something snazzy, such as MySource. You could then use a formula such as the following to return the random string of characters:

=MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)
& MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)
& MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)
& MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)
& MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)
& MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)
& MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)
& MID(MySource,RANDBETWEEN(1,LEN(MySource)),1)

The formula is long; it has been broken into individual lines for clarity, but it is still a single formula. It concatenates eight characters pulled from the source you entered into cell B7.

Another approach is to create a table that contains all the characters you would want in your random text string. Start by placing the numbers 1 through 62 in a column, one number in each row. To the left of these numbers place your characters—A, B, C, D, etc. (This should be the same characters you placed in cell B7 in the previous technique.) Select both columns of the 62 rows and give it a name, such as MyTable. You can then use the following formula to generate the random characters:

=VLOOKUP(RANDBETWEEN(1,62),MyTable,2)
& VLOOKUP(RANDBETWEEN(1,62),MyTable,2)
& VLOOKUP(RANDBETWEEN(1,62),MyTable,2)
& VLOOKUP(RANDBETWEEN(1,62),MyTable,2)
& VLOOKUP(RANDBETWEEN(1,62),MyTable,2)
& VLOOKUP(RANDBETWEEN(1,62),MyTable,2)
& VLOOKUP(RANDBETWEEN(1,62),MyTable,2)
& VLOOKUP(RANDBETWEEN(1,62),MyTable,2)

Again, remember that this is a single formula, although it is a bit shorter than the previous formula.

Each of the approaches presented so far has one drawback: they are regenerated each time your worksheet is recalculated. Thus, it is hard to have a single generated random string that won't change on a regular basis. The best way around this is to use a macro, but you don't necessarily want to use a user-defined function. Why? Because it, too, would change its result every time the worksheet was recalculated. Instead, you need a macro that will put the random strings into your workbook starting at a specific cell location. The following is an example of such a macro:

Sub MakeRandom()
    Dim J As Integer
    Dim K As Integer
    Dim iTemp As Integer
    Dim sNumber As String
    Dim bOK As Boolean

    Range("D4").Activate
    Randomize
    For J = 1 To 50
        sNumber = ""
        For K = 1 To 8
            Do
                iTemp = Int((122 - 48 + 1) * Rnd + 48)
                Select Case iTemp
                    Case 48 To 57, 65 To 90, 97 To 122
                        bOK = True
                    Case Else
                        bOK = False
                End Select
            Loop Until bOK
            bOK = False
            sNumber = sNumber & Chr(iTemp)
        Next K
        ActiveCell.Value = sNumber
        ActiveCell.Offset(1, 0).Select
    Next J
End Sub

Run the macro, and whatever is in cells D4:D53 is overwritten by the random values. If you want the values written into a different location, change the Range statement near the beginning of the macro.

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (3872) applies to Microsoft Excel 97, 2000, 2002, and 2003. You can find a version of this tip for the ribbon interface of Excel (Excel 2007 and later) here: Generating Random Strings of Characters.