[RESOLVED] Logic With days

[Oliver1]

Hi All,

We have a system were a user chooses a combination of days from a calander. We then insert a values in the databse based on the following

Sunday - 1
Monday - 2
Tuesday - 4
Wednesday - 8
Thursday - 16
Friday - 32
Saturday - 64

So if they picked Monday and Friday we would insert 34, or if they picked Monday,Tuesday and Thursday we would insert 22.

Now my question is how do I reverse this easily in code, to find out what days they have picked. So I get fed 22 from the database how do I figure out what combination of days make up that value.

Sorry it sounds a bit like a homework project bit it really isn't.
The only thing I can think of is creating the mother of all case statements but I know there must be a better way.

[jmcilhinney]

You don't have to but I would suggest defining an enumeration to represent the days.

http://www.vbforums.com/showthread.php?t=502644

The logic is still pretty much the same even if you don't use an enumeration, i.e. bitwise operations, e.g.

Dim days As Integer = 2 Or 32 'Monday and Friday
 
If (days And 1) = 1 Then
    MessageBox.Show("Sunday")
Else
    MessageBox.Show("Not Sunday")
End If
 
If (days And 2) = 2 Then
    MessageBox.Show("Monday")
Else
    MessageBox.Show("Not Monday")
End If
 
If (days And 4) = 4 Then
    MessageBox.Show("Tuesday")
Else
    MessageBox.Show("Not Tuesday")
End If
 
If (days And 8) = 8 Then
    MessageBox.Show("Wednesday")
Else
    MessageBox.Show("Not Wednesday")
End If
 
If (days And 16) = 16 Then
    MessageBox.Show("Thursday")
Else
    MessageBox.Show("Not Thursday")
End If
 
If (days And 32) = 32 Then
    MessageBox.Show("Friday")
Else
    MessageBox.Show("Not Friday")
End If
 
If (days And 64) = 64 Then
    MessageBox.Show("Saturday")
Else
    MessageBox.Show("Not Saturday")
End If

[tsungik]

    Dim tmp as Integer = 64 ' Change this to the value from the Database 
    Dim strDays as String = String.Empty
    If tmp >= 64 Then
        strDays = "Sat"
        tmp = tmp Mod 64
    End If
    If tmp >= 32 Then
        strDays = "Fri" + strDays
        tmp = tmp Mod 32
    End If
    If tmp >= 16 Then
        strDays = "Thu" + strDays
        tmp = tmp Mod 16
    End If
    If tmp >= 8 Then
        strDays = "Wed" + strDays
        tmp = tmp Mod 8
    End If
    If tmp >= 4 Then
        strDays = "Tue" + strDays
        tmp = tmp Mod 4
    End If
    If tmp >= 2 Then
        strDays = "Mon" + strDays
        tmp = tmp Mod 2
    End If
    If tmp >= 1 Then
        strDays = "Sun" + strDays
        tmp = tmp Mod 1
    End If
    MessageBox.Show(strDays)

There is better code out there, but this will sure help you figure out to solve your problem.

[Oliver1]

Thanks both.
Have never used "and" and "or" Bitwise Operators for anything other than booleans and that was a long time ago.
Anyway I lied a little by posting this in the vb.net forum, as ideally I want to do this all at the tsql level (though worst come to worst I would have done it in code), quick google search gave me.

http://sqlfool.com/2009/02/bitwise-operations/

Which shows me how to do the same thing in sql.
So thanks for your help.

[jmcilhinney]

Have never used "and" and "or" Bitwise Operators for anything other than booleans and that was a long time ago

If you used them with Booleans then they were logical operators, not bitwise operators. Bitwise operations are basically logical operations at the bit level, where 1 is true and 0 is false. When you perform a bitwise operation on two values you basically perform a logical operation on each pair of corresponding bits.