What does Xor mean?

Code:

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NowPlaying = NowPlaying Xor True : PictureBox1.Invalidate()

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Is this like php?

Code:

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$variable = ($variable2 = false) ? true : false

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?

http://msdn.microsoft.com/en-us/libr...a6(VS.80).aspx

Code:

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Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Debug.WriteLine("---Xor---")
        Debug.WriteLine(False Xor False) 'False
        Debug.WriteLine(False Xor True) 'True
        Debug.WriteLine(True Xor False) 'True
        Debug.WriteLine(True Xor True) 'False
        Debug.WriteLine("---Or---")
        Debug.WriteLine(False Or False) 'False
        Debug.WriteLine(False Or True) 'True
        Debug.WriteLine(True Or False) 'True
        Debug.WriteLine(True Or True) 'True
End Sub

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I believe the "X" actually stands for Exclusion.

Xor returns True if just one of the conditions is True

As such, NowPlaying = NowPlaying Xor True is the same as this:

Code:

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If NowPlaying = True Then
  NowPlaying = False
Else
  NowPlaying = True
End If

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or this:

Code:

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NowPlaying = Not(NowPlaying)

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Quote:

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Xor returns True if just one of the conditions is True

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and returns False when both conditions are Same whether True or False

Code:

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Condition1  Condition2    Result
True          True            False  ---> note this result
False          False            False
True          False            True
False          True            True

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the above is same what ForumAccount has told its just the another way to show u the working of XOR

a lot of times where xor is handy (where you will see it used a lot), is when you are trying to parse a combined flags enum to get individual values.

Quote:

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a lot of times where xor is handy (where you will see it used a lot), is when you are trying to parse a combined flags enum to get individual values.

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hey kleinma
please provide a sample for the same

XOR is actually eXclusive-OR.
One and only one conditions of the two must be true to make it return true. i.e. one is true and other is false.
Besides XORing two boolean expressions you can XOR two numbers also and it would return a number with their individual bits XORed.

Have a look here to know more about it.

@steel - what they are doing is toggling NowPlaying between true and false. A clearer way to do that is using Not

Code:

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        Dim toggle As Boolean
        toggle = False
        Debug.WriteLine(toggle.ToString)

        toggle = toggle Xor True
        Debug.WriteLine(toggle.ToString)

        toggle = toggle Xor True
        Debug.WriteLine(toggle.ToString)

        toggle = Not toggle
        Debug.WriteLine(toggle.ToString)

        toggle = Not toggle
        Debug.WriteLine(toggle.ToString)

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Code:

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    Enum bi As Byte
        t0 = CByte(2 ^ 0)
        t1 = CByte(2 ^ 1)
        t2 = CByte(2 ^ 2)
        t3 = CByte(2 ^ 3)
        t4 = CByte(2 ^ 4)
        t5 = CByte(2 ^ 5)
        t6 = CByte(2 ^ 6)
        t7 = CByte(2 ^ 7)
        tAll = &HFF
    End Enum
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'sample data
        Dim flags As Byte = bi.t0 Or bi.t3 Or bi.t4 Or bi.t6 'turn some bits on for example
        'create a mask
        Dim TwoBitMask As Byte = bi.tAll Xor (bi.t3 Or bi.t4)
        'turn only bit 3 & 4 off
        flags = flags And TwoBitMask
    End Sub

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Quote:

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Originally Posted by su ki

hey kleinma
please provide a sample for the same

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Sure... lets use the keys enum as an example (because I just had to do this the other day)

so keys is an enum value at system.windows.forms.keys

the enum has all the different values for the possible keys on a keyboard that windows would recognize, and each enum member has its own value that corresponds to the given key.

So

keys.a (the a key) has a value of 65
keys.esc (the escape key) has a value of 27
keys.shiftkey (the shift key) has a value of 16
but the actual modifier of the shift key being pressed is
keys.shift which has a value of 65536

anyways, the keys enumeration (any enumeration that has the <flags> attribute on it), can be used for bitwise combination.

This means I could say

Code:

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Dim myKeys as Keys = Keys.shift or keys.a

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using or is how you combine bits, so the value of myKeys would be both the shift modifier key and the a key.

So the problem is, how do I know that?

Well if I want to test if the shift key was pressed along with whatever other key i pressed, it is easy enough to write code like this with an or keyword which will return true if myKeys contains shift (regardless of what other keys it contains)

Code:

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if myKeys = (myKeys or keys.shift) then
  'myKeys contains shift modifier
else
    'myKeys does not contain shift modifier
end if

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so you can say, ok great, I know the shift modifier key was pressed, but what actual key was pressed along with it? well you could write a huge long if statement using or to test every possible key combination, or you could use xor like this:

Code:

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Dim KeyPressedAlongWithShift as keys = CType((myKeys Xor keys.shift), Keys)

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and the value of KeyPressedAlongWithShift would be 65 also known as keys.a

I hope that makes sense.. bit shifting can be a bit confusing...

Code:

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    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim i As Integer
        i = 42
        i = -i 'i=negative 42

        'two-complement the hard way
        i = 42
        i = (i Xor &HFFFFFFFF) + 1 'i=negative 42
    End Sub

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So,

If A xor B

is the same as

If A = True or B = True and A <> B

In terms of Boolean logic yes, but it would usually be written more like this:

Code:

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If (A = True And B = False) or (A = False And B = True) Then

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As such, the original code (which had a hard-coded 'True') was basically the same as this:

Code:

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A = (A = True And True = False) or (A = False And True = True)

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..because the first half will always evaluate to False, it can be reduced to this:

Code:

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A = (A = False And True = True)

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..and because the second half of that will always evaluate to True, it can then be reduced to either of these:

Code:

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A = (A = False)
A = Not(A)

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The bitwise uses are more complex, but are based on the same principles.

A million thanks..

will remember this tool, its the same as PHP

except its

($var = true) ? 'var is true' : 'var is false'

I will just point out that if you want to toggle each bit of a value then using Xor makes sense but if you want to toggle a boolean value then it makes far more sense to simply negate it:

vb.net Code:

NowPlaying = Not NowPlaying

hey kleinma

thanks for the example :-)