Bitwise Operations

  • Thread starter Thread starter Christopher Weaver
  • Start date Start date
C

Christopher Weaver

I know that the bitwise AND of 8 and 4 will return 0 or false and the
bitwise AND of 8 and 9 will return 1 or true but I don't know how to write
the synax for it in C#. I have a value that ranges from 0 to 15 and I need
to compare it to 15 in order to find if it contains the values 1, 2, 4, or
8.

To represent it more graphically, the value 1010 when ANDed with 1000 or
0010 will produce either true or a value greater than 0. I believe the
operator I need to use is &. Can anyone tell me how this is done?
 
Christopher,

if ((value & 0x01) == 0x01)
Contains1();
if ((value & 0x02) == 0x02)
Contains2();

etc.

Regards - Octavio
 
This works where Status is the value containing the various options and
STATUS_CANCELLED is a constant that represents one of them:

if ((Status & STATUS_CANCELLED) >0)
{
ckbCancelled.Checked = true;
}
else
{
ckbCancelled.Checked = false;
}

Any other ideas?
 
Actually, 8 AND 9 gives 8 not 1.

AFAIK, for the CLR 0 is FALSE and anything NOT FALSE is TRUE (ie.
anything not zero)

i.e.
System.Convert.ToBoolean(8) = True

So 8 AND 9 is true but becuase it is not equal to 0, not because it is
equal to 1.


Back on topic: the bitwise AND and OR operators are & and |

8 AND 9 ==> 8 & 9

for any bit test

(( value & bit) == bit)

e.g.

((15 & 4) == 4) is true, thus 4 is turned on
((9 & 4) == 4) is false, thus 4 is turned off


The simplest way to do this is to set up a lookup table.

bool[][] lookup = new bool[16][]{ new bool[4]{false,
false,false,false},
new bool[4]{false,
false,false,true},
...
new bool[4]{true,
true,true,true}};


Indexing into the table e.g. lookup[9] will give you a bool[4]
indicating which bits are turned on.


hth,

Alan.
 
Christopher Weaver said:
I know that the bitwise AND of 8 and 4 will return 0 or false and the
bitwise AND of 8 and 9 will return 1 or true but I don't know how to write
the synax for it in C#. I have a value that ranges from 0 to 15 and I need
to compare it to 15 in order to find if it contains the values 1, 2, 4, or
8.

To represent it more graphically, the value 1010 when ANDed with 1000 or
0010 will produce either true or a value greater than 0. I believe the
operator I need to use is &. Can anyone tell me how this is done?
Click to expand...

int x = 8 & 9; // x=1
int y = 8 & 4; // y=0;

Does that help?
 
Just as an added measure, you might want to use a flag enum in your
code to make this code section more readable.

private void Form1_Load(object sender, System.EventArgs e)
{
int someValue = 7;
Settings settings = (Settings) someValue;
if (settings == Settings.None)
{
MessageBox.Show("None");
}
if ((settings | Settings.Setting1 | Settings.Setting2 |
Settings.Setting3 | Settings.Setting4) > 0)
{
MessageBox.Show("One of the settings is set.");
}
}

[Flags()]
private enum Settings
{
None = 0,
Setting1 = 1,
Setting2 = 2,
Setting3 = 4,
Setting4 = 8,
All = Setting1 | Setting2 | Setting3 | Setting4
}


http://www.csvreader.com
 
Hi shriop,

I agree that an enum is appropriate, but I'm not sure about "[Flags()]".
MSDN says it should be used, but doesn't say why. I've used bitwise enums
without Flags, and they work fine. Any info?

Cheers,

Javaman.
 
I'm repeating what a few have said already, but I hope that this sums it up...
I know that the bitwise AND of 8 and 4 will return 0 or false and the
bitwise AND of 8 and 9 will return 1 or true but I don't know how to write
the synax for it in C#. I have a value that ranges from 0 to 15 and I need
to compare it to 15 in order to find if it contains the values 1, 2, 4, or
8.
Click to expand...

In c/c++ true and false can be used as ints, but in c#, true and false are
of type bool, which is not compatible with int. To get a bool from a number,
you must do a omparison with 0.

eg.
if ((0x11 & 0x01) != 0)
{
Console.WriteLine("True");
}

Note that you should almost always use != 0 to test for true, rather than ==
1 (or == 0x10, or whatever).

C# is so strict about this, that it doesn't even permit a cast from int to
bool. (eg. (bool) 5 won't compile). WTG.

To represent it more graphically, the value 1010 when ANDed with 1000 or
0010 will produce either true or a value greater than 0. I believe the
operator I need to use is &. Can anyone tell me how this is done?
Click to expand...

The operators you need are & and |, for the bitwise operations, and == and
!= to get the boolean.

You should use an enum to define your bitwise constants.

This program shows how it's down...note particularly the function
"Eligible", which I discuss below...

class BitwiseExample
{
// Declare an enum with fixed bit settings
enum EmploymentBasis
{
None = 0,
FullTime = 1,
PartTime = 2,
Casual = 4,
Temporary = 8,
}

// Use bitwise operations to test set inclusion.
// Return true if the Employment Basis is FullTime
// or ParTime.
static bool Eligible(EmploymentBasis e)
{
return ((e &
(EmploymentBasis.FullTime |
EmploymentBasis.PartTime))
!= 0);
}

public static void Main()
{

// Use our bitwise test for set inclusion.

EmploymentBasis emp1 = EmploymentBasis.FullTime;
EmploymentBasis emp2 = EmploymentBasis.Casual;

Console.WriteLine(
"emp1 eligible for retirement benefits = " + Eligible(emp1));

Console.WriteLine(
"emp2 eligible for retirement benefits = " + Eligible(emp2));

}
}

Output:

emp1 eligible for retirement benefits = True
emp2 eligible for retirement benefits = False

This function shows how to use the &, | and != operators. We use | to build
up a set (FullTime | PartTime), we & to test for inclusion in the set, and we
use != to get a boolean.

static bool Eligible(EmploymentBasis e)
return ((e &
(EmploymentBasis.FullTime |
EmploymentBasis.PartTime))
!= 0);


I've seen in the help files the [Flags] attribute, which is intended for
bitwise enums, and would be used like this
[Flags]
enum EmploymentBasis
{
}

but it is not clear what it does. My example program worked fine without it.

Cheers,

Javaman
 
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