What are bitfields?

[akanevsky]

What are bitfields and how do they work? Esspecially, what does "&" operator do? Thanks.

[Adrian Schneider]

They are fields that contain a binary number such as 37 (100101), which contains:
32 +
16
8
4 +
2
1 +
= 37


They are used for permissions in vB, as well as checkbox profile fields (probably others too). You basically work backwards to see if it contains a value or not.

Here:
http://ca.php.net/manual/en/language.operators.bitwise.php

[deathemperor]

You may want to see this thread:

https://vborg.vbsupport.ru/showthread.php?t=76297

hope that help.

[akanevsky]

Well, every programmer already knows this but for anyone new to the fun here's some quick clean code to determine if a number is odd or even.

function oddeven($x){
if($x & 1) return "odd";
else return "even";
}

I don't get this...
What does it mean - work backwars to see if it contains a value or not.
Say, 3 does not contain 1... Then how is it an odd number?

You may want to see this thread:

https://vborg.vbsupport.ru/showthread.php?t=76297

hope that help.

Not really... I still don't understand how one variable can contain another.. Maybe I need a guide on how binary works...

And what are bit variables? are they arrays? Or varibles that store data in a special form? Or just variables with binary-encoded data?

[Adrian Schneider]

It basically goes through every 2^x where X is less than the # and if it can subtract the X, do so, then proceed - if it is < 0 then skip that subtraction and return false

So say for that function oddeven thing:

if the number is 20 (x = 20)
20 - 16 ( >= 0) x & 16 = true
4 - 8 ( < 0) x & 8 = false
4 - 4 ( >= 0) x & 4 = true
0 - 2 ( < 0) x & 2 = false
0 - 1 ( < 0) x & 1 = false

[akanevsky]

I am confused...
"where X is less than the #" - what #?
"if it can subtract the X" - what is "it"?

Regarding the oddeven thing..
Please show me the process by which 5 & 1 determines that 5 is odd... Thanks.

32 +
16
8
4 +
2
1 +
= 37

Why does it skip of 2,8,16?

[Adrian Schneider]

37-32 >= 0 so change the number to 5 (which is what is left over)
5 - 16 < 0, so ignore
5 - 8 < 0, so ignore
5 - 4 >= 0, so change the number to 1 (which is what is left over)
1 - 2 < 0, so ignore
1 - 1 >= 0, so change the number to 0

$num = 64;

for ($i=6; $i>0; $i--)
{
$result = pow(2, $i);
if ($num & $result)
{
echo 1;
}
else
{
echo 0;
}
}

outputs 100000 (same thing as what i just posted really)

[Andreas]

A Bitfield is a Variable/Database Field where each Bit is being as a On-Off Setting.
This way you can store may settings in one integer.

Decimal 5 is Binaray 101 (1*2^0 + 0*2^1 + 1*2^2).
& is a binary AND operator, which can be used to check if a Bit is set: 101 & 001 = 001, eg. the Bit is set.

[akanevsky]

Thanks, KirbyDE.

Could someone please explain how does this function work?

function oddeven($x){
if($x & 1) return "odd";
else return "even";
}

Decimal 5 is Binaray 101 (1*2^0 + 0*2^1 + 1*2^2).
Hmm...
2 in zero power multiplied by 1 is 1...
2 in first power multiplied by 0 is 0....
But 2 in second power multiplied by 1 is 5, not 1... (wtf?)

[Andreas]

Could someone please explain how does this function work?

Well ... all Powers of 2 except 2^0 are even.
So if a number is odd, it must have Bit 0 (2^0=1) set, so this functions checks for this Bit being set.


But 2 in second power multiplied by 1 is 5, not 1... (wtf?)
1*2^0 = 1*1 = 1
0*2^1 = 0*2 = 0
1*2^2 = 1*4 = 4
In Total: 5
:)

[deathemperor]

let me try to explain.

like the above example:

32
16
8
4
2
1

this is 65, in your database you have a field called permissionwhatever to store that number (65), and if a usergroup has that value 65 it means they have full permission, each bit represent for a permission. if it's 64, then the value you don't have must be 1, 60 means without 1 and 4 and it cannot be any other number. so that's the way it realizes you don't have what permission.

[akanevsky]

Well ... all Powers of 2 except 2^0 are even.
So if a number is odd, it must have Bit 0 (2^0=1) set, so this functions checks for this Bit being set.

I don't understand the connection between 0 power and Bit 0? =//

1*2^0 = 1*1 = 1
0*2^1 = 0*2 = 0
1*2^2 = 1*4 = 4
In Total: 5

How do you convert decimals to binarys?

let me try to explain.

like the above example:

32
16
8
4
2
1

this is 65, in your database you have a field called permissionwhatever to store that number (65), and if a usergroup has that value 65 it means they have full permission, each bit represent for a permission. if it's 64, then the value you don't have must be 1, 60 means without 1 and 4 and it cannot be any other number. so that's the way it realizes you don't have what permission.

I understand this, thanks...

[Marco van Herwaarden]

Also have a look at this post: https://vborg.vbsupport.ru/showpost.php?p=734362&postcount=4

Might help in understanding

[akanevsky]

Okay, I already completely understand how the vB permissions work :)

What I do not understand is this:

Well ... all Powers of 2 except 2^0 are even.
So if a number is odd, it must have Bit 0 (2^0=1) set, so this functions checks for this Bit being set.
I don't understand the connection between 0 power and Bit 0? =//

[Andreas]

Hmm, what exactly do you not understand?
2^0 = 1
2^1 = 2
2^2 = 4
2^3 = 8
2^4 = 16
...
As you can see, all numbers are even - except 1. So if you add them together, you will always get an even number if 1 is not included.
1 is 2^0, which means this bit must be set if a number is odd.

[akanevsky]

Oh!!! Yeah, now I understand.
When a number is an even number, it consists of such sub-numbers, as 2,4,8,16, but not 1. And when a number is odd, it has 1 included, so 3 & 1 works :) This is so cool...

Thanks everyone.